Key role of surface oxidation and reduction processes in the coarsening of Pt nanoparticles

The surface chemistry of the atomic layer deposition of metal thin films

In this perspective we discuss the progress made in the mechanistic studies of the surface chemistry associated with the atomic layer deposition (ALD) of metal films and the usefulness of that knowledge for the optimization of existing film growth processes and for the design of new ones. Our focus is on the deposition of late transition metals. We start by introducing some of the main surface-sensitive techniques and approaches used in this research. We comment on the general nature of the metallorganic complexes used as precursors for these depositions, and the uniqueness that solid surfaces and the absence of liquid solvents bring to the ALD chemistry and differentiate it from what is known from metalorganic chemistry in solution. We then delve into the adsorption and thermal chemistry of those precursors, highlighting the complex and stepwise nature of the decomposition of the organic ligands that usually ensued upon their thermal activation. We discuss the criteria relevant for the selection of co-reactants to be used on the second half of the ALD cycle, with emphasis on the redox chemistry often associated with the growth of metallic films starting from complexes with metal cations. Additional considerations include the nature of the substrate and the final structural and chemical properties of the growing films, which we indicate rarely retain the homogeneous 2D structure often aimed for. We end with some general conclusions and personal thoughts about the future of this field.

Controlling Pt nanoparticle sintering by sub-monolayer MgO ALD thin films

Metal nanoparticle (NP) sintering is a major cause of catalyst deactivation, as NP growth reduces the surface area available for reaction. A promising route to halt sintering is to deposit a protective overcoat on the catalyst surface, followed by annealing to generate overlayer porosity for gas transport to the NPs. Yet, such a combined deposition-annealing approach lacks structural control over the cracked protection layer and the number of NP surface atoms available for reaction. Herein, we exploit the tailoring capabilities of atomic layer deposition (ALD) to deposit MgO overcoats on archetypal Pt NP catalysts with thicknesses ranging from sub-monolayers to nm-range thin films. Two different ALD processes are studied for the growth of MgO overcoats on Pt NPs anchored on a SiO2 support, using Mg(EtCp)2 and H2O, and Mg(TMHD)2 and O3, respectively. Spectroscopic ellipsometry and X-ray photoelectron spectroscopy measurements reveal significant growth on both SiO2 and Pt for the former process, while the latter exhibits a drastically lower growth per cycle with an initial chemical selectivity towards Pt. These differences in MgO growth characteristics have implications for the availability of uncoated Pt surface atoms at different stages of the ALD process, as probed by low energy ion scattering, and for the sintering behavior during O2 annealing, as monitored in situ with grazing incidence small angle X-ray scattering (in situ GISAXS). The Mg(TMHD)2-O3 ALD process enables exquisite coverage control allowing a balance between physically blocking the Pt surface to prevent sintering and keeping Pt surface atoms free for reaction. This approach avoids the need for post-annealing, hence also safeguarding the structural integrity of the as-deposited overcoat.

Nanoscale Chemical Diversity of Coke Deposits on Nanoprinted Metal Catalysts Visualized by Tip-Enhanced Raman Spectroscopy

Coke formation is the prime cause of catalyst deactivation, where undesired carbon wastes block the catalyst surface and hinder further reaction in a broad gamut of industrial chemical processes. Yet, the origins of coke formation and their distribution across the catalyst remain elusive, obstructing the design of coke-resistant catalysts. Here, the first-time application of tip-enhanced Raman spectroscopy (TERS) is demonstrated as a nanoscale chemical probe to localize and identify coke deposits on a post-mortem metal nanocatalyst. Monitoring coke at the nanoscale circumvents bulk averaging and reveals the local nature of coke with unmatched detail. The nature of coke is chemically diverse and ranges from nanocrystalline graphite to disordered and polymeric coke, even on a single nanoscale location of a top-down nanoprinted SiO2 -supported Pt catalyst. Surprisingly, not all Pt is an equal producer of coke, where clear isolated coke "hotspots" are present non-homogeneously on Pt which generate large amounts of disordered coke. After their formation, coke shifts to the support and undergoes long-range transport on the surrounding SiO2 surface, where it becomes more graphitic. The presented results provide novel guidelines to selectively free-up the coked metal surface at more mild rejuvenation conditions, thus securing the long-term catalyst stability.

Metal Nanocatalyst Sintering Interrogated at Complementary Length Scales

Metal nanoparticle (NP) sintering is a prime cause of catalyst degradation, limiting its economic lifetime and viability. To date, sintering phenomena are interrogated either at the bulk scale to probe averaged NP properties or at the level of individual NPs to visualize atomic motion. Yet, "mesoscale" strategies which bridge these worlds can chart NP populations at intermediate length scales but remain elusive due to characterization challenges. Here, a multi-pronged approach is developed to provide complementary information on Pt NP sintering covering multiple length scales. High-resolution scanning electron microscopy (HRSEM) and Monte Carlo simulation show that the size evolution of individual NPs depends on the number of coalescence events they undergo during their lifetime. In its turn, the probability of coalescence is strongly dependent on the NP's mesoscale environment, where local population heterogeneities generate NP-rich "hotspots" and NP-free zones during sintering. Surprisingly, advanced in situ synchrotron X-ray diffraction shows that not all NPs within the small NP sub-population are equally prone to sintering, depending on their crystallographic orientation on the support surface. The demonstrated approach shows that mesoscale heterogeneities in the NP population drive sintering and mitigation strategies demand their maximal elimination via advanced catalyst synthesis strategies.

In Situ TEM Observation of the Atomic Transport Process during the Coalescence of Au Nanoparticles

In practical applications, the coalescence of metal nanoparticles (NPs) is a major factor affecting their physical chemistry properties. Currently, due to a lack of understanding of the atomic-level mechanisms during the nucleation and growth stages of coalescence, the correlation between the different dynamic factors in the different stages of NP coalescence is unclear. In this study, we used advanced in situ characterization techniques to observe the formation of atomic material transport channels (Au chains) during the initiation of coalescence nucleation. We focused on the movement and migration states of Au atoms and discovered an atomic ordered arrangement growth mechanism that occurs after the completion of nucleation. Simultaneously, we used density functional theory to reveal the formation principle of Au chains. These findings improve our understanding of the atomic-scale coalescence process, which can provide a new perspective for further research on coalescence atomic dynamics.

Designing Nanoparticles and Nanoalloys for Gas-Phase Catalysis with Controlled Surface Reactivity Using Colloidal Synthesis and Atomic Layer Deposition

Supported nanoparticles are commonly applied in heterogeneous catalysis. The catalytic performance of these solid catalysts is, for a given support, dependent on the nanoparticle size, shape, and composition, thus necessitating synthesis techniques that allow for preparing these materials with fine control over those properties. Such control can be exploited to deconvolute their effects on the catalyst's performance, which is the basis for knowledge-driven catalyst design. In this regard, bottom-up synthesis procedures based on colloidal chemistry or atomic layer deposition (ALD) have proven successful in achieving the desired level of control for a variety of fundamental studies. This review aims to give an account of recent progress made in the two aforementioned synthesis techniques for the application of controlled catalytic materials in gas-phase catalysis. For each technique, the focus goes to mono- and bimetallic materials, as well as to recent efforts in enhancing their performance by embedding colloidal templates in porous oxide phases or by the deposition of oxide overlayers via ALD. As a recent extension to the latter, the concept of area-selective ALD for advanced atomic-scale catalyst design is discussed.

In situ study of the thermal stability of supported Pt nanoparticles and their stabilization via atomic layer deposition overcoating

Downscaling of supported Pt structures to the nanoscale is motivated by the augmentation of the catalytic activity and selectivity, which depend on the particle size, shape and coverage. Harsh thermal and chemical conditions generally required for catalytic applications entail an undesirable particle coarsening, and consequently limit the catalyst lifetime. Herein we report an in situ synchrotron study on the stability of supported Pt nanoparticles and their stabilization using atomic layer deposition (ALD) as the stabilizing methodology against particle coarsening. Pt nanoparticles were thermally annealed up to 850 °C in an oxidizing environment while recording in situ synchrotron grazing incidence small angle X-ray scattering (GISAXS) 2D patterns, thereby obtaining continuous information about the particle radius evolution. Al2O3 overcoat as a protective capping layer against coarsening via ALD was investigated. In situ data proved that only 1 cycle of Al2O3 ALD caused an augmentation of the onset temperature for particle coarsening. Moreover, the results showed a dependence of the required overcoat thickness on the initial particle size and distribution, being more efficient (i.e. requiring lower thicknesses) when isolated particles are present on the sample surface. The Pt surface accessibility, which is decisive in catalytic applications, was analyzed using the low energy ion scattering (LEIS) technique, revealing a larger Pt surface accessibility for a sample with Al2O3 overcoat than for a sample without a protective layer after a long-term isothermal annealing.

Thermal atomic layer deposition of gold nanoparticles: controlled growth and size selection for photocatalysis

Gold nanoparticles have been extensively studied for their applications in catalysis. For Au nanoparticles to be catalytically active, controlling the particle size is crucial. Here we present a low temperature (105 °C) thermal atomic layer deposition approach for depositing gold nanoparticles on TiO2 with controlled size and loading using trimethylphosphino-trimethylgold(iii) and two co-reactants (ozone and water) in a fluidized bed reactor. We show that the exposure time of the precursors is a variable that can be used to decouple the Au particle size from the loading. Longer exposures of ozone narrow the particle size distribution, while longer exposures of water broaden it. By studying the photocatalytic activity of Au/TiO2 nanocomposites, we show how the ability to control particle size and loading independently can be used not only to enhance performance but also to investigate structure-property relationships. This study provides insights into the mechanism underlying the formation and evolution of Au nanoparticles prepared for the first time via vapor phase atomic layer deposition. Employing a vapor deposition technique for the synthesis of Au/TiO2 nanocomposites eliminates the shortcomings of conventional liquid-based processes opening up the possibility of highly controlled synthesis of materials at large scale.

Understanding chemical and physical mechanisms in atomic layer deposition

Atomic layer deposition (ALD) is a powerful tool for achieving atomic level control in the deposition of thin films. However, several physical and chemical phenomena can occur which cause deviation from "ideal" film growth during ALD. Understanding the underlying mechanisms that cause these deviations is important to achieving even better control over the growth of the deposited material. Herein, we review several precursor chemisorption mechanisms and the effect of chemisorption on ALD growth. We then follow with a discussion on diffusion and its impact on film growth during ALD. Together, these two fundamental processes of chemisorption and diffusion underlie the majority of mechanisms which contribute to material growth during a given ALD process, and the recognition of their role allows for more rational design of ALD parameters.

Surface mobility and impact of precursor dosing during atomic layer deposition of platinum:in situmonitoring of nucleation and island growth

The nucleation rate and diffusion-driven growth of Pt nanoparticles are revealed within situX-ray fluorescence and scattering measurements during ALD: the particle morphology at a certain Pt loading is similar for high and low precursor exposures.

Creation of gallium acid and platinum metal sites in bifunctional zeolite hydroisomerization and hydrocracking catalysts by atomic layer deposition

Active sites in bifunctional zeolite catalysts were engineered using atomic layer deposition (ALD). Gallium acid and platinum metal sites were introduced to zeolites via ALD and investigated for a hydroconversion reaction.

Formation and Functioning of Bimetallic Nanocatalysts: The Power of X-ray Probes

Bimetallic nanocatalysts are key enablers of current chemical technologies, including car exhaust converters and fuel cells, and play a crucial role in industry to promote a wide range of chemical reactions. However, owing to significant characterization challenges, insights in the dynamic phenomena that shape and change the working state of the catalyst await further refinement. Herein, we discuss the atomic-scale processes leading to mono- and bimetallic nanoparticle formation and highlight the dynamics and kinetics of lifetime changes in bimetallic catalysts with showcase examples for Pt-based systems. We discuss how in situ and operando X-ray spectroscopy, scattering, and diffraction can be used as a complementary toolbox to interrogate the working principles of today's and tomorrow's bimetallic nanocatalysts.