Periodic Reporting for period 1 - FASINA (Fabrication and Synthesis of Noble Metal Nanoparticle Arrays for Single Particle Catalysis)
Période du rapport: 2021-05-01 au 2023-04-30
Noble metal nanoparticles find use as catalysts in, for example, sensing, energy technology, and environmental clean-up applications. The structural and compositional complexity of such nanosized systems offers many degrees of freedom for tuning their catalytic, as well as other such as mechanical or optical/plasmonic, properties. Nevertheless, structural heterogeneity at the individual particle level is still prevalent in most studies, which commonly use billions of particles at a time. This heterogeneity hampers deeper understanding of how particle size, shape and composition affect their catalytic activity and selectivity, as these parameters directly determine the active sites exposed to the reactants. Revealing such behavior, as well as identifying champion or spectator nanoparticles, requires single nanoparticle-level measurements. To this end, impressive advances have been made in the last years, and a number of approaches now exist that can monitor processes on single nanoparticles, including electrochemical methods, single molecule fluorescence microscopy, plasmonic nanospectroscopy, X-ray microscopy, and surface-enhanced Raman spectroscopy.
However, because of this recent methodological progress, it has also become apparent that one of the major remaining bottlenecks in the development of these and other techniques is the challenge to position the nanoparticles rationally and precisely on the surface. This is the consequence of the fact that – to date – shape- and size-selected nanoparticles are first made by colloidal synthesis in suspensions and then self-assembled, or randomly deposited, onto the substrate surface. On one hand, this provides access to the exquisite chemical control offered by colloidal synthesis, which produces sophisticated nanoparticles in terms of morphology, faceting, and composition, which often express a single-crystal character. On the other hand, and as the key point here, it comes with a significant inherent lack of control during nanoparticle deposition on the substrate once the synthesis is completed.
Hence, recognizing that all existing nanofabrication and synthesis approaches independently have severe limitations, but that they collectively would satisfy all the key requirements necessary for the fabrication of nanocrystal arrays, it was the main aim of this project to develop strategies in which both nanofabrication and colloidal synthesis methods are synergistically combined into a new paradigm. Specifically, the aim was to first use nanolithography to define arrays of relatively simple particles (seeds) on a surface and subsequently transform them through colloidal chemistry into more sophisticated nanostructures. It was also the aim of this project to make these nanostructured surfaces compatible with a plasmonic nanospectroscopy platform to monitor catalytic processes at the single-particle level.
However, because of this recent methodological progress, it has also become apparent that one of the major remaining bottlenecks in the development of these and other techniques is the challenge to position the nanoparticles rationally and precisely on the surface. This is the consequence of the fact that – to date – shape- and size-selected nanoparticles are first made by colloidal synthesis in suspensions and then self-assembled, or randomly deposited, onto the substrate surface. On one hand, this provides access to the exquisite chemical control offered by colloidal synthesis, which produces sophisticated nanoparticles in terms of morphology, faceting, and composition, which often express a single-crystal character. On the other hand, and as the key point here, it comes with a significant inherent lack of control during nanoparticle deposition on the substrate once the synthesis is completed.
Hence, recognizing that all existing nanofabrication and synthesis approaches independently have severe limitations, but that they collectively would satisfy all the key requirements necessary for the fabrication of nanocrystal arrays, it was the main aim of this project to develop strategies in which both nanofabrication and colloidal synthesis methods are synergistically combined into a new paradigm. Specifically, the aim was to first use nanolithography to define arrays of relatively simple particles (seeds) on a surface and subsequently transform them through colloidal chemistry into more sophisticated nanostructures. It was also the aim of this project to make these nanostructured surfaces compatible with a plasmonic nanospectroscopy platform to monitor catalytic processes at the single-particle level.
During the project, regular arrays of polycrystalline and single-crystal nanoparticles, mainly of Pd but also of other metal (Au, Ag and Pt), of different sizes and shapes have been successfully grown in-situ on oxidized silicon surfaces.
The developed methodology uses electron beam lithography (EBL) to generate a surface templated with perfectly positioned seed particles that are subsequently transformed in solution by adapting some of the most common colloidal synthesis protocols for particle size and shape control available in the literature. The main advantage of EBL is its nearly endless possibilities to craft ‘arbitrary’ nanostructure arrays with nanometric resolution in the position of each unit. Furthermore, as a key new feature, the protocol features a thin poly(methyl methacrylate) (PMMA) protective layer that is deposited onto the seed arrays previous to their exposure to the growth solution. The thickness of this PMMA layer is adjusted to fully cover the substrate surface and expose only the seeds to the growth solution. In this way, the PMMA layer allows the growth in solution of the seeds while it, at the same time, effectively prevents contamination of the substrate surface by secondary nanoparticles (nucleating either at off-pattern sites on the surface or in the growth solution), which is an ever-present concern in substrate-supported nanoparticle synthesis.
Interestingly, taking advantage of the exceptional control achieved through EBL on the position of the particles, and the possibility to grow them in-situ, we have revealed the critical role of the array lattice parameters in determining the kinetics of particle growth. By having access to this kinetic control, we have then proven that it is possible to manipulate globally and locally the growth rate of supported-nanoparticles in an unreported way, simply by modifying the array parameters, introducing array defects, or changing the array periodicity. The result is the preparation of shape-selected nanoparticle arrays with rational and systematic variations in particle size on surfaces.
Finally, the applicability of the project has been demonstrated by plasmonically monitoring the kinetics of hydride formation in arrays of polycrystalline Pd flower-like and star-like nanoparticles, and in single crystal Pd nanocube arrays, to establish structure-activity correlations at the single-particle level.
As for the potential exploitable results, the developed methodology is sufficiently robust to be extended to a range of nanostructures and nanostructure compositions on surfaces that goes beyond those prepared in the project. The outputs of the research also prove how in-situ grown nanoparticle arrays can be used to study process on single-nanoparticles, but, by no means, the applicability is restricted to this specific case.
The developed methodology uses electron beam lithography (EBL) to generate a surface templated with perfectly positioned seed particles that are subsequently transformed in solution by adapting some of the most common colloidal synthesis protocols for particle size and shape control available in the literature. The main advantage of EBL is its nearly endless possibilities to craft ‘arbitrary’ nanostructure arrays with nanometric resolution in the position of each unit. Furthermore, as a key new feature, the protocol features a thin poly(methyl methacrylate) (PMMA) protective layer that is deposited onto the seed arrays previous to their exposure to the growth solution. The thickness of this PMMA layer is adjusted to fully cover the substrate surface and expose only the seeds to the growth solution. In this way, the PMMA layer allows the growth in solution of the seeds while it, at the same time, effectively prevents contamination of the substrate surface by secondary nanoparticles (nucleating either at off-pattern sites on the surface or in the growth solution), which is an ever-present concern in substrate-supported nanoparticle synthesis.
Interestingly, taking advantage of the exceptional control achieved through EBL on the position of the particles, and the possibility to grow them in-situ, we have revealed the critical role of the array lattice parameters in determining the kinetics of particle growth. By having access to this kinetic control, we have then proven that it is possible to manipulate globally and locally the growth rate of supported-nanoparticles in an unreported way, simply by modifying the array parameters, introducing array defects, or changing the array periodicity. The result is the preparation of shape-selected nanoparticle arrays with rational and systematic variations in particle size on surfaces.
Finally, the applicability of the project has been demonstrated by plasmonically monitoring the kinetics of hydride formation in arrays of polycrystalline Pd flower-like and star-like nanoparticles, and in single crystal Pd nanocube arrays, to establish structure-activity correlations at the single-particle level.
As for the potential exploitable results, the developed methodology is sufficiently robust to be extended to a range of nanostructures and nanostructure compositions on surfaces that goes beyond those prepared in the project. The outputs of the research also prove how in-situ grown nanoparticle arrays can be used to study process on single-nanoparticles, but, by no means, the applicability is restricted to this specific case.
In addition to the unique nanoparticle arrays prepared in the project, two main things have progressed beyond the state of the art.
One is the protecting PMMA layer, which represents an easy applicable alternative to current strategies aimed at protecting the growing seed arrays from being contaminated by secondary nanoparticles in solution. The control of the thickness of this PMMA layer also adds a new and hitherto unexplored synthetic parameter; the possibility to precisely adjust the fraction of a seed particle that will be exposed to the growth solution.
On the other hand, our direct manipulation of the array lattice parameters has revealed the critical role that interparticle interactions play in kinetically controlling the growth process of the nanoparticles. When accessing to this kinetic control, it is possible to simultaneously grow on the same surface and at predefined positions nanoparticles at different growth rates.
Hence, the project has laid the foundation of a unique platform for mechanistic studies in nanoparticle synthesis and provides new synthetic strategies for the design of nanostructured surfaces. By creating a platform in which nanostructures can be designed to express specific properties based on sophisticated architectures, the work also opens the door to the fabrication of functional chemically and optically active surfaces whose applicability certainly extends beyond those for which they were initially intended in the project.
One is the protecting PMMA layer, which represents an easy applicable alternative to current strategies aimed at protecting the growing seed arrays from being contaminated by secondary nanoparticles in solution. The control of the thickness of this PMMA layer also adds a new and hitherto unexplored synthetic parameter; the possibility to precisely adjust the fraction of a seed particle that will be exposed to the growth solution.
On the other hand, our direct manipulation of the array lattice parameters has revealed the critical role that interparticle interactions play in kinetically controlling the growth process of the nanoparticles. When accessing to this kinetic control, it is possible to simultaneously grow on the same surface and at predefined positions nanoparticles at different growth rates.
Hence, the project has laid the foundation of a unique platform for mechanistic studies in nanoparticle synthesis and provides new synthetic strategies for the design of nanostructured surfaces. By creating a platform in which nanostructures can be designed to express specific properties based on sophisticated architectures, the work also opens the door to the fabrication of functional chemically and optically active surfaces whose applicability certainly extends beyond those for which they were initially intended in the project.