Abstract
It is hard to predict the future of science. For example, when C60 and its structure were identified from the mass spectra of gas phase carbon clusters, few could have predicted the era of carbon nanotechnology which the discovery introduced. The solubilization and functionalization of C60, the identification and then synthesis of carbon nanotubes, and the generation and physics of graphene have made a scale of impact on the international R&D (and to some extent industrial) landscape which could not have been foreseen. Technology emerged from a search for molecules of astrochemical interest in the interstellar gas. This little sketch provides the authors with the confidence to present here a status report on progress toward another radical future-the synthesis of nanoparticles (typically metals) on an industrial scale without solvents and consequently effluents, without salts and their sometimes accompanying toxicity, with minimal prospects for unwanted nanoparticle escape into the environment, with a high degree of precision in the control of the size, shape and composition of the nanoparticles produced and with applications from catalysts and sensors to photonics, electronics and theranostics. In fact, our story begins in exactly the same place as the origin of the nanocarbon era-the generation and mass selection of free atomic clusters in a vacuum chamber. The steps along the path so far include deposition of such beams of clusters onto surfaces in vacuum, elucidation of the key elements of the cluster-surface interaction, and demonstrations of the potential applications of deposited clusters. The principal present challenges, formidable but solvable, are the necessary scale-up of cluster beam deposition from the nanogram to the gram scale and beyond, and the processing and integration of the nanoclusters into appropriate functional architectures, such as powders for heterogeneous catalysis, i.e., the formulation engineering problem. The research which is addressing these challenges is illustrated in this Account by examples of cluster production (on the traditional nanogram scale), emphasizing self-selection of size, controlled generation of nonspherical shapes, and nonspherical binary nanoparticles; by the scale-up of cluster beam production by orders of magnitude with the magnetron sputtering, gas condensation clustersource, and especially the Matrix Assembly Cluster Source (MACS); and by promising demonstrations of deposited clusters in gas sensing and in heterogeneous catalysis (this on the gram scale) in relevant environments (both liquid and vapor phases). The impact on manufacturing engineering of the new paradigm described here is undoubtedly radical; the prospects for economic success are, as usual, full of uncertainties. Let the readers form their own judgements.
Highlights
By definition, nanoparticles (NPs) or nanomaterials are small objects in the range of 1−100 nm,[1] usually defined as an agglomeration of atoms or molecules which exhibit a wide range of size-dependent properties.[2]
The chemical approaches are the most widely used, as they offer a high control over the size, shape and chemical composition.[9−11] the chemical approaches often involve chemical agents associated with environmental toxicity
A core−shell morphology with a highly crystalline core is revealed after air exposure, as shown in the inset to Figure 4a, in high-angle annular dark field image (HAADF) in scanning TEM (STEM) mode
Summary
Nanoparticles (NPs) or nanomaterials are small objects in the range of 1−100 nm,[1] usually defined as an agglomeration of atoms or molecules which exhibit a wide range of size-dependent properties.[2]. For binary NPs, three main types of NP morphologies (Figure 1) have been reported, which differ in the atomic arrangements of the two elements (A and B) within the same NP: (i) NP alloys (AB or BA), which can be either random[17] or ordered;[18,19] (ii) core@shell NPs (A@B),[20] which consist of one type of atoms (B) surrounding a core of another type of atoms (A); and (iii) the commonly named Janus NPs (A-B), which consist of two parts of different elements sharing a common interface.[21] Discussion of these morphologies has invoked thermodynamic considerations which determine the driving forces and stability of the different NPs.[22,23] a more in-depth understanding of the underlying mechanisms of NP growth has highlighted kinetic effects, for example, to explain the formation mechanism of cuboid shapes in gas-phase synthesis.[24] Among these different morphologies, CBD methods offer the ability to size-control the NPs by postgrowth separation phase (Figure 1), such as time-of-flight[25,26] (TOF) or quadrupole mass-filtration.[27] size separation challenges the utilization of gas-phase condensation techniques for large scale production of nanoparticles.[25] In this Account, we highlight methods that focus on accurate determination of nanoparticle sizes during the growth, instead of producing a wide size-dispersion first followed by mass-filtration afterward. We consider the potential of nanoparticle selfarrangement during gas-phase growth, and a new class of CBD source, the Matrix Assembly Cluster Source (MACS) that promises a breakthrough in large-scale production of clusters
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