Gas-phase synthesis of functional nanomaterials: Challenges to kinetics, diagnostics, and process development
Introduction
For decades, combustion research has focused on gaining understanding of the complex interaction of high-temperature reactions and fluid flows. This fundamental knowledge has successfully been used in the context of optimizing practical processes for energy conversion with the side condition of high efficiency and low pollutant formation. This has led to a wide range of sophisticated experimental and theoretical methods that help to tackle the related multi-scale and multi-phase problems that connect elemental processes from the molecular to the device scale and consider processes such as fuel evaporation, gas-phase reaction, and – in the context of soot – particle nucleation and growth.
Experimental and computational tools used by combustion researchers can be applied to gain insight into the research in gas-phase synthesis of nanomaterials providing the predictive and process control capabilities that could revolutionize nanomaterials production. Hydrocarbon/air combustion systems are primarily limited to C, H, O, and N, while nanomaterials require the addition of metals, metalorganics, and organometallics. Thus, modeling and quantitative diagnostics of nanoparticle-forming systems requires significant research to expand the fundamental chemical kinetics and spectroscopy databases.
In contrast to classical combustion, the reactive systems contain “unusual” species (such as a wide range of elements and their oxides as well as the related metal-organic and organometallic species) and they might be operated under different operating conditions. In contrast to the generation of materials by wet-chemical routes, the interplay between thermodynamics and kinetics enables the production of metastable materials in terms of composition and crystal structure by steep temperature gradients, e.g., by rapidly quenching high-temperature phases after their formation in the reaction zone. Also in contrast to wet-phase chemistry, gas-phase synthesis can generate “naked” particles without surface layers from solvents or reactants (ligands or capping agents) that can direct particle nucleation and growth. While the wet-phase methods are usually batch processes, typically ending up in the formation of thermodynamically stabilized materials, gas-phase synthesis – being a continuous flow process – can be scaled to continuous, large-scale production as demonstrated in industrial processes that are established for a limited number of species such as fumed silica, titania, and carbon black [1].
Nanoparticle synthesis therefore is a highly attractive – and challenging – field for fundamental studies towards practical processes with high industrial potential. This, of course, is related to expanding from C, H, O, N as the building blocks of combustion-related chemistry to a wide range of elements – strongly increasing the number of species that participate in the underlying reactions, for which thermodynamics and kinetics data are required. Also, the rich variability on particle structure and morphology (amorphous, crystalline, polymorphs, kinetically vs. thermodynamically stable, …) increases the parameter space.
Nanomaterials are of specific interest to many applications because they enable the modification of materials characteristics beyond the properties set by the bulk composition. The structure size on the nanometer scale influences the melting temperature (e.g., enabling sintering at much reduced temperature [2]), the magnetic properties (e.g., creating superparamagnetic particles that do not interact in the absence of a magnetic field and thus can be dispersed in liquids but can be collected again by applying magnetic fields [3], [4]), the electronic properties (e.g., band-gap tuning for electronics and photovoltaic applications or for modifying optical properties in quantum dots [5], [6]), and often enables compositional variation far beyond the stability limit of the respective bulk materials. In many cases, the mere structure size is of interest in providing materials with large active surface-to-volume ratios for instance in catalysis (to maximize the active surface area [7], for high density of phase boundaries (e.g., to tune the thermal conductivity in thermoelectrics materials [8], or for providing short distances for solid-state diffusion in battery materials [9], [10]. Small structures are often more tolerant to stress in the lattice structure, as, e.g., introduced by intercalation of ions in battery materials [11], [12], [13], [14] or by substitutional doping and formation of mixed oxides in crystal structures that are of interest in fine-tuning electronic and catalytic materials properties [15].
Of specific interest are energy-related applications. Highly potent energy-storage materials for next-generation batteries and supercapacitors, noble metal-free catalysts, and highly-efficient electrocatalysts for energy conversion and production of novel fuels are just few examples. Over the last years, the understanding how ideal materials should look like has dramatically improved and many promising highly elaborate examples have been demonstrated. In many cases, it is required to combine nanoparticle-based functionality (such as large surface area, short diffusion lengths, high defect density, mechanical/structural flexibility that enables high density for intercalation, e.g., of Li ions in a host in lithium-ion batteries) with porosity, and electrical conductivity in a single complex nanostructured material. Apart from the composition and size of the primary particles that determine ion storage and catalytic activity, secondary (e.g., aggregates, core-shell structures), and tertiary structures (e.g., nanocomposites) determine the practical applicability and durability. They ensure (and even synergistically support) species storage and enable transport from and towards the active particles without limitation through diffusive processes. They are able to provide flexible matrices with good electrical conductivity that can for instance accommodate volume changes inherent to battery materials.
For these electrically-conducting supporting structures, carbon-based materials are highly attractive. They can be formed from pyrolyzed polymers [16], [17] or added as, e.g., graphene and carbon nanotubes [12], [18], [19] that can also be generated from gas-phase processes [20], [21]. While mixing with carbon-based materials typically requires subsequent process steps, co-synthesis of carbonaceous materials has been demonstrated in some cases [22], which remains one mostly unexplored opportunity for future research towards the direct synthesis of complex nanostructured materials that combine nanoparticle-based functionality, porosity, and electrical conductivity.
This intricacy sets the stage for application-motivated fundamental research towards the gas-phase synthesis of functional nanomaterials that can benefit tremendously from combustion research. It also indicates that empirical strategies will not be able to fully exploit the potential of the synthesis toolbox because of the combinatorial complexity of the problem. The demonstrated successes of Edisonian discoveries merely indicate what is possible by fully mastering synthesis through understanding the kinetics of the related (often competing) processes. A detailed insight into the interaction of the reaction and the fluid flow is required as this interaction determines the species-concentration–temperature history a reacting volume element experiences on its way through a reactor.
Many of the materials of interest cater to potentially large-scale applications. Therefore, process scalability is an important issue – that again can benefit from the analogy to combustion research, where complex large-scale multiscale problems have been solved successfully based on an integration of the relevant sub-mechanisms in computational fluid dynamics (CFD) simulations. Underlying fundamental data needs to be generated in specific experiments such as flow reactors and shock tubes. Thus, models can be developed, reduced, and optimized through experiments in well-controlled standardized reactor systems with increasing complexity that enable a close interaction of advanced experimental techniques and numerical simulation.
There are, however, also aspects that simplify the description of nanoparticle synthesis processes in comparison to practical combustion. The process pressure is typically limited to atmospheric pressure (or slightly below to prevent the uncontrolled release of aerosols in case of leakages). Reacting flows are often laminar or limited to low levels of turbulence, and the structure of the particles is less influenced by molecular features (as in the case of soot) but directly lead to compact liquid or solid particles. The interaction (growth, coalescence, agglomeration) of these primary particles can often be well described by aerosol dynamics [7].
Section snippets
Organization of this paper
There are excellent recent reviews on various aspects of flame synthesis of nanoparticles and this paper tries to avoid overlap by focusing specifically on the underlying kinetics and diagnostics challenges. In this way it builds on the fundamental approach of the topical review Paul Roth presented on the 2006 Symposium [23]. The review by Strobel and Pratsinis focuses on combustion-synthesized materials [24]. In contrast, the review Pratsinis presented on the 2016 Symposium has a strong focus
Pathways of gas-phase synthesis of nanoparticles
In gas-phase synthesis of nanomaterials, so-called precursors – after being exposed to high temperature – decompose to condensable species in a highly supersaturated state that then leads to particle nucleation and growth. The ultimate aim for nanoparticle synthesis in the gas phase is to provide full control on the materials composition (pure, doped, mixed materials) that can either be present as homogeneous materials, as nanocomposites of materials with distinct compositions, or as structured
Kinetics experiments in shock tubes
The kinetics of the precursor chemistry and the initial particle formation strongly depend on temperature and operate on the micro- to millisecond time scale. Therefore, experimental investigations require rapid heating of the reactive gas mixture to defined temperatures and the application of time-resolved diagnostics (Fig. 3). Shock-tube experiments are ideally suited for studying the kinetics of ultra-fast gas-phase reactions at high temperatures, since near-ideal reaction conditions can be
Scale-up
Scale-up of gas-phase processes for nanoparticle synthesis based on in situ measurements and simulations that incorporate detailed information from fundamental experiments has been a core topic of our group for years resulting in unique facilities on the pilot-plant scale. They outperform pilot-plant-scale research facilities in industry due to their additional capabilities for in situ optical measurements. Silicon nanopowder produced in these facilities [30] has enabled research for batteries
Conclusions and future research needs
While there has been significant progress in recent years to move gas-phase synthesis of nanomaterials from empirical development to physics- and chemistry-based understanding, there is a wide range of complex research needs that must be addressed in the near future. While many fundamental methods of modeling and measurement can be transferred from combustion science, the main task is to expand the understanding of high-temperature processes on a wide variety of elements. This dramatically
Acknowledgments
The authors acknowledge funding by the German Research Foundation through a variety of projects, especially those within the Research Unit FOR2284 and the Priority Program SPP1980, and the European Union's Horizon 2020 Research and Innovation Program, under Grant Agreement No. 646121 (NanoDome). CS thanks all the members of his research group that have contributed to the work reviewed in this article.
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