Elucidating and understanding nanoparticle synthesis by nanosecond repetitively pulsed plasma discharges – NanoByNano

The portable, on-demand, and point-of-use synthesis of nanoparticles (NP) can improve the feasability of portable applications requiring such materials. Currently, many synthesis techniques require fixed-site, complex facilities, hence prohibiting on-demand NP production at a remote location. As a result, mobile devices must store and deliver pre-made NP, which adds cost and complexity. Stored NP can agglomerate and sediment, becoming unstable. Delivery to the point of use may require transformation of the NP storage medium, which could be energetically expensive and slow the responsiveness of the device. Furthermore, certain applications may require a variety of NP properties to adapt to changing usage conditions, which would complexify on-board storage/delivery systems. There exists thus a growing need to develop a new NP synthesis paradigm that can achieve fast and efficient synthesis using a simple and compact platform capable of exerting fine control over NP properties.

Plasmas can meet these challenges by producing physico-chemical conditions that are difficult to achieve otherwise. However, existing NP synthesis techniques using plasmas suffer from certain weaknesses. Low energy density techniques operating far from local thermodynamic equilibrium (LTE) have difficulty breaking down solid precursors, which are ideal for mobile applications in terms of compactness and ease of storage. High energy density techniques such as spark ablation at LTE rely on very high temperature on the order of 10000 K, which can negatively impact product yield and thermal engineering of the reactor.

We aim to achieve gas-phase synthesis of NPs by using nanosecond repetitively pulsed (NRP) plasmas, which achieve strongly non-LTE conditions that direct energy towards ionization and dissociation instead of heating and other losses, potentially leading to high energy efficiency. Shortening the pulse duration to the 10-ns scale may resolve several of the issues facing spark ablation by strongly limiting the thermal plasma state. Operating at high pulse repetition frequency above 10 kHz lowers the energy cost of ionization. NRP discharges also create unique thermal conditions for NP nucleation and growth, including very fast heating and cooling rates.

Although plasma techniques have been highly successful at synthesizing NP, gaining insight into synthesis mechanisms has relied largely on ex situ materials characterization of the produced NP. However, without direct observation of nucleation and growth inside the reactor, the understanding of these plasma-driven processes lacks detail and remains mostly limited to qualitative analyses. Validation of detailed, quantitative models would require in situ measurements of NP, chemical species, and reaction environments.

To address these challenges, NANObyNANO aims to develop a fundamental and detailed understanding of NP synthesis by plasma. Our experimental approach will feature state-of-the-art in-situ laser diagnostics of spatiotemporal NP growth, using coherent anti-Stokes Raman spectroscopy and coherent Rayleigh-Brillouin scattering, as well as optical diagnostics of plasma properties. With the detailed spatiotemporal input from these diagnostics, we will develop a theoretical model of nanoparticle formation in NRP discharges. Ultimately, the fundamental understanding developed by NANObyNANO will enable the use of NRP discharges for the production of rationally designed NP with well-defined properties (morphology, composition) in a simple, highly energy-efficient way.

The results of NANObyNANO can eventually lead to a future mobile platform for the on-demand NP synthesis directly at the point of use, eliminating the need for on-board storage/delivery systems. The process can be single-step, highly efficient in its use of energy and precursor material, and enable on-demand adjustment of NP properties.