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Communication Dans Un Congrès Année : 2021

In-situ optical diagnostics of plasma-water interfaces for applications in graphene synthesis

Diagnostics optiques in situ des interfaces plasma-eau pour des applications dans la synthèse du graphène

Résumé

Many approaches to graphene synthesis can require high temperature, strong/toxic reducing agents, or are expensive. The microplasma-electrochemical reactor (MEC), composed of an atmospheric-pressure microplasma with an aqueous solution as an electrode, may overcome these difficulties by providing physico-chemical conditions that are difficult to achieve otherwise. They can initiate non-equilibrium electrochemistry and nucleation in solution without additional heating or reducing agents, improving the efficiency of synthesis. In addition, the technological barrier to implementing MECs is low. MECs have successfully synthesized graphene quantum dot (GQD) synthesis in aqueous solution [1,2]. One of the main challenges going forward is to develop a detailed mechanism of GQD growth, which is a general difficulty in plasma-based nanomaterials synthesis due to the complexity of non-equilibrium plasma chemistry and interactions with surfaces. So far the relevant species and reactions have mainly been inferred from ex situ or macroscopic effects, with a limited degree of detail and certainty. The liquid-phase diagnostics of the plasma-water interfacial region can provide new insight into how the MEC transforms the precursor into GQDs. Current experimental techniques used for the analysis of liquid chemistry suffer from a lack of selectivity and/or degradation of dyes, chemical probes, or spin traps/probes introduced into the liquid. The spatial distribution of species is not often accessible. Most importantly, the majority of the diagnostics must be performed ex situ, removed from the plasma reactor and after treatment. To address this challenge, we have developed an in situ multi-diagnostics approach to encompass a wide range of physical and chemical properties at the plasma-water interface. The centerpiece of this platform is in situ spontaneous Raman microspectroscopy, which offers several important advantages over the aforementioned diagnostic tools: non-intrusiveness, selectivity, versatility, and straightforward calibration. By developing a light-sheet technique, we have experimentally investigated the interfacial region with a spatial resolution as high as several tens of microns. To gain insight into the physical state of the solvent, we tracked the Raman spectrum of water. In particular, changes to the –OH stretch band shape indicate a weakening of the hydrogen bonding network of water with. These changes to the Raman spectra over the course of plasma treatment occur at both fast and slow time scales and become more pronounced as the detection volume approaches the interface. We also tracked the aqueous species H2O2 and NO3-, whose concentrations both increase when approaching the interface to within several tens of µm [3]. An interfacial layer of excess NO3- concentration was found to extend 28 µm in depth. Similar interfacial layers have been modeled for transient species such as OH radicals but not for NO3-, a stable product of plasma-activated water. Raman spectroscopy of the liquid environment was complemented by in situ photoluminescence (PL) spectroscopy to track the appearance of GQDs in real time. The PL spectrum evolves differently according to the depth of detection in the liquid, and so particle image velocimetry of the liquid flow field was performed to gain an understanding of possible transport mechanisms. These measurements of the liquid phase were complemented by optical emission spectroscopy of plasma properties such as the electron number density and presence of excited species as a function of the distance from the interface. Together, these experimental results represent the fullest description to date of the physico-chemical environment enabling GQD synthesis and mark an important step towards the discovery of the reaction mechanism. Acknowledgments Financial support: ANR grants ANR-15-CE06-0007-01 and ANR-11-LABX-0017-01, PHC Orchid 40938YL, CNRS-IEA “GRAFMET”. References [1] Orrière, T., Kurniawan, D., Chang, Y. C., Pai, D. Z., & Chiang, W. H. (2020). Nanotechnology 31 (485001). [2] Yang, J. S., Pai, D. Z., & Chiang, W. H. (2019). Carbon 153, 315-319. [3] Pai, D. Z. (2021) J. Phys. D. : Appl. Phys. 54, 355201
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Dates et versions

hal-04488801 , version 1 (05-03-2024)

Identifiants

  • HAL Id : hal-04488801 , version 1

Citer

David Z Pai, Orrière Thomas, Caeilli Francesca, Thyagarajan Karthik, Kurniawan Darwin, et al.. In-situ optical diagnostics of plasma-water interfaces for applications in graphene synthesis. Materials Research Society Fall 2021 Meeting – Symposium EQ09: Cutting-Edge Plasma Processes for Next-Generation Materials Science Applications, Nov 2021, Boston (MA), United States. ⟨hal-04488801⟩
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