Study by Optical Spectroscopy of Bismuth Emission in a Nanosecond-Pulsed Discharge Created in Liquid Nitrogen.

Anna V Nominé,Valentin A Milichko,Thierry Belmonte,Cédric Noel,Thomas Gries,Alexandre Nominé

doi:10.3390/molecules26237403

Abstract

Time-resolved optical emission spectroscopy of nanosecond-pulsed discharges ignited in liquid nitrogen between two bismuth electrodes is used to determine the main discharge parameters (electron temperature, electron density and optical thickness). Nineteen lines belonging to the Bi I system and seven to the Bi II system could be recorded by directly plunging the optical fibre into the liquid in close vicinity to the discharge. The lack of data for the Stark parameters to evaluate the broadening of the Bi I lines was solved by taking advantage of the time-resolved information supported by each line to determine them. The electron density was found to decrease exponentially from 6.5 ± 1.5 × 1016 cm−3 200 ns after ignition to 1.0 ± 0.5 × 1016 cm−3 after 1050 ns. The electron temperature was found to be 0.35 eV, close to the value given by Saha’s equation.

Highlights

Processes based on discharges in liquids are commonly used to synthesize original nanoobjects with high yields [1,2,3,4,5]
By applying a nanosecond-pulsed high voltage to bismuth electrodes submerged in liquid nitrogen, the erosion process leads to the production of metallic nanosheets
This value came up naturally, higher values leading to unrealistic broadenings. It is lower than electron temperatures usually estimated in the same kinds of discharges that lie rather between 0.5 and 1 eV. The reason for this is due to the conditions that prevail in these kinds of discharges: assuming that the medium is close to the local thermodynamic equilibrium (LTE), Saha’s equation applied to bismuth indicates that if the electron density is 1 × 1016 cm−3, the electron temperature should be

Summary

Introduction

Processes based on discharges in liquids are commonly used to synthesize original nanoobjects with high yields [1,2,3,4,5] In liquid nitrogen, they are interesting, as they lead to the synthesis of two-dimensional structural materials like nanosheets [6,7,8], exhibiting high levels of selectively exposed reactive facets and the fast separation of electrons and holes. Liquid nitrogen evaporates, and the nanosheets, exposed to air, get oxidized and are transformed into Bi2 O3 Diagnosing these discharges is not an easy task, as they are very small (tens of micrometres) and pretty fast (hundreds of nanoseconds) [13]. Such investigations are essential to assess the plasma parameters, which is important to improve our control of the process [14,15,16]

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Conclusion