7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Near surface Langmuir probe characterization ofsilver reduction via plasma irradiation
A.D. Montallana1, M.R. Vasquez Jr.2, and M. Wada1
Graduate School of Science and Engineering, Doshisha University
2
Department of Mining, Metallurgical, and Materials Engineering, University of the Philippines
e-mail (speaker): cyjg2301@mail4.doshisha.ac.jp
1

Metal catalyst preparation can be realized by plasma
technology. These catalysts can be widely used forenvironmental treatment and energy production
applications.[1] Studies on the reduction of metal ionsusing plasma began with the utilization of hydrogen (H2)
as the carrier gas, as it was believed that the highly
reactive H2 plasma could produce species required for thereduction process to occur.[2] With time, more studies on
different plasmas for the reduction of noble metals fromnoble metal precursors were explored and it became
apparent that the plasma reduction process is also possible
with the use of other inert gases such as argon (Ar) andhelium (He). Different types of plasmas and plasma
devices have been reported for the reduction of metalNPs.[1-3]

Prior to plasma exposure, AgNO3(aq) was pipetted on
silicon (Si) substrates. The system was evacuated to basepressure (5x10-3 Pa) to allow solvent evaporation andretain the Ag precursor on the substrate. 1.0 Pa Ar wasthen introduced into the system. A tungsten (W) wire (l =50 mm, d = 0.30 mm) was resistively heated to produce adischarge via electron impact ionization. The discharge
current (Id) and voltage (Vd) were set to 50 mA and 100 V,respectively. To measure the plasma density (n0), ionsaturation current (Isat), electron temperature (Te), andplasma potential (Vp) during the plasma reduction of Ag,an electrostatic probe was positioned 10 mm away fromthe sample surface. The probe was swept from -40 to 20
V and I-V traces were obtained at 5-min intervals for 30mins.

Plasma contains many active species. Electrons, positiveions, negative ions, neutrals, and radicals present in theplasma may lead to a rapid reduction of metal compounds.
Due to its complex and dynamic nature, there is difficulty
in determining which species in the plasma are influential,
hence the mechanism of plasma-based reduction reactions
remains unclear.[4]Figure 1(a-d) show the trends of the plasma parameters
as a function of time at varying substrate bias, while Fig.
1(e) shows the X-ray diffractograms of the samples after
30 mins of plasma exposure. The plasma characteristics
can be correlated with the structure and properties of theplasma-reduced Ag.

Langmuir probe is one of the simplest but the moststraightforward method for in situ characterization of thelocal plasma.[5] In this study, the temporal evolution of theplasma characteristics near the surface holding Ag
compound during plasma-induced reduction of Ag wasinvestigated.

References[1] Z. Wang et al., ACS Catal. 8 (2018) 2093-2110.
[2] M. Darwish et al., Green Chem. 24 (2022) 8142-8154.
[3] A.D. Montallana et al., J. Vac. Sci. Technol. B 41(2023) 042204.
[4] C. Liu et al., Chinese J. Catal. 37 (2016) 340-348.
[5] R.L Merlino, Am. J. Phys. 75 (2007) 1078-1085.

Figure 1. Trends of (a) Isat, (b) Vp, (c) Te, and (d) n0 as a function of plasma exposure time at varying substrate bias and(e) X-ray diffractograms of the samples exposed to plasma for 30 mins at varying substrate bias.