In-Liquid Plasma Process for Size- and Shape-Controlled Synthesis of Silver Nanoparticles by Controlling Gas Bubbles in Water

Abstract

Most methods controlling size and shape of metal nanoparticles are chemical methods, and little work has been done using only plasma methods. Size- and shape-controlled synthesis of silver nanoparticles (Ag NPs) is proposed based on adjusting the gas bubble formation produced between two silver electrodes. The application of a voltage waveform with three different pulse widths during a plasma process in water can generate different gas bubble formations. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of Ag NPs synthesized using three different bubble formations reveal that spherical Ag NPs are synthesized when very tiny bubbles are generated between two electrodes or when only the grounded electrode is enveloped with large gas bubbles, but Ag nanoplates are synthesized when both electrodes are completely enveloped with large gas bubbles.

Keywords: controlled synthesis; gas bubble formation; silver nanoparticle; silver nanoplate; voltage waveform.

Figure 1

Schematic diagram of liquid plasma reactor and measurement setup used in this study.

Figure 2

(a) Intensified charge-coupled device (ICCD) images of bubbles; (b) detailed images of gas bubble formation times using high speed camera; and (c) ICCD plasma intensities during liquid plasma discharge when applying three different bipolar pulses in Figure 1 to powered electrodes (P), where G is grounded electrode.

Figure 3

(a) Electrical model of liquid plasma region and (b) three voltage waveforms with related breakdown voltages and discharge currents (V_p-I_p) measured during one cycle with different on-times; 20 μs for case I, 40 μs for case II, and 60 μs for case III.

Figure 4

Optical emission spectra (OES) for plasma generated by three bubble formations shown in Figure 2 with exposure time of 200 ms.

Figure 5

Scanning electron spectroscopy (SEM) images of synthesized Ag nanoparticles (NPs) through solution plasma process (or liquid plasma process) SPP (or LPP) for three cases.

Figure 6

Transmission electron microscopy (TEM) images in (a) case I, (b) case II, and (c) case III. Top-right corner insets in (a–c) are energy dispersive X-ray spectroscopy (EDS). Bottom-left corner insets in (a–c) are selected area electron diffraction (SAED) patterns of synthesized Ag NPs for three cases.

Figure 7

Color changes of Ag NP suspensions for three different cases according to plasma process time.

Figure 8

UV-Vis absorption spectra for six different Ag NP suspensions prepared by LPP: (a) UV-Vis absorption spectra and ((b) inset) corresponding colors of Ag NP suspensions.

Figure 9

Dynamic light spectroscopy (DLS) measurement of synthesis Ag NPs (a) case I and (b) case II.

Figure 10

(a) Microscope and (b) SEM images of changes in Ag electrode tips exposed to discharges after 10-min LPP with three different bubble formations induced by adjusting pulse width.