**Synthesis of Nanomaterials by Solution Plasma Processing**

Osamu Takai, Maria Antoaneta Bratescu, Tomonaga Ueno and Nagahiro Saito

**1. Introduction**

Recently, plasmas in the liquid phase have attracted a great attention because ofits applications to industrial materials processing [1]. In particular, glow discharge in the liquid phase ( ʺsolution plasmaʺ ‐ SP) is a useful tool for the synthesis of nanomaterials. We named the synthesis process using plasma in liquids as Solution Plasma Processing (SPP).

Plasma in water has been produced in 1899 by Dr. Wilsing between different types of metal electrodes, in order to study the spectral properties in connection with the astronomical observations [2]. The earliest works on discharge in liquids have studied the arc and spark discharge in water and salt solutions [3].

The detailed structure of the solution plasma is still unclear at present. Currently, SPs are generated by nanoseconds pulsed or dc voltages. Also, ac excitations with frequencies raging from 50 Hz up to MHz were used [4‐7].

In our research group, we focused on the fundamental aspects of the solution plasma diag‐ nostics related with the synthesis of novel nanomaterisls processed by SP. Figure 1 shows a model of the solution plasma. The characteristic regions of SP are the plasma gas phase, the liquid phase, the interface between plasma gas and liquid, and the interface between electrode surface and gas plasma. The emission center of plasma is located in the gas phase which is surrounded by the liquid phase. Near the gas/liquid interface an ion sheath is formed. The plasma is confined by a condensed phase, which produces unique features of the solution plasma. This solution plasma provides extremely rapid reactions using activated chemical species and radicals under high pressure [1].

SPP is a new useful and simple method for the metal nanoparticles (NPs) synthesis since this non‐equilibrium plasma can provide extremely rapid reactions due to the reactive chemical

© 2013 Takai et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Takai et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

species, radicals, and UV radiation produced in an atmospheric pressure plasma [1]. The most important merits of the SPP forthe NPs synthesis, as compared with chemical methods, consist in the short processing time (in the range from few minutes to several tens of minutes), preparation in room temperature and pressure conditions, and low energy of plasma. The novelty of the SPP method used in our laboratory consists in the fact that plasma operates in glow discharge limits, offering a suitable medium to control the chemical reactions inside the solutions [10, 11]. This is possible because plasma offers a new reaction medium, where hydrogen, hydroxyl, and oxygen radicals are produced, where the hydrogen radical is the most responsible for the reduction reaction of the gold ion to the neutral atom, and therefore a reduction agent is not necessary. The SPP method seems well‐suited for the NPs synthesis offering the possibility to control the size by controlling the surrounding chemistry of the gold NPs, adding thus another level of utility of this procedure to material science [12, 13].

**2. Conditions for SP generation**

ns to 3 μs [10, 16, 18‐20].

plasma, and synthesis of nanomaterials.

electrochemical reactions dominate [20].

A typical experimental set‐up for the generation of SP, with time‐dependent electrical and optical diagnostics of plasma and synthesis of nanomaterials is shown schematically in Figure 2. A typical power supply for SP generation has the following characteristics: bipolar pulse type with a maximum voltage and current of 5 kV and 10 A, respectively, a variable repetition frequency in the range from 5 to 60 kHz, and a variable pulse width from 500

Synthesis of Nanomaterials by Solution Plasma Processing 111

**Figure 2.** Typical experimental set-up for SP generation, with time-dependent electrical and optical diagnostics of

We investigated the dependence of SP characteristics on the inter‐electrode distance. In SP, the formation of differentradicals, the excited atoms and molecules are strongly influenced by the geometry and the input electrical power in the system. Figure 3 shows various regions of SP depending on the inter‐electrode distance and the applied pulsed high voltage. Typicalregions are the glow discharge regime, when the inter‐electrode space is less than 2 mm and the peak voltage is more than 2 kV, the corona discharge regime, when the inter‐electrode distance increases and the peak high voltage is also high, and the pre‐breakdown regime where

In nanomaterials processing, an important factor consists in controlling of the solution plasma stability. The value of pH and the conductivity of the solution also determine the operation regime of SP. In the diagram from Figure 4 various conditions of plasma determined by solution conductivity, bipolar pulse width and frequency, generate a glow or an arc discharge. If the solution conductivity is high, more than 1 mS/cm, the ionic current through the liquid is high, and at the same input electrical power, plasma is instable, if the pulse width is smaller

SPP was successfully used for loading metal NPs on carbon materials to prepare composite structures which can improve the catalytic activity of fuel cells [14].

Many other applications of SPP in nanomaterial technology have been performed in our group related with template removal in mesoporous silica synthesis process [15], decomposition of organic dyes or compounds [16, 17], surface modification of metals [19], and sterilization of water [1].

**Figure 1.** Model of Solution Plasma. The main regions are: plasma gas phase, gas phase, liquid phase and the interfa‐ ces between plasma and gas, and gas and liquid.

In the followings we will discuss about the conditions for SP generation [16, 19‐21], the role of radicals, ions and surrounding chemistry in NPs synthesis [11, 12, 17, 22], and synthesis of nanomaterials with enhanced catalytic activity [1, 14].
