**2. Conditions for SP generation**

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

Micro-Nano Mechatronics — New Trends in Material, Measurement, Control, Manufacturing and Their Applications in

NPs, adding thus another level of utility of this procedure to material science [12, 13].

structures which can improve the catalytic activity of fuel cells [14].

water [1].

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ces between plasma and gas, and gas and liquid.

nanomaterials with enhanced catalytic activity [1, 14].

SPP was successfully used for loading metal NPs on carbon materials to prepare composite

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

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

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 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 ns to 3 μs [10, 16, 18‐20].

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

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 electrochemical reactions dominate [20].

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

than 2 μs. If the pulse width increases, the input electrical power increases and even the solution conductivity is high, a stable glow discharge can be obtained.

**Figure 3.** Dependence of SP characteristics on the inter-electrode distance and the applied peak high voltage.

**Figure 5.** (a) Optical absorption spectra of the OH radical measured in HCl, KCl, and KOH solutions, on the positive applied voltage, under the same experimental conditions (discharge voltage of 700 V, frequency of 25 kHz, and pulse width of 4 μs). (b) The time evolution of the OH radical density measured in HCl, KCl, and KOH solutions, under the

The optical absorption spectra of the OH radical measured in different solution with HCl, KCl, and KOH, under a positive value of the high voltage, and the corresponding time‐ dependent signals of the emission line of the OH radical, are represented in Figure 5 (a) and (b), respectively. The OH radical number density measured by broad band absorp‐

voltage pulses were applied to the electrodes. KOH is highly basic and can be an impor‐ tant source of hydroxyl radicals, but in this experiment the density was the lowest for this

The main chemical reactions responsible for the generation of the reactive species in SPP

H2O → ∙H + ∙OH 2H2O → O2 + 2H2 O2 → 2∙O O2 → 2∙H H+e → H\* + e O+e → O\* + e

) when positive

Synthesis of Nanomaterials by Solution Plasma Processing 113

(1)

same experimental conditions above mentioned. Lines are used as guides for the eyes.

) [24].

type of solution (~5x1016 cm‐<sup>3</sup>

are [4, 5]:

tion spectroscopy was highest for the HCl solution plasma (2x1017 cm‐<sup>3</sup>

**Figure 4.** Influence of solution conductivity and input electrical parameters of the pulsed power supply on SP stability. Different operation regimes of SP: glow discharge and arc discharge. (a) Stability conditions of SPP in a solution of LiCl with 4 mM concentration. (b) Stability conditions of SPP in a solution of LiCl with 40 mM concentration.

Independently of the solution conductivity, if the pulse width is higher than 2.5 μs and the pulsed power supply frequency is 30 kHz, the plasma generated in solution switches fast to arc discharge regime.

The optical emission spectra strongly depend on SP regime working. Corona discharge is characterized by a strong emission of the OH radical as compared with the glow discharge regime, when the inter‐electrode distance is around 1 mm [20].

than 2 μs. If the pulse width increases, the input electrical power increases and even the

Micro-Nano Mechatronics — New Trends in Material, Measurement, Control, Manufacturing and Their Applications in

**Figure 3.** Dependence of SP characteristics on the inter-electrode distance and the applied peak high voltage.

**Figure 4.** Influence of solution conductivity and input electrical parameters of the pulsed power supply on SP stability. Different operation regimes of SP: glow discharge and arc discharge. (a) Stability conditions of SPP in a solution of LiCl

Independently of the solution conductivity, if the pulse width is higher than 2.5 μs and the pulsed power supply frequency is 30 kHz, the plasma generated in solution switches fast to

The optical emission spectra strongly depend on SP regime working. Corona discharge is characterized by a strong emission of the OH radical as compared with the glow discharge

with 4 mM concentration. (b) Stability conditions of SPP in a solution of LiCl with 40 mM concentration.

regime, when the inter‐electrode distance is around 1 mm [20].

arc discharge regime.

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solution conductivity is high, a stable glow discharge can be obtained.

**Figure 5.** (a) Optical absorption spectra of the OH radical measured in HCl, KCl, and KOH solutions, on the positive applied voltage, under the same experimental conditions (discharge voltage of 700 V, frequency of 25 kHz, and pulse width of 4 μs). (b) The time evolution of the OH radical density measured in HCl, KCl, and KOH solutions, under the same experimental conditions above mentioned. Lines are used as guides for the eyes.

The optical absorption spectra of the OH radical measured in different solution with HCl, KCl, and KOH, under a positive value of the high voltage, and the corresponding time‐ dependent signals of the emission line of the OH radical, are represented in Figure 5 (a) and (b), respectively. The OH radical number density measured by broad band absorp‐ tion spectroscopy was highest for the HCl solution plasma (2x1017 cm‐<sup>3</sup> ) when positive voltage pulses were applied to the electrodes. KOH is highly basic and can be an impor‐ tant source of hydroxyl radicals, but in this experiment the density was the lowest for this type of solution (~5x1016 cm‐<sup>3</sup> ) [24].

The main chemical reactions responsible for the generation of the reactive species in SPP are [4, 5]:

$$\begin{aligned} \mathrm{H\_2O} &\rightarrow \cdot \mathrm{H} + \cdot \mathrm{OH} \\ 2\mathrm{H\_2O} &\rightarrow \mathrm{O\_2} + 2\mathrm{H\_2} \\ \mathrm{O\_2} &\rightarrow 2\cdot \mathrm{O} \\ \mathrm{O\_2} &\rightarrow 2\cdot \mathrm{H} \\ \mathrm{H^+e} &\rightarrow \mathrm{H^+e} \\ \mathrm{O^-e} &\rightarrow \mathrm{O}^+ + \mathrm{e} \end{aligned} \tag{1}$$
