**2. Experimental details and methodology**

## **2.1 Experimental designs**

The design of experimental setups mainly depends on two electrode configurations: pin-to-pin and pin-to-plane. In the first one, two pointed electrodes are placed vertically towards each other and in the second one, a pointed electrode is positioned vertically to a planar (plate) electrode. The advantage of pin-to-pin and pin-to-plane electrode configuration is to attain maximum electric field at a desired electrode gap. Various types of voltage waveforms such as DC, pulsed DC, AC, RF (radio-frequency) and microwave are used to generate plasma. **Figure 2** (a) and (b) shows the generation of plasma using the pin-to-plane and pin-to-pin electrode configurations respectively, in our laboratory. For pin-to-plane, it is observed that

**Figure 2.** *Generation of plasma using (a) pin-to-plane and (b) pin-to-pin electrode configurations.*

*In-Liquid Plasma: A Novel Tool for Nanofabrication DOI: http://dx.doi.org/10.5772/intechopen.98858*

the plasma channel diverse from the pin electrode towards the plane electrode and for pin-to-pin, the plasma expands at the central region from the tip of the two electrodes. Hattori et al. employed a pin-to-plane electrode configuration to generate a plasma between a metallic electrode and a copper plate using radio-frequency plasma [20]. Whereas Lange et al. reported the use of pin-to-pin electrode configuration to generate a plasma between two vertically pointed graphite electrodes using a DC power supply [8].

Many different types of experimental setups have been designed so far for the fabrication of nanomaterials in various liquids. Ishigami et al. designed the experimental setup for the continuous production and transportation of carbon nanotubes [5]. They dipped a graphite anode and a short copper or graphite cathode having pin-to-pin electrode geometry into a vessel containing liquid nitrogen. After the generation of plasma, carbon nanotubes were formed due to the erosion of the anode material. They made funnel-shaped bottom of the vessel and sealed it with a valve to operate continuously. The valve opens periodically to transfer the nanotubes from the vessel. In a pin-to-pin or pin-to-plane electrode configuration, erosion of anode is much higher than the cathode erosion [32]. Hence, the resultant nanomaterial is made of anode material.

### **2.2 Mechanism of plasma generation**

Usually, a pulsed high voltage power supply with a voltage rise time shorter than the Maxwellian relaxation time of the liquid is required to generate plasma inside a liquid [33]. When a high voltage is applied between the two electrodes inside a liquid, it induces a current and redistribution of the electric field. Subsequently, Joules heating at the tip of the electrodes takes place, which initiates the bubble formation due to the evaporation of the liquid. Eq. (1) provides an expression for the theoretical maximum of the electric field in pin-to-plane electrode configuration [34]:

$$E\_m = \frac{2V\_i}{R\ln(2\mathcal{D}/\_{\mathbb{R}})} \tag{1}$$

Where, *Vi*, *R* and *D* represent the applied voltage, the radius of curvature of the pin electrode tip and the distance between the two electrodes respectively. Therefore, to attain the maximum electric field (*Em*) at a constant voltage, both the radius of curvature of the pin electrode tip and the distance between the two electrodes should be as minimum as possible. When the applied voltage is increased to a specific value, the high electric field initiates the discharge inside the bubbles. When the bubbles are bridged together, a continuous plasma channel is formed between the two electrodes. The formation of plasma or the conductive channel depends on the Joules heating. When it is larger than a threshold value, instability occurs, which stimulates the immediate evaporation of the liquid followed by thermal breakdown. Hence, plasma is generated between the two electrodes. However, when Joules heating is smaller than the threshold value, only electrolysis takes place. The mechanism of plasma discharge inside liquid also depends on its polarity. As water is a polar medium, it can conduct current and the plasma discharge occurs using the mechanism as explained above. However, for a non-polar medium, the discharge mechanism is slightly different. As non-polar medium cannot conduct electricity hence, bubble formation does not take place. Therefore, plasma generation can only be possible when the electric field between the two electrodes is high enough to trigger the dielectric breakdown of the medium. The dielectric breakdown can be defined as the sharp reduction in the electric resistance of a medium,

when the electric field is higher than the dielectric constant of the medium. Li et al. compared the mechanism of plasma discharge in polar (tap water) and non-polar (benzene) solution [35]. The dielectric constant of benzene is �106 V/m. They observed the discharge inside benzene after applying the voltage between the two electrodes (electrode gap 0.5 mm) reached 1.5 kV. However, for tap water, the plasma discharge channel was observed by applying less than 1 kV.

### **2.3 Plasma diagnostics and influence of reactive species on material fabrication**

As the plasma inside the liquid is confined to a tiny region, spectroscopic diagnostics is mainly employed to determine the plasma parameters such as plasma density and temperature. Optical emission spectroscopy (OES) helps to identify the presence of various reactive species in the plasma zone. The emission spectrum obtained from the plasma zone is the superposition of the continuous spectra of electron radiation and band or line spectra of various molecules, atoms and radicals. Stark broadening of spectral lines and line intensity ratios are employed to determine the plasma density and temperatures respectively [36]. The temperature of the plasma zone is very crucial to fabricate various nanoparticles from the electrode materials. When two or more spectral lines of the same element (atom or ion) are present in the emission spectrum, then the electron/excitation temperature of the plasma from the line intensity ratio can be expressed as [37].

$$T\_{\epsilon} = \frac{E\_2 - E\_1}{k \ln\left(\frac{I\_1 A\_2 \varrho\_2 \lambda\_1}{I\_2 A\_1 \varrho\_1 \lambda\_2}\right)}\tag{2}$$

Where, subscripts 1 and 2 denote two different spectral lines of the same element. *I*, *E*, *A*, λ, *g*, and *k* are the relative intensity, the energy of upper level, transition probability, the wavelength of the emission line, statistical weight and Boltzmann constant respectively. When the temperature exceeds the boiling point of the electrode material, there is a high probability of the vaporisation of the electrode material to form the nanoparticles. Dunleavy et al. observed the presence of two well-defined regions of plasma [38]. A central core having high temperature � (16000 � 3500) K with high electron density Ne � 5 � 1017 cm�3 . The region is at local thermodynamic equilibrium (LTE). As for LTE, the minimum electron density is 2 � 1017 cm�3 . The other is the low density (Ne � 1015 cm�3 ) peripheral region, which is much cooler having temperature � 3500 K. In a unique work of deposition of anti-corrosion layer using plasma electrolytic carbonitriding on pure aluminium, the electron density and temperature were calculated to be 6 � 1015 cm�3 and 4000 K respectively [39].

The generation of various reactive species such as hydrogen (H\* ), oxygen (O\* ), hydroxyl (OH\* ) and superoxide (*O*∗ � 2 ) radicals in the plasma region can be explained using the following reactions [2]:

$$\rm H\_2O + e^- \rightarrow \rm 2H^\* + O^\* + e^- \tag{3}$$

$$H\_2O + e^- \rightarrow H^\* + OH^\* + e^- \tag{4}$$

$$\bullet \bullet^\* + \bullet^\* \to \bullet\_2 \tag{5}$$

$$H^\* + O\_2 \to HO\_2^\* \leftrightarrow O\_2^{\*-} + H^+ \tag{6}$$

During the formation of metal nanoparticles from the electrolytic solution, the expression for the reduction of metal ions (Mn+) dissolved in the solution by energetic electrons in the plasma zone is as follows:

*In-Liquid Plasma: A Novel Tool for Nanofabrication DOI: http://dx.doi.org/10.5772/intechopen.98858*

$$M^{n+} + n\varepsilon^{-} = M\tag{7}$$

The mechanism of the formation of Au nanoparticles using the gold precursor solution (HAuCl4) can be understood by considering the Eqs. (8)-(12) reported by Ashkarran et al. [40] and Bratescu et al. [41]. Here 0 < j < 4 and the replacement of *Cl*� by *OH*� depends on the pH of the solution.

$$\text{HAuCl}\_4 \to H^+ + AuCl\_4^- \tag{8}$$

$$AuCl\_4^- + 3e^- \rightarrow Au^0 + 4Cl^- \tag{9}$$

$$\rm AuCl\_4^- + jOH^- \leftrightarrow AuCl\_{4-j}^-(OH)\_j^- + jCl^- \tag{10}$$

$$\rm{AuCl}\_4^- + \rm{3H}^\* \rightarrow \rm{Au}^0 + \rm{4Cl}^- + \rm{3H}^+ \tag{11}$$

$$\rm{Au(OH)}\_{4}^{-} + \rm{3H}^{\*} \rightarrow \rm{Au}^{0} + \rm{OH}^{-} + \rm{3H}\_{2}\rm{O} \tag{12}$$

Klapkiv et al. reported a simulation study on the synthesis of Al2O3 by considering the plasma channel into three zones [42]. In the central zone of the plasma channel, the temperature ranges from 7000 to 10000 K and the density of electrons in the order of 1022 cm�3 . Here, the evaporated anodic materials (made of aluminium, Al) are partially ionised and all the other species are in a monoatomic state. In this zone, Al reacts with singlet oxygen to form AlO, Al2O, and AlO2 as given by the following reactions:

$$Al + O \leftrightarrow AlO\tag{13}$$

$$\text{2Al} + \text{O} \leftrightarrow \text{Al}\_2\text{O} \tag{14}$$

$$Al + 2O \leftrightarrow AlO\_2 \tag{15}$$

In the next zone, the temperature is about 5400 K and the reactive molecular species can exist. The following reactions are possible in this zone:

$$Al + OH \leftrightarrow AlO + \psi\_2H\_2 \tag{16}$$

$$2Al + OH \leftrightarrow Al\_2O + \text{\textquotedblleft}\_2\text{H}\_2\tag{17}$$

The temperature of the third zone is around 2327 K and the formation of Al2O3 is possible using the following equations:

$$\text{2Al} + \text{3O} \leftrightarrow \text{Al}\_2\text{O}\_3 \tag{18}$$

$$2AlO + O \leftrightarrow Al\_2O\_3 \tag{19}$$

$$Al\_2O + 2O \leftrightarrow Al\_2O\_3 \tag{20}$$

$$2Al + 3OH \leftrightarrow Al\_2O\_3 + 3\downarrow\_2H\_2\tag{21}$$

After this region, the temperature of the liquid medium falls to around 300 K (room temperature of the liquid). During this drastic temperature change, the polymorphic transition in the oxide phases is possible.

Optical Emission Spectroscopy also provides the emission spectra of electrode materials; therefore, one can get an idea about the formation of nanoparticles from the electrode material before going through the other material characterisation techniques. To detect the plasma species, Lu et al. used OES, where they observed the emission of Cu atoms along with the other plasma species such as OH, Hα, Hβ, O and Na [43]. From the emission of Cu atoms, they suggested that at first, copper foil anode is oxidised to form Cu2+, which then move towards the cathode due to the

external electric field inside the plasma region. During their movement, they react with high-energy electrons and H atoms to form Cu atoms. As copper atoms are highly reactive in water, they are easily oxidised to form CuO nanoparticles. However, Saito et al. reported the synthesis of CuO nanoflower by considering a slightly different mechanism [44]. Firstly, copper hydroxide [Cu(OH)2] is formed at the surface of the copper electrode. The temperature of the electrolyte covering the electrode goes beyond 260°, which is sufficient to melt the surface of the electrode. Secondly, Cu(OH)2 interacts with plasma-generated OH to form tetrahydroxocuprate(II) anions [Cu(OH)4] 2. Lastly, as the temperature drops, precipitation of CuO (s) occurs by releasing H2O and OH. Preferential growth of crystal plane along with a specific direction forms spindle structures.