*Plasma Science and Technology*

liquid surface and the other is immersed into the liquid in such a way that the liquid surface acts as the counter electrode. The liquid (precursor) acts as the nanomaterial source i.e., plasma interact with the liquid solution to synthesise nanomaterials.

In-Liquid Plasma mainly consists of three zones. The central zone is the plasma region, where the temperature goes beyond thousands of kelvin. The next one is the gaseous region, which is formed due to the evaporation of water/solvent. The outermost zone is the liquid medium, where the temperature is slightly more than the room temperature. Besides these zones, there are two interfacial regions called the plasma/gas and gas/liquid interface. These are very significant regions for nanomaterial synthesis, where many physical and chemical activities occur. A broad range of active radicals such as *OH* <sup>∗</sup> , *H***2***O***2**, *O***3**, *NO*, *e*� ð Þ *aq* , UV radiations and shock

waves are formed in these interfacial zones [4]. **Figure 1** shows the presence of the three zones (plasma, gas and liquid) and the interfacial regions (plasma/gas and gas/liquid) during the generation of plasma inside a liquid. This chapter will mainly discuss the In-Liquid Plasma, i.e., plasma generation inside liquid to synthesise various nanomaterials. A review of nanofabrication by In-Liquid Plasma over the last two decades has been discussed in the following paragraphs.

Since the beginning of the 21st century, In- Liquid Plasma has been vigorously involved in the synthesis of various nanomaterials. In the early years, researchers have mainly focused on the synthesis of carbon-based nanomaterials [5–8]. In 2000, Ishigami et al. reported the continuous synthesis of multi-walled carbon nanotubes by generating arc discharge between two graphite electrodes inside liquid nitrogen [5]. Whereas Sano et al. in 2001 investigated the synthesis of carbon onions in water [6] and in 2004, single-walled carbon nanotubes with nanohorns in liquid nitrogen [9]. Bera et al. synthesised palladium nanoparticles filled carbon nanotubes using arc discharge in palladium chloride (PdCl2) solution in 2004 [10]. After these pioneering investigations, researchers began to work on the synthesis of noble and

### **Figure 1.**

*Schematic representation of the generation of plasma inside a liquid showing the presence of plasma/gas and gas/liquid interfaces.*

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

transition metal nanomaterials and their composites. Lo et al. synthesised Copper (Cu) – based nanofluids and silver nanofluids using submerged arc nanoparticle synthesis system (SANSS) in 2005 and 2007 respectively [11, 12]. Lung et al., in 2007, reported the synthesis of gold (Au) nanoparticles in water by arc discharge [13]. Ashkarran et al. synthesised tungsten trioxide (WO3), Zinc oxide (ZnO) and Zirconium oxide (ZrO2) nanoparticles inside water in 2008 [14], 2009 [15] and 2010 [16] respectively. In 2009, Omurzak et al. reported the synthesis of blue amorphous titanium oxide (TiO2) and TinO2n-1 nanoparticles by generating plasma between two titanium electrodes [17].

In the meantime, besides the erosion of electrode materials, the researchers investigated the reduction of metallic salt solutions by the In-Liquid Plasma for the synthesis of various nanomaterials. Saito et al., in 2009, synthesised Au nanoparticles by generating plasma between two tungsten electrodes inside Chloroauric acid (HAuCl4) [18]. HAuCl4 acts as the metal precursor for Au nanoparticles. Pootawang et al. investigated the synthesis of silver/platinum (Ag/Pt) bimetallic nanocomposites by producing plasma between a silver and a platinum electrode inside a mixture of solution containing sodium dodecylsulfonate (SDS) and sodium chloride (NaCl) using a unipolar pulse power supply in 2012 [19]. Synthesis of WO3, Ag and Au nanoparticles by generating plasma between a metal electrode (tungsten or silver or gold) and a copper plate was investigated by Hattori et al. in 2013 [20]. Lee et al., in 2014, synthesised tin (Sn) and tin oxide (SnO2) nanoparticles by the reduction of tin chloride dehydrate (SnCl2. 2H2O) [21]. Fabrication of manganese (Mn) oxide/activated carbon composites was investigated by Lee et al. in 2015 [22]. They used a mixture of manganese chloride tetrahydrate (MnCl2.4H2O) and activated carbon powder as the solution. The former and latter act as the precursor for manganese oxide and carbonaceous material respectively. Synthesis of bimetallic Nickel (Ni)/Copper (Cu) nanoparticles by generating plasma inside a mixture of nickel nitrate hexahydrate (Ni(NO3)2.6H2O) and copper nitrate tetrahydrate (Cu(NO3)2.4H2O) solution was reported by Sun et al. in 2016 [23]. To enhance catalytic activity towards oxygen reduction reaction (ORR), Panomsuwan et al. reported the synthesis of metal-free composite of nitrogen-doped carbon nanoparticles (NCNP)/carbon nanofiber (CNF) using solution plasma in 2016 [24]. The composites were obtained by generating plasma inside a mixture of CNF and 2 – cyanopyridine (C6H4N2), where the latter act as the source of nitrogen. Fabrication of bead-chain-like nanostructures of ZnO from the oriented attachment of spherical Zn/ZnO nanoparticles by generating DC plasma between two Zinc electrodes inside deionised water was reported by Ziashahabi et al. in 2017 [25]. Fabrication of nitrogen-doped activated carbon-supported iron oxide (Fe2O3) nanocomposites for supercapacitor applications was investigated by Lee et al. in 2018 [26]. They first prepared the nitrogen-doped carbon (NC) by Liquid Phase Plasma (LPP) inside a solution containing ammonium chloride (NH4Cl) and activated carbon (AC) powder, where the former act as the precursor for nitrogen. After that, the resultant particles were mixed in iron chloride (FeCl2) and Cetyltrimethyl ammonium bromide (CTAB) solution. The LPP reaction then gives the iron oxide/NC composites (IONCC). The specific capacitance and cyclic stability of NC and IONCC were found superior to the bare AC. Synthesis of Cu – Ni/CuO – NiO (CNO) nanocomposites by generating a plasma between a copper and nickel electrodes inside water using a bipolar pulse high voltage power supply was reported by Yang et al. in 2020 [27]. They found superior catalytic activity of CNO towards methanol electrocatalytic oxidation in alkaline media than the other transition metal or metal oxide based catalyst. Boruah et al. in 2021, reported a novel single-step synthesis method of Au/CuO micro/nanocomposites by generating plasma between two copper electrodes inside a solution of HAuCl4 [28]. The

copper electrodes acted as the source of CuO particles and the HAuCl4 acted as the precursor of Au nanoparticles.

Moreover, researchers have also focused on the fabrication of nanomaterials having various defect states to enhance the catalytic activity of the materials during the last few years. Panomsuwan et al., in 2015, reported the synthesis of defectinduced black titanium oxide (H-TiO2-x) nanoparticles by generating plasma between two titanium electrodes inside water [29]. They observed a higher photocatalytic performance of H-TiO2-x (90%) than the commercial TiO2 particles (18%) for the degradation of Methylene blue (MB) dye under visible light irradiation. Moreover, about 51% of MB molecules adsorbed on the surface of H-TiO2-x under dark, whereas for commercial TiO2, the adsorption was about only 9%. The same group in 2018 fabricated defect–induced heterophase anatase/brookite TiO2-x nanocrystals by generating plasma inside a solution containing commercially available TiO2 powder [30]. Active radicals present in the plasma interact with the TiO2 particles to form defective sites. A higher gaseous photocatalytic activity towards acetaldehyde degradation to CO2 of the plasma-treated particles (TiO2-x) (91.1%) than the untreated commercial TiO2 particles (51%) was observed. Boruah et al. in 2020 synthesised narrow bandgap tungsten oxide (WO3-x) nanoparticles by generating plasma inside deionised water [31]. The reason behind the formation of narrow bandgap nanoparticles was investigated to be the presence of higher amount of oxygen vacancies. They observed higher photocatalytic performance of WO3-x nanoparticles (77%) than the commercial nano WO3 (62%) and bulk WO3 (50%) particles under a solar simulator.
