**2. What is the cold plasma?**

As it is known, in the Universe, matter exists in four different states from the molecular interrelations point of view, namely: solid, liquid, gas and plasma. Simply speaking, plasma is a kind of ionized gas, into which sufficient energy is provided to free electrons from atoms or molecules and to allow both species, ions and electrons, to coexist. Generally, the plasma state can be divided into two main types (Fig. 1): low-temperature plasma – that is the state in which only a part of gas molecules is ionized and the gas is a mixture of electrons, ions, free radicals, excided and neutral molecules – and high-temperature plasma, in which all atoms are fully ionized. The latter type of plasma can be found, for example, in

 1 Division of Molecular Engineering (DME), Faculty of Process and Environmental Engineering, Technical University of Lodz, Wolczanska 213, 90-924 Lodz, Poland, E-mail: jatyczko@wipos.p.lodz.pl

Cold Plasma – A Promising Tool for the Development of Electrochemical Cells 107

chemical constitution, molecular construction and nanostructure can be obtained in this way

The second way consists in the modification of conventional materials, performed by their cold plasma treatment. Generally, such a treatment triggers three basic processes occurring mainly on the surface. It can create new functional groups by implantation of atoms present in the plasma, it can generate free radicals that then react with atmospheric oxygen and water molecules giving additional functional groups or can be used in grafting processes, and finally it can modify the microporous structure by the etching and degradation effects

The cold plasma is most often generated in laboratories and industry by an electric glow discharge under low pressure using various frequencies of the applied electric field: audio frequencies (AF, mainly in the range of 10–50 kHz), radio frequencies (RF, mainly 13.56 MHz), and microwave frequencies (MW, mainly 2.45 GHz). Sometimes, a direct current (DC) discharge is also used. An example of typical parallel plate plasma reactor, one of those

At first, the reactor chamber is evacuated down to 10−1 –10−3 Pa. Then, a precursor of plasma deposition is introduced to the chamber in the form of gas or vapor under controlled flow. Organic and inorganic gases, sublimating solids and evaporating liquids can be used as precursors. They are supplied as pure compounds or as mixtures with an inert carrier gas (e.g. argon). The carrier gas enables to generate plasma in the presence of compounds with very low vapor pressure. It is also possible to perform the plasma deposition process using a mixture of two or more precursors. Suitable selection of these compounds and their concentrations in the reactor chamber make it possible to control the molecular structure of deposited films, and consequently – properties of the films. In the reactor chamber with

RFpower

being used in our laboratory for deposition of thin films, is sketched in Fig. 2.

Fig. 2. A sketch of a typical parallel plate plasma reactor.

(Gordillo-Vázquez et al., 2007; Konuma, 1992).

(Inagaki, 1996).

the Sun or in laboratories involved in nuclear fusion research, but this type of plasma is rather not interesting as a technology for the preparation of new materials.

Fig. 1. Classification of the plasma types.

The low-temperature plasma can be divided, in turn, into two further types (Fig. 1): equilibrium and non-equilibrium plasmas. In the equilibrium plasma, often called the thermal plasma, electrons and the rest of plasma species have nearly the same temperature (*Te* ≈ *Tgas*), much higher than the room temperature. Such plasma is generated, for example, in plasma jets and torches. On the other hand, the non-equilibrium plasma, called sometimes the cold plasma, is characterized by the lack of thermal equilibrium between electrons and the rest of plasma species. In this case the electron temperature is in the range of 104–105 K, whereas the rest of the species are at temperature close to the room temperature. Under such conditions, chemical processes (e.g. chemical synthesis of new materials) can be performed at the room temperature using energetic electrons to cleavage covalent bonds in the gas molecules. By contrast, very high temperature of all the species in the thermal plasma considerably limits its application for the chemical syntheses and the surface modifications of thermal-degradable materials.

As one can see, among the various types of plasmas, the cold plasma is especially recognized as a promising tool on the road towards the search for new materials. The creation of such materials by the cold plasma technology can be carried out in two ways (Fig. 1). The first one is the deposition of completely new materials in the form of thin films, which is mainly accomplished by plasma polymerization processes (sometimes not quite correctly called plasma-enhanced chemical vapor deposition (PECVD)), and also, but relatively more rarely, by reactive sputtering processes. Thin-film materials with unusual

the Sun or in laboratories involved in nuclear fusion research, but this type of plasma is

The low-temperature plasma can be divided, in turn, into two further types (Fig. 1): equilibrium and non-equilibrium plasmas. In the equilibrium plasma, often called the thermal plasma, electrons and the rest of plasma species have nearly the same temperature (*Te* ≈ *Tgas*), much higher than the room temperature. Such plasma is generated, for example, in plasma jets and torches. On the other hand, the non-equilibrium plasma, called sometimes the cold plasma, is characterized by the lack of thermal equilibrium between electrons and the rest of plasma species. In this case the electron temperature is in the range of 104–105 K, whereas the rest of the species are at temperature close to the room temperature. Under such conditions, chemical processes (e.g. chemical synthesis of new materials) can be performed at the room temperature using energetic electrons to cleavage covalent bonds in the gas molecules. By contrast, very high temperature of all the species in the thermal plasma considerably limits its application for the chemical syntheses and the

As one can see, among the various types of plasmas, the cold plasma is especially recognized as a promising tool on the road towards the search for new materials. The creation of such materials by the cold plasma technology can be carried out in two ways (Fig. 1). The first one is the deposition of completely new materials in the form of thin films, which is mainly accomplished by plasma polymerization processes (sometimes not quite correctly called plasma-enhanced chemical vapor deposition (PECVD)), and also, but relatively more rarely, by reactive sputtering processes. Thin-film materials with unusual

rather not interesting as a technology for the preparation of new materials.

Fig. 1. Classification of the plasma types.

surface modifications of thermal-degradable materials.

chemical constitution, molecular construction and nanostructure can be obtained in this way (Gordillo-Vázquez et al., 2007; Konuma, 1992).

The second way consists in the modification of conventional materials, performed by their cold plasma treatment. Generally, such a treatment triggers three basic processes occurring mainly on the surface. It can create new functional groups by implantation of atoms present in the plasma, it can generate free radicals that then react with atmospheric oxygen and water molecules giving additional functional groups or can be used in grafting processes, and finally it can modify the microporous structure by the etching and degradation effects (Inagaki, 1996).

The cold plasma is most often generated in laboratories and industry by an electric glow discharge under low pressure using various frequencies of the applied electric field: audio frequencies (AF, mainly in the range of 10–50 kHz), radio frequencies (RF, mainly 13.56 MHz), and microwave frequencies (MW, mainly 2.45 GHz). Sometimes, a direct current (DC) discharge is also used. An example of typical parallel plate plasma reactor, one of those being used in our laboratory for deposition of thin films, is sketched in Fig. 2.

Fig. 2. A sketch of a typical parallel plate plasma reactor.

At first, the reactor chamber is evacuated down to 10−1 –10−3 Pa. Then, a precursor of plasma deposition is introduced to the chamber in the form of gas or vapor under controlled flow. Organic and inorganic gases, sublimating solids and evaporating liquids can be used as precursors. They are supplied as pure compounds or as mixtures with an inert carrier gas (e.g. argon). The carrier gas enables to generate plasma in the presence of compounds with very low vapor pressure. It is also possible to perform the plasma deposition process using a mixture of two or more precursors. Suitable selection of these compounds and their concentrations in the reactor chamber make it possible to control the molecular structure of deposited films, and consequently – properties of the films. In the reactor chamber with

Cold Plasma – A Promising Tool for the Development of Electrochemical Cells 109

takes part in chemical processes after sputtering and finally a new converted material is

deposited. This is the so-called reactive plasma sputtering.

Fig. 4. A schematic diagram of a typical set-up for sputtering deposition.

far, still dominating in the cold plasma technology.

**3. Plasma deposition of new materials** 

For the sake of formality, it should be added that recently more and more attention has been focused on the cold plasma processes performed under atmospheric pressure conditions (Belmonte et al., 2011). However, plasma processes carried out under low pressure are, so

**substrate**

Since the first literature reports describing – nearly 140 years ago – the formation of solid products during electrical discharge in a tube filled with acetylene, many researches working in the field of plasma chemistry have observed the presence of high molecular weight materials as reaction by-products. These products were usually considered disadvantageous – they were deposited on the reactor walls and, due to their good adhesion to glass and insolubility in organic solvents, were not easily removable. They began to stimulate a scientific interest as late as in the sixties of the 20th century only, after Goodman (Goodman, 1960) had reported a successful application of plasma polymerized styrene films as an insulating layer in nuclear batteries. A vast amount of literature concerning plasma deposited films (plasma polymers), their properties, structure and mechanism of formation as well as potential application in various technologies has been published since then. The application ability of plasma polymers originates from both their often unique properties

and relative simplicity of their production (Biederman, 2004; and references therein).

At the beginning, the interest was mainly limited to classical monomers, i.e. substances known to be able to polymerize in the conventional way (e.g. ethylene, styrene, butadiene). Hence, at that time the term "plasma polymer" was introduced as the result of the supposed analogy to conventional polymerized materials. In soon turned out, however, that many other low molecular weight organic, organometallic and inorganic compounds undergo plasma polymerization as well. Plasma polymers and conventional polymers, even though

reactive gases, the glow discharge is generated between two internal metal electrodes by means of an appropriate generator. As a result of chemical processes proceeding in the plasma, a thin film of a new material, commonly referred to as "plasma polymer", is deposited on the electrode surfaces as well as on any substrate being in the plasma region. The same reactor can be also used for the modification of conventional materials. In this procedure, however, only "non-polymerizable" gases (e.g. Ar, O2, N2, NH3) are utilized as precursors of plasma processes.

Another important plasma reactor employed in our study for the preparation of very interesting new materials is the so-called "three-electrode AF reactor". A schematic view of this reactor is shown in Fig. 3. A small electrode, on which films are deposited, is placed horizontally between two main perpendicular electrodes maintaining a glow discharge (10– 50 kHz). The small electrode is coupled with the powered main electrode by a variable capacitor. The coupling capacitance controls the sheath voltage of the small electrode (*V*(-)) and, in consequence, the impact energy of ions bombarding the growing film, independently of the plasma chemistry processes proceeding in the gas phase. It is especially striking that in some cases a very small variation of the bombarding ion energy in a defined range of its values is sufficient to create a drastic change in the electronic structure of deposited films (Tyczkowski, 1999). Thin films of this type will be discussed later in this Chapter (*Sec. 4.1.*).

Fig. 3. A schematic diagram of the three-electrode AF reactor.

A useful variant of the fabrication of new materials by the cold plasma technique is the reactive sputtering. A schematic diagram of a typical set-up for sputtering deposition is sketched in Fig. 4. Positive ions that are produced in RF plasma generated in an inert gas, for example Ar, bombard the target surface (supplied with the negative self-bias) and cause the sputtering of its material. The sputtered material condenses on the substrate that is located out of the plasma region. If we use some reactive gas (e.g. O2, N2, CH4), the target material

reactive gases, the glow discharge is generated between two internal metal electrodes by means of an appropriate generator. As a result of chemical processes proceeding in the plasma, a thin film of a new material, commonly referred to as "plasma polymer", is deposited on the electrode surfaces as well as on any substrate being in the plasma region. The same reactor can be also used for the modification of conventional materials. In this procedure, however, only "non-polymerizable" gases (e.g. Ar, O2, N2, NH3) are utilized as

Another important plasma reactor employed in our study for the preparation of very interesting new materials is the so-called "three-electrode AF reactor". A schematic view of this reactor is shown in Fig. 3. A small electrode, on which films are deposited, is placed horizontally between two main perpendicular electrodes maintaining a glow discharge (10– 50 kHz). The small electrode is coupled with the powered main electrode by a variable capacitor. The coupling capacitance controls the sheath voltage of the small electrode (*V*(-)) and, in consequence, the impact energy of ions bombarding the growing film, independently of the plasma chemistry processes proceeding in the gas phase. It is especially striking that in some cases a very small variation of the bombarding ion energy in a defined range of its values is sufficient to create a drastic change in the electronic structure of deposited films (Tyczkowski, 1999). Thin films of this type will be discussed later in this

precursors of plasma processes.

Chapter (*Sec. 4.1.*).

Fig. 3. A schematic diagram of the three-electrode AF reactor.

A useful variant of the fabrication of new materials by the cold plasma technique is the reactive sputtering. A schematic diagram of a typical set-up for sputtering deposition is sketched in Fig. 4. Positive ions that are produced in RF plasma generated in an inert gas, for example Ar, bombard the target surface (supplied with the negative self-bias) and cause the sputtering of its material. The sputtered material condenses on the substrate that is located out of the plasma region. If we use some reactive gas (e.g. O2, N2, CH4), the target material takes part in chemical processes after sputtering and finally a new converted material is deposited. This is the so-called reactive plasma sputtering.

Fig. 4. A schematic diagram of a typical set-up for sputtering deposition.

For the sake of formality, it should be added that recently more and more attention has been focused on the cold plasma processes performed under atmospheric pressure conditions (Belmonte et al., 2011). However, plasma processes carried out under low pressure are, so far, still dominating in the cold plasma technology.
