Studies also showed the improved self-cleaning property by using silicon dioxide sol modifying the wettability of acrylate polyurethane from hydrophobicity to hydrophilicity. Transparent hydrophilic photocatalytic TiO2/SiO2 thin films were also explored in polycarbonate substrate which was precoated by an intermediate SiO2 layer. The coated surfaces displayed considerable photocatalytic activity and superhydrophilicity. The self- cleaning coatings usually suffer from photodegradation caused by the nanofiller's photoactivity. From the previous study, it was found that coating of SiO2 on the surface of TiO2 nanoparticles influenced the photocatalytic activity of the formed PU composite films, resulting in reduced photodegradation. Recent studies report the integration of SiO2-coated TiO2 nanostructures into a PU matrix for the synthesis of self-cleaning coatings [27].

Photochemical additives based on C60 fullerene were incorporated into polyurethane coatings to investigate their coating compatibility and ability to impart chemical decontaminating capability to the coating surface. C60 fullerene molecules have also been observed to exhibit intriguing photochemical properties, including oxidative capabilities, which hold exciting potential for development of a self-decontaminating coating [28].

A combination of easy cleaning with self-cleaning of oleophobic and hydrophilic stains has been recently demonstrated in PU system by using a combination approach of hydrophilic thermo-responsive hydrogel coating and the self-cleaning from the embedded non-metallic photocatalyst g-C3N4 [29]. Due to the existence of strong hydrogen bonds between the hydroxyl groups in the hybrid hydrogel coating and the hydroxyl/carboxyl groups in the plasma-treated PU, the hybrid hydrogel coating is very stable on PU. Simultaneously, the acrylamide network in the hybrid hydrogel coating enhances its mechanical strength. Because the transition temperature of

oligo(ethylene glycol) methyl ether methacrylate (OEGMA300) is well above the room temperature, the cross-linked coating remains hydrophilic in ambient conditions. Thus, oleophilic stains, such as oil and grease, can be easily removed from the coating surface. In addition, the embedded photocatalyst g-C3N4 in the hybrid hydrogel coating introduces the extra capability of decomposing organic compounds under sunshine, which favors the removal of hydrophilic stains such as dyes and wines.

#### **4.3 Oleophobic surfaces for self-cleaning**

Oleophobic surfaces play a crucial role in self-cleaning and anti-fouling applications, particularly in industries such as oil, steel, and marine environments where oil spills can cause significant damage [30]. Super-oleophobic surfaces, which exhibit strong repellency towards oil and organic liquids with lower surface tension, have garnered interest for their potential in preventing oil adhesion and facilitating easy cleaning. To create super-oleophobic surfaces, one approach is to design solid surfaces with a lower surface energy than that of oil. By reducing the surface energy of the material below the surface tension of the oil, the oil droplets tend to minimize contact with the surface, leading to oil repellency. Hydrophobic and oleophobic coatings can be formulated using ceramic particles such as SiO2 (silicon dioxide), SiO (silicon monoxide), and Al2O3 (aluminum oxide). These ceramic particles offer thermal and chemical durability, making them suitable for various applications. Such coatings serve as alternatives to materials like Teflon, which is known for its oil-repellent properties but may have limitations in certain environments. Super-oleophobic coatings are also developed by employing techniques like spray casting of nanoparticlepolymer suspensions. This process allows for the deposition of a thin film containing ceramic nanoparticles onto a substrate, creating a surface with excellent oil-repellent properties [31].

#### **4.4 Amphiphobic surfaces for self-cleaning**

Amphiphobic coatings repel both water and oil, hence surface combines both hydrophobicity and oleophobicity. Inspired by nature, researchers have fabricated superhydrophobic-superoleophobic surfaces. A stable superamphiphobic coating using electrospinning technique was fabricated on glass substrate with nano/ mesostructured TiO2 [32]. Also, a novel hydrophobic and oleophobic surfaces using polyurethane with hydrogenated polyisoprene soft segment have been fabricated and reported recently [33].

#### **5. Applications of polyurethane self-cleaning coatings**

Polyurethane based self-cleaning coatings are popular in different applications such as antifogging, anti-icing, anti-reflection, corrosion resistance, drag reduction, sensors, solar cells, and textiles etc. Potential application sectors include but not limited to textile industry (self-cleaning clothing), automobile industry (self-cleaning windshield glass, car bodies and mirrors), optical industry (cameras, sensors, lenses and telescopes), marine industry (anticorrosion protection) and aerospace industry (non-sticky surfaces) and so on. Moreover, self-cleaning coatings can also be used in windows (window coatings), solar modules (self-cleaning coatings for solar modules) and in paints (exterior paints with self-cleaning properties) etc. Commonly desired

properties for such applications include durability, water and dirt repellency, easy cleaning, scratch resistance, chemical resistance, UV resistance, etc.

#### **5.1 Anti-fogging**

Polyurethane-based self-cleaning coatings can be designed to provide an antifogging effect, which can be particularly useful in applications where visibility is important. This is achieved through the hydrophobic properties of the coating, which prevent moisture from condensing on the surface. When a surface is cooled below the dew point, moisture in the air can condense on the surface, leading to the formation of fog. Interestingly, a hydrophobic polyurethane coating can prevent this from happening by repelling water droplets and preventing them from coalescing into larger droplets that can cause fogging. By preventing fogging, these coatings can help to improve safety and performance, as well as reduce the need for frequent cleaning.

Along with hydrophobicity, the polyurethane formulations for self-cleaning coatings may also contain anti-fogging agents that help to further reduce the formation of fog. These agents work by lowering the surface tension of the water droplets, causing them to spread out and evaporate more quickly. This type of polyurethane-based selfcleaning anti-fog coatings can be particularly beneficial in applications such as automotive windshields, eyewear, and medical equipment where visibility is crucial.

A steel surface coated using polyurethane, SiO2 nanoparticles and hexadecyltrimethoxysilane by a spin-coating technique has been reported to show excellent superhydrophobic and anti-fogging effect [34]. Highly transparent antifogging polyurethane coatings have been fabricated via a UV-assisted cross-linking method by Li et al. [35].

#### **5.2 Anti-icing**

In cold regions, especially during winter season, layers of ice get deposited on solid materials exposed to open environments. Polyurethane-based self-cleaning coatings when applied to surfaces, the coatings create a water-repellent layer due to the hydrophobic nature that prevents water from adhering to the surface. This can help to reduce the formation of ice significantly well [36]. Self-cleaning polyurethane superhydrophobic coating having excellent anti-icing property at �10°C could be a viable strategy for transmission line and wind turbines icing mitigation [37].

#### **5.3 Anti-reflective**

The anti-reflective effect of polyurethane-based self-cleaning coatings can be particularly beneficial in applications such as eyewear, camera lenses, and electronic displays, where clarity and visibility are important. By reducing glare and reflection, these coatings can improve the performance and usability of these devices. Polyurethane-based self-cleaning coatings can also provide an anti-reflective effect, which can improve visibility and reduce glare. This is achieved through the use of special coatings that are designed to reduce the reflection of light on the surface. When light hits a surface, some of it is absorbed while the rest is reflected back. This reflection can cause glare and reduce visibility, particularly in bright sunlight or under certain lighting conditions. However, an anti-reflective coating can reduce the amount of light that is reflected back, allowing more light to pass through the surface and improving visibility. In addition, some formulations of self-cleaning polyurethane

coatings may also contain anti-reflective agents that further reduce the reflection of light. These agents work by altering the refractive index of the coating, causing light to be refracted in a way that reduces the amount of reflection. A polyurethane coating with photocatalytic properties can help to reduce the accumulation of ice and snow on the coated surface. Ko et al. [38] have reported a replication route to non-planar, three dimensional microlens arrays with an antireflective poly(urethane) surface with "moth-eye" nanopattern.

#### **5.4 Enhanced corrosion resistance coatings**

Polyurethane can directly adhere to metals without the use of adhesion promotors. Hence, polyurethanes are used in direct-to-metal coatings. Polyurethane-based selfcleaning coatings can provide an effective barrier against corrosion and protect metal surfaces from environmental damage. The hydrophobic and/or photocatalytic properties of these coatings prevent the accumulation of moisture and contaminants on the surface, which can potentially cause corrosion. The formulations of self-cleaning polyurethane coatings can be designed to contain corrosion inhibitors that provide additional protection against corrosion. These inhibitors work by forming a protective layer on the surface of the metal, preventing corrosive agents from penetrating the surface. The corrosion resistance effect of polyurethane-based self-cleaning coatings can be particularly useful in applications such as automotive, marine, and aerospace industries where metal components are exposed to very harsh environments. By protecting these components from corrosion, these coatings can help to extend the lifespan of the equipment and reduce maintenance costs. Nosrati et al. [39] have prepared anti-corrosive polyurethane coating modified with titanium dioxide/ polyaniline/halloysite nanotube/carbon nanotube nanocomposite having excellent effect to improve environmental protection. Ye et al. [40] reported that ZnO/polyurethane nanocomposite coatings can enhance the corrosion resistance of stainlesssteel. Zhang et al. [41] fabricated slippery coating by spraying silicone-oil soaked SiO2 with irregular coral cluster structure on the acrylic polyurethane having corrosion resistance with self-cleaning and good coating stability. Fluorine-modified hyperbranched waterborne polyurethane resin applied to a water-based nano anticorrosion self-cleaning finish paint has been reported to show excellent anti-corrosion property [42].

#### **5.5 Drag reduction**

Frictional drag is a major source of resistance that affects the speed and efficiency of vehicles and equipment. By reducing friction, a polyurethane-based self-cleaning coating can improve the flow of fluids around the surface, reducing drag and increasing speed and efficiency. Polyurethane-based self-cleaning coatings can also provide a drag reduction effect, which can improve the performance of vehicles and equipment that move through fluids, such as air or water. The hydrophobic properties of these coatings reduce the surface tension of the fluid, allowing it to slide more easily over the surface and reducing friction. In addition, some formulations of self-cleaning polyurethane coatings may also contain additives that further reduce drag, such as nanoparticles that modify the flow of the fluid around the surface.

The drag reduction effect of polyurethane-based self-cleaning coatings can be particularly beneficial in applications such as aerospace, marine, and automotive

industries, where reducing drag can improve fuel efficiency and reduce operating cost. By improving the performance of these systems, these coatings can help to reduce environmental impact and improve sustainability. Superhydrophobic PDMS assisted immobilized fluorine functionalized SiO2 nanoparticles have been reported to improve coating adhesion to substrate materials [43].

#### **5.6 Sensors**

Polyurethane-based self-cleaning coatings can be applied in various sensor applications to improve their performance and longevity. In optical sensors, the antireflective properties of the coating can reduce the reflection of light and improve the accuracy of the sensor. Similarly, in gas sensors, the hydrophobic and/or photocatalytic properties of the coating can prevent the accumulation of moisture and contaminants on the surface of the sensor, reducing interference with the sensor's performance [44]. The anti-fouling properties of polyurethane-based self-cleaning coatings can help to prevent the buildup of biofilms and other materials on the surface of sensors used in medical and biological applications. A group of researchers fabricated a pressure sensor consisting of PU mesoscaled dome arrays embedded with gradient-distributed silver nanowire (AgNW) for flexible electronic applications [45].

#### **5.7 Solar**

Polyurethane-based self-cleaning coatings can be applied to solar cells to improve their efficiency & reduce maintenance requirements. The coatings can provide several benefits, including self-cleaning, anti-reflective, anti-scratch, hydrophobic and UVstability. Interpenetrated polymer networks prepared using polyurethane- acrylic colloidal suspension when applied on glass covers of solar cell panels is reported to give excellent super-hydrophobicity, transparency, and durability [46].

#### **5.8 Textile**

Textiles coated with these types of coatings can resist dirt, dust, and other contaminants, making them easier to clean and maintain.

The hydrophobic properties of polyurethane-based coatings can prevent the absorption of moisture and liquids, making textiles more resistant to water damage.

Aliphatic Polyurethane-based coatings can provide protection against UV radiation, which can cause damage to textiles and fade their colors over time.

The textiles coated with these types of coatings can be more durable, with improved resistance to wear and tear, stretching, and abrasion. Chen et al. [47] fabricated coating with self-cleaning and pH-controllable oil/water separating ability on account of the pH-responsive UV-cured PU having good super-hydrophobicity and can maintain their self-cleaning ability even after mechanically damaged, seawater immersing and UV irradiation.

#### **5.9 Others: electrowetting and other functions**

Electrowetting is a phenomenon that occurs when an electric field is applied to a liquid droplet on a surface, causing the droplet to spread or contract. Incidentally, polyurethane based self-cleaning coatings can exhibit electrowetting behavior, which can be beneficial in certain applications. By applying an electric field to the surface of a polyurethane based self-cleaning coating, the contact angle between the droplet and the surface can be modified. This can change the wetting behavior of the droplet and allow for precise control of its movement and positioning on the surface. This behavior can be useful in microfluidic systems and lab-on-a-chip devices, where precise control of droplet movement is important.

#### **6. Current challenges and possible solutions**

Along with versatility of polyurethane self-cleaning coatings, there are still several challenges that must be addressed to develop multi-functional robust and effective self-cleaning coatings to take the next level. The current challenges and possible solutions are highlighted below:

#### **6.1 Durability**

One of the main challenges with aromatic polyurethane based self-cleaning coatings is maintaining their durability over time, particularly in the UV light-based environments. Possible solutions to this challenge include incorporating additives that enhance the coating's UV resistance, abrasion and scratch resistance or modifying the coating's structure to improve its UV resistance and adhesion and cohesion properties.

#### **6.2 Compatibility with different substrates**

Although polyurethane is versatile, another challenge is developing coatings that can be applied to a wide range of substrates, including metals, plastics, and ceramics. Possible solutions to this challenge include optimizing the coating's composition and application process to improve its adhesion to different substrates, or developing surface pre-treatments that improve the substrate's surface energy and adhesion properties.

#### **6.3 Environmental impact**

Many current self-cleaning coatings contain toxic chemicals or are not biodegradable, which can have negative environmental impacts. Possible solutions to this challenge include using eco-friendly raw materials and production processes, or developing coatings that are biodegradable or recyclable.

#### **6.4 Cost**

Polyurethane based self-cleaning coatings can be more expensive than traditional coatings, which can limit their adoption in some applications. Possible solutions to this challenge include developing more cost-effective production processes, or identifying applications where the benefits of self-cleaning coatings outweigh the higher cost.

#### **6.5 Performance under extreme conditions**

Self-cleaning coatings may not perform as well under extreme conditions, such as high temperatures (>200°C), humidity, or exposure to chemicals. Possible solutions to this challenge include developing coatings with improved thermal stability, resistance to chemical exposure, or moisture resistance.

#### *Self-Cleaning Surfaces of Polyurethanes DOI: http://dx.doi.org/10.5772/intechopen.111929*

Overall, developing a robust, high performance and cost-effective polyurethane based self-cleaning coatings requires a combination of material chemistry, and engineering. In order to provide a solution-based approach to these challenges, researchers can continue to improve the performance and applicability of self-cleaning coatings in a wide range of industries and applications.

#### **7. Characterization of polyurethane self-cleaning surfaces**

The self-cleaning property of a polyurethane surface is mainly governed by its *phobic* and *philic* nature towards oil and water. The exact characterization is made by quantitatively measuring (1) surface energy, (2) surface roughness and (3) liquid droplet contact angle by using various characterization techniques and correlate this with the self-cleaning effect. Typically, higher surface energy polymers offer to be potential adhesives since, higher their surface free energy higher will be their ability to interact with the substrate. The surface energy aspects can be simulated with different mathematical models to design new surfaces.

#### **8. Conclusions**

In conclusion, all the research efforts, as we have seen here, are explored to mimic the supreme strategies perfected by nature over billions of years. The self-cleaning surface on naturally occurring leaves and wings of certain insects is multipurpose in achieving self-cleaning, anti-reflective, camouflage and various other functionalities which has got researchers across the globe take stock and attempt to mimic. Though the self-cleaning surfaces designed by them are yet to match their naturally occurring counterparts, the fabrication techniques have indeed evolved into more environmentally compatible and cost-effective and polyurethane is one amongst them.

Polyurethane is a well-accepted polymer-based coating material in the market. Polyurethane polymers can be tailored very easily to meet the *phobic* and *philic* nature to oils and water. Therefore, polyurethane-based self-cleaning coatings have potential for reducing maintenance and cleaning costs in various industries. Polyurethane coatings can be tuned to utilize hydrophobic and/or photocatalytic properties to repel dirt, dust, and other contaminants, which can then be easily washed away as exemplified in this chapter. Polyurethane based applications for self-cleaning coatings are currently in automotive, aerospace, and marine industries.

As the market expands more effort on building a strong structure performance analysis is critical to develop a robust self-cleaning surface for different applications. The future outlook for self-cleaning polyurethane coatings is promising, with potential for further improvements in the smartness, durability, efficiency, sustainability/eco-friendliness and circularity of the coatings along with the develop-

ment of easily scalable manufacturing processes. As and when more industries adopt these coatings, there may be an increased demand for customized formulations to meet specific application requirements in this domain.

#### **Acknowledgements**

The authors would like to acknowledge Rob Kerkhofs for his contribution to providing literature support.

### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Kamlesh Panwar, Neelam Vyas, Roopali Rai\* and Shreedhar Bhat\* Huntsman Solutions India Pvt. Ltd., Mumbai, India

\*Address all correspondence to<EMAIL_ADDRESS>and<EMAIL_ADDRESS><EMAIL_ADDRESS>
© 2023 The Author(s). Licensee IntechOpen. This chapter is 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.

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#### **Chapter 4**

## A Self-Cleaning Approach Utilizing Metal Oxide Thin Films and Nanocomposites

*Abdullah Al Nahid*

#### **Abstract**

Self-cleaning technology mimics the natural self-cleaning abilities of plants and animals such as lotus effect, to create a surface that is hydrophobic and oleophobic, meaning it repels water and oil. The resultant surface is resistant to dirt and grime, making it easier to clean and maintain, reducing labor costs and time consumption. However, it is not only limited to the superhydrophobic surface for making the water roll off instead of sliding but also modern research focuses on incorporating photocatalysts to break down organic compounds during daylight at outdoor applications. In addition, self-cleaning surfaces and coatings are attracting research attention due to their ability to self-disinfect. This review highlights the use of metal oxide-based nanocomposite for self-cleaning purposes. This chapter provides an outlook of different metal oxide and metal-metal oxide nanocomposites in advancing self-cleaning properties, durability, and other mechanical properties. This chapter aims to give a general overview of a variety of polymeric metal oxide-based systems and methods that enhance self-cleaning behavior as well as the projection toward future research.

**Keywords:** hydrophobicity, metal oxide, nanocomposite, self-cleaning, photocatalytic activity

#### **1. Introduction**

Self-cleaning materials have become increasingly popular in recent years due to their remarkable ability to remove dirt, contaminants, and even microorganisms from their surfaces. The materials hold great promise for a variety of applications, including architectural coatings, automotive surfaces, medical devices as well as self-cleaning exterior construction materials, interior furnishing materials, road construction materials, solar cells, car mirrors, textiles, utensils, roof tiles, etc. Among the various strategies employed to achieve self-cleaning properties, the use of metal oxide thin films and nanocomposites has emerged as a particularly promising approach.

Metal oxides, such as titanium dioxide (TiO2), zinc oxide (ZnO), and iron oxide (Fe2O3), have been extensively studied for their photocatalytic properties and their ability to generate reactive oxygen species (ROS) upon exposure to ultraviolet (UV) light. These ROS possess strong oxidizing power and can break down organic compounds, rendering them highly effective in the degradation of dirt, organic

pollutants, and even bacteria and viruses [1]. For example, ZnO is an n-type semiconductor with band gap Eg = 3.37 eV. CuO is a p-type, narrow band gap (Eg = 1.2 eV) semiconductor capable of large absorption of light in spite of its low photocatalytic efficiency [2]. Through their light absorption capabilities, incorporating metal oxide thin films onto surfaces or nanoparticles into composite materials can impart selfcleaning properties to a wide range of substrates.

In this book chapter, we delve into the world of self-cleaning materials and explore the potential of metal oxide thin films and nanocomposites for achieving this functionality. We begin by providing a comprehensive overview of the principles underlying self-cleaning mechanisms and the unique properties of metal oxides that make them ideal candidates for self-cleaning applications. We discuss the photocatalytic activity of metal oxides, their role in generating ROS, and the mechanisms through which dirt and contaminants are degraded and removed from the surface.

Moreover, we explore the diverse range of applications that can benefit from self-cleaning materials. From self-cleaning glass windows that maintain their transparency and clarity to self-cleaning textiles that repel stains and odors, the potential of metal oxide thin films and nanocomposites is vast. We discuss the challenges and opportunities associated with scaling up these materials for realworld applications and address the environmental implications of self-cleaning technologies.

This review aims to provide an in-depth understanding of the self-cleaning approach utilizing metal oxide thin films and nanocomposites. By exploring the fundamental principles, synthesis techniques, and potential applications, we hope to contribute to the advancement of self-cleaning materials and inspire further research and development in this exciting field.

#### **2. Origin of self-cleaning technology**

The history of nature-inspired self-cleaning can be traced back to the early twentieth century when scientists and engineers began studying natural phenomena that exhibited self-cleaning properties. Self-cleaning refers to the spontaneous removal of contaminants, such as dirt, organic compounds, and pollutants, from the surface of a material without the need for external cleaning agents or mechanisms. However, in the 1960s, Swiss engineer Georges de Mestral invented Velcro, a fastening system inspired by the way burrs clung to his clothes and dog's fur. This innovation sparked interest in biomimicry, the practice of emulating nature's designs and processes in engineering and design [3].

In the 1970s, Lotus leaf's self-cleaning ability became a subject of scientific inquiry. Researchers discovered that the leaf's microscale surface structure, consisting of tiny wax crystals, repelled water and prevented dirt particles from adhering [4]. This discovery laid the foundation for the development of superhydrophobic and self-cleaning surfaces.

#### **3. Mechanism of self-cleaning**

There are two mechanisms responsible for surface cleaning, that is, hydrophilic and hydrophobic.

*A Self-Cleaning Approach Utilizing Metal Oxide Thin Films and Nanocomposites DOI: http://dx.doi.org/10.5772/intechopen.111971*

#### **3.1 Hydrophilic and superhydrophilic surfaces**

Hydrophilic and superhydrophilic surfaces utilize the photocatalytic activity of the coated materials in achieving the self-cleaning property. A surface is said to be hydrophilic if water spread over the surface evenly without the formation of droplets. Usually ordinary surfaces with typical wetting have contact angle 30° to 90°, whereas surfaces with contact angles less than 30° are termed hydrophilic, since the forces of interaction between water and the surface are almost equal to the forces of bulk water and water does not drain cleanly from these surfaces. However, when the advancing contact angle of water on the surface is less than 5°–10° [5–7] or the time for complete moisture to be achieved by small drops of water has to be under 0.5 seconds then the surface is designated as superhydrophilic [8]. However, superhydrophilic surfaces have a strong affinity for water due to the low surface energy and the micro/ nanostructured surface of the metal oxide thin film [9]. As the water spreads over a surface, dirt particles are picked up and carried away, effectively cleaning the area.

Nevertheless, photocatalytic self-cleaning metal oxide thin films, such as titanium dioxide (TiO2) and zinc oxide (ZnO), exhibit photocatalytic properties, enabling them to undergo self-cleaning under light. When exposed to ultraviolet (UV) or visible light, the metal oxide thin film absorbs photons and generates electron-hole pairs. The holes at the valence band (VB) can accept electrons from the surrounding moisture and generate hydroxyl radicals (OH• ) (**Figure 1c**) which are capable of acting as a detergent for the troposphere and breaking down organic pollutants into small fragments with low adhesion to the surface. Furthermore, excited electrons in the conduction band (CB) can react with atmospheric oxygen and produce superoxide radicals (O2 •–) (**Figure 1b**) that can oxidize organic compounds and break them down into harmless compounds. This photocatalytic oxidation process effectively removes contaminants from the surface but generates highly reactive oxygen species (ROS), such as hydroxyl radicals (OH• ) and superoxide radicals (O2 •–) resulting in self-cleaning. As a result, either a sprinkle of water or rainwater can easily remove the dirt from the surface without using any laborious cleaning.

#### **Figure 1.**

*Illustration of photocatalytic self-cleaning mechanism of MO thin film. (a). Window glass coated with MO thin film; (d). using rainwater or sprinklers can remove dirt with no effort or extensive cleaning.*

Furthermore, the thin film promotes self-drying through the photocatalytic generation of hydroxyl radicals, which dissolve water molecules, further, eliminate contaminants, and facilitate quick evaporation. The photocatalytic self-cleaning action of metal oxide thin film can be simply illustrated as shown in **Figure 1**.

#### **3.2 Superhydrophobic self-cleaning**

Another approach to achieving self-cleaning property is to create a surface superhydrophobic and a water contact angle above 150° with a sliding age below 5° [10, 11]. Normal hydrophobic surfaces have a water contact angle between 90° to 120°. However, in order to make a surface super hydrophobic, coating and nanostructure engineering are required. For this purpose, polymer nanocomposites can be designed to exhibit this property by incorporating hydrophobic nanoparticles or modifying the surface with a hydrophobic coating. A superhydrophobic self-cleaning mechanism is inspired by nature, specifically the lotus leaf [4] and certain insect wings [12], which are extremely water-repellent. These mechanisms are designed to repel water and prevent the adhesion of dirt and other contaminants, enabling surfaces to remain clean with minimal maintenance [13].

When water droplets come into contact with a superhydrophobic surface, they form spherical beads and roll off, carrying away dirt and contaminants with them. As water droplets roll off the superhydrophobic surface, they pick up and carry away any dust, dirt, or contaminants present on the surface. The low adhesion of the water droplets to the surface prevents the contaminants from sticking, effectively cleaning the surface in the process. **Figure 2** shows a visual representation of this phenomenon.

Superhydrophobic coatings can be applied to many everyday surfaces, such as windshields, bodies of vehicles, windows and doors, skyscrapers, solar cell panels, fabrics, sports shoes, metals, paper, sponges, woods, marbles, and the list is endless [14]. It will be advantageous for the architecture, automotive, electronics, and textile industries to use superhydrophobic coatings in the future, as they will increase their profitability, lucrativeness, and durability by preventing fouling due to contaminants.

The surface to be made superhydrophobic is coated with a material that has low surface energy, such as a hydrophobic coating or a nanoparticle-based solution. This coating alters the surface properties, making it highly water-repellent. Then the

#### **Figure 2.**

*Visual representation of self-cleaning action of a superhydrophobic surface. (a). Rough surface resembling a lotus leaf; (b). water droplets. (c & d). Rolling off water droplets while capturing dirt inside and draining away.*

#### *A Self-Cleaning Approach Utilizing Metal Oxide Thin Films and Nanocomposites DOI: http://dx.doi.org/10.5772/intechopen.111971*

surface is engineered with microscale or nanoscale structures, such as micro-ridges or nanopillars [15]. These structures create a rough surface texture, increasing the effective surface area and reducing contact between water droplets and the surface. The combination of the surface coating and the micro/nanostructures mimics the lotus leaf's characteristics [3]. When water droplets come into contact with the superhydrophobic surface, they tend to bead up and roll off due to the reduced contact area and the low surface energy of the coating. This behavior is known as the "lotus effect."

#### **4. Examples of some metal oxides self-cleaning**

The self-cleaning mechanism of TiO2 can be applied in various applications, such as building materials that includes paints, coatings, or materials to create self-cleaning surface. TiO2 coatings can be applied to solar panels to prevent the accumulation of dust and dirt, which can decrease the efficiency of the panels. The self-cleaning property of TiO2 ensures that the panels remain clean and maximize their energy generation [16]. TiO2 coatings on glass surfaces make them self-cleaning. This application is particularly useful for windows, facades, and glass surfaces in outdoor environments. TiO2 nanoparticles can be used in air filters or coatings to remove airborne pollutants and improve indoor air quality. It is worth noting that the self-cleaning efficiency of TiO2 can vary depending on factors such as the specific TiO2 formulation, the intensity of UV light, the presence of contaminants, and the surface area of TiO2 exposed to the light.

Other semiconductor materials, such as zinc oxide (ZnO), tungsten oxide (WO3), and cadmium selenide (CdSe), are all examples of self-cleaning [17]. Different classes of metal oxides used for self-cleaning coating material can be categorized as follows, as shown in **Figure 3**.

The photocatalytic property of most metal oxides can be greatly enhanced by combining them with other metal oxides, even though few metal oxides do not possess self-cleaning properties. A good example is ZrO2, which is not intrinsically selfcleaning but can be used as part of composite systems or modified to act as catalysts [18]. Furthermore, metal-doped metal oxides such as aluminum doped zinc oxide (AZO) and copper oxide (CuO) thin film may have potential applications in photovoltaics like solar cells [19, 20]. Other metal ions such as Cr, V, Fe, Pb, Cu, and Al [17] and nonmetals like C, N, and F doped metal oxide like TiO2 have shown potential self-cleaning surfaces [21]. The self-cleaning ability helps to maintain the catalytic activity by continuously removing adsorbed contaminants, resulting in efficient and long-lasting remediation processes. Therefore, metal oxide thin films may have potential applications in environmental remediation and solar panels. These panels can maintain optimal light absorption and maximize their energy by incorporating self-cleaning metal oxide thin films.

Nevertheless, the self-cleaning properties of metal oxides such as ZrO2 + TiO2 and Al2O3 + ZrO2 nanocomposites have found numerous applications in various fields [18, 20]. In architecture, self-cleaning coatings on building facades can maintain their esthetic appearance and reduce maintenance costs. In automotive and aerospace industries, self-cleaning coatings protect surfaces from environmental contaminants, improving performance and longevity. Additionally, self-cleaning metal oxide nanocomposites have shown promise in water purification, air pollution control, and biomedical applications.

**Figure 3.** *Various types of metal oxides for self-cleaning coating.*

### **5. Examples of some polymer nanocomposites self-cleaning**

Polymer nanocomposites, which consist of polymer matrices reinforced with nanoparticles, offer a promising solution for the development of self-cleaning materials. This chapter explores some of the important polymer nanocomposites used for self-cleaning purposes, their properties, fabrication methods, and potential applications. Polymer nanocomposites provide an avenue to replicate the nature inspire self-cleaning properties on a commercial scale. However, important polymer nanocomposites can be categorized as shown in **Figure 4**, and their details are discussed in Sections 5.1–5.4 focusing on the self-cleaning properties.

#### **5.1 Silica-based polymer nanocomposites**

Silica nanoparticles are widely employed in polymer nanocomposites for their hydrophobic and oleophobic properties. These nanoparticles are incorporated into polymer matrices such as polyethylene, polypropylene, or poly (methyl methacrylate) (PMMA), hyperbranched polyester acrylate to enhance their surface energy and reduce adhesion of contaminants. Silica-based nanocomposites exhibit excellent self-cleaning properties due to their low surface energy and high contact angle with water and oils [22].

*A Self-Cleaning Approach Utilizing Metal Oxide Thin Films and Nanocomposites DOI: http://dx.doi.org/10.5772/intechopen.111971*

**Figure 4.** *Various polymer nanocomposites for self-cleaning coating.*

#### **5.2 Metal-based polymer nanocomposites**

Metal-based polymer nanocomposites are widely used for self-cleaning due to their antibacterial and catalytic activity. Silver-polyethylene glycol (Ag-PEG) exhibits antimicrobial properties, Ag-chitosan shows antifungal properties, Ag-polyacrylonitrile (Ag-PAN) offers antibacterial and antimicrobial activity, ultraviolet blocking, and catalytic activities [23, 24]. Zinc can also be combined with chitosan, cellulose, and polyvinyl alcohol and is shown to have photocatalytic and antimicrobial properties, making it widely used in food contact surfaces and packaging [25]. The combination of gold with several polymers such as chitosan, polyoxoborate matrix, heparin-polyvinyl alcohol, and carboxyl methylcellulose exhibits antimicrobial and wound healing properties that can be used in biomedicine, medical devices, and sensors as well as photovoltaics [26, 27].

#### **5.3 Metal oxide-based polymer nanocomposites**

Metal oxide-based polymer nanocomposites such as A coating made of Aramid zyrconia nanocomposites, that is, ZrO2 combined with 1, 4-phenylene diamine, demonstrated remarkable protection by controlling charge transfer at the interface between steel alloy and electrolyte, preventing alloy dissolution [28]. TiO2 and polyurethane nanocomposites have shown self-cleaning and antibacterial activities of textiles [29, 30]. Moreover, polydimethylsiloxane (PDMS) incorporated with ZrO2 can produce superhydrophobic surfaces on cotton fabrics [31]. Additionally, the

photocatalytic printed film of titania polypyrrole (PPy/TiO2) nanocomposites shows excellent photocatalytic, self-cleaning, and antibacterial functionalities [32]. It is, therefore, possible to create self-cleaning surfaces using a suitable polymer together with other metal oxides.

#### **5.4 Carbon-based polymer nanocomposites**

Carbon-based nanomaterials such as graphene, carbon nanotubes (CNTs), and their derivatives, nanodiamond and fullerenes, offer unique self-cleaning properties due to their low surface energy, high electrical conductivity, and high mechanical strength. Carbonaceous materials have been used in polymer matrixes as filler or reinforcement to form carbon polymer composites. When incorporated into polymer matrices, carbonaceous nanomaterials can create self-cleaning surfaces that repel water, oils, and dirt particles. Additionally, their conductivity allows for generating electrostatic charges, which can repel contaminants from the surface. However, several carbon-based polymer nanocomposites include reduced graphene oxide-polyurethane (GO-PU), graphene oxide-polyacrylic acid (GO-PAA), graphene oxide-polyurethan acrylate (GO-PUA) coating, etc. [33].

Polymer nanocomposites derived from carbon offer the potential for individual and hybrid development of a wide range of innovative materials for a wide range of applications, such as thermal materials, shielding for electromagnetic interfaces, sensors, and energy storage [34].

However, polymer nanocomposite coating has some advantages because polymers are excellent host materials for nanoparticles. When the nanoparticles are embedded or encapsulated in polymer, the polymer acts as a surface capping agent and generates proper coating adhesion to the surface [35]. Metal/metal oxide nanoparticle-polymer composites are versatile materials containing dispersion of nanosized filler materials within the polymer matrix. Owing to their nanoscale size features and high surfaceto-volume ratios, they possess unique combination of multifunctional properties such as barrier properties and high ductility without strength loss. For example, if a drop of water trying to get through the film made with nanocomposite, it would face more barriers than in case of conventional composites because the distance between fillers in nanocomposite is much smaller. Additionally, Embedding nanoparticles inside polymer thin films facilitates immobilization and organization of the metal nanoparticles and tuning of their electronic and optical responses by the dielectric environment.

#### **6. Overview of preparing nanocomposites and its fabrications**

The most convenient routes of embedding the metallic nanoparticles into the polymer involve the following two ways. In situ method – growth of nanoparticles into polymer matrices through reduction or decomposition of precursors; ex situ method – incorporation of premade particles into a premade polymer matrix using a common blending solvent. In situ polymerization involves the simultaneous synthesis of polymer and nanoparticle formation within a reaction medium. This method allows for better control over nanoparticle dispersion and interfacial interactions, leading to enhanced self-cleaning properties. In situ polymerization is particularly suitable for the synthesis of polymer nanocomposites with specialized applications.

*A Self-Cleaning Approach Utilizing Metal Oxide Thin Films and Nanocomposites DOI: http://dx.doi.org/10.5772/intechopen.111971*

#### **Figure 5.**

*Graphical demonstration of the general routes of polymer nanocomposite preparation and its popular fabrication processes. (a). Nanomaterials precursor solution; (b). prepared nanoparticles or nano-colloid; (c). few techniques for nanocomposite fabrication.*

Whereas in ex situ method, solution mixing involves dispersing nanoparticles in a solvent and subsequently adding the polymer matrix to form a homogeneous mixture. The mixture is then cast or coated onto a substrate and dried to form a nanocomposite film. Solution mixing is a simple and scalable method for producing polymer nanocomposites with self-cleaning properties. Either method can be suitable for preparing composite material depending on the criteria of polymer and nanomaterials. There are also several easy and inexpensive ways to apply these coatings including sol–gel, immersion, spin coating, printing, and others. A general overview of nanocomposites preparation and its fabrication can be outlined as illustrated in **Figure 5**.

#### **7. Challenges and future directions**

The main challenges were twofold for engineering nanostructures in order to get better properties. First, the addition of nanosized particles into low-viscosity oligomers usually leads to a problematic liquid-to-solid transition at very low particle loadings, which may have a catastrophic effect on the low-pressure, nanoscale replication fidelity. Secondly, the dense network of nanoparticles may act as a filter for the migration of the surfactant molecules and therefore suppress the superhydrophobic effect.

It is true that metal oxide thin films and polymer nanocomposite materials possess unique properties that enable them to repel dirt, water, and other contaminants, resulting in less maintenance and cleaning. For their self-cleaning capabilities to be further enhanced, however, there are several challenges to overcome, as described below, and **Figure 6** depicts its summary outline.

#### **7.1 Durability and stability**

One of the primary challenges is ensuring the long-term durability and stability of metal oxide thin films and polymer nanocomposites. These materials may undergo degradation over time due to exposure to environmental factors, such as UV radiation,

#### **Figure 6.**

*A simple overview of current challenges and future projections for enhancing properties of MO/nanocomposites.*

humidity, and temperature fluctuations. Future research should focus on improving the stability of these materials to maintain their self-cleaning properties over extended periods.

#### **7.2 Scalability and manufacturing**

To make self-cleaning materials commercially viable, it is crucial to develop scalable manufacturing processes. Currently, the fabrication techniques for metal oxide thin films and polymer nanocomposites often involve complex and expensive procedures, limiting their large-scale production. Future directions should explore cost-effective and scalable manufacturing methods that can be easily implemented in various industries.

#### **7.3 Enhanced self-cleaning efficiency**

While metal oxide thin films and polymer nanocomposites demonstrate selfcleaning properties, there is room for improvement in terms of efficiency. Researchers can explore novel nanostructures, surface modifications, or coating techniques to optimize the self-cleaning effect. Increasing the contact angle, reducing surface energy, and enhancing the photocatalytic activity of metal oxide films are some potential avenues to pursue.

#### **7.4 Selective self-cleaning**

Current self-cleaning materials typically repel both water and oil-based contaminants. However, there may be applications where selective self-cleaning is desired. Future research could focus on developing materials with tailored properties to selectively repel specific types of contaminants, allowing for targeted cleaning capabilities in specific environments or industries.

#### **7.5 Environmental considerations**

As with any emerging technology, it is crucial to consider the environmental impact of self-cleaning materials. Future research should focus on developing sustainable and eco-friendly fabrication processes, reducing the use of harmful chemicals, and exploring recyclability and biodegradability of these materials.

#### **7.6 Real-world applications**

While self-cleaning materials have been studied extensively in the lab, their practical implementation in real-world applications is still limited. Future research should aim to bridge this gap by focusing on developing prototypes and conducting field tests in various industries, such as architecture, automotive, and healthcare, to demonstrate the feasibility and benefits of using self-cleaning materials in different settings.

#### **8. Conclusion**

Metal oxide thin film and polymer nanocomposites hold great promise as self-cleaning materials, offering a range of applications and potential benefits due to having several advantages including flexibility, lightweight, and ease of processing. However, there are still challenges that need to be addressed for widespread practical implementation. Continued research and development in this field such as enhancing the efficiency of photocatalytic processes, optimizing stability, and exploring sustainable synthesis, are necessary to overcome existing challenges and unlock the full potential of these materials for practical use in various industries. Continued research and innovation will drive the development of advanced self-cleaning materials with enhanced efficiency and broader practicality in the near future.

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Author details**

Abdullah Al Nahid Bangladesh Council of Science and Industrial Research, Dhaka, Bangladesh

\*Address all correspondence to<EMAIL_ADDRESS>
© 2023 The Author(s). Licensee IntechOpen. This chapter is 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.

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[34] Paul DR, Robeson LM. Polymer nanotechnology: Nanocomposites. Polymer (Guildf). 2008;**49**(15): 3187-3204. DOI: 10.1016/j.polymer. 2008.04.017

[35] Caseri WR. In situ synthesis of polymer-embedded nanostructures. In: Nanocomposites: In Situ Synthesis of Polymer-Embedded Nanostructures. USA: John Wiley & Sons, Inc.; 2013. pp. 45-72. DOI: 10.1002/9781118742655.ch2

#### **Chapter 5**

## Plasma Based Approaches to Achieve Self-Cleaning Surfaces

*Deepanjana Adak and Raghunath Bhattacharyya*

#### **Abstract**

The role of gaseous plasma has proven to be very beneficial in creating self-cleaning of various surfaces. Few references are there, in the published literature, on plasma enhanced hydrophilicity/hydrophobicity behavior of surfaces. A range of atmospheric pressure plasma spray systems are gaining popularity for creating self-cleaning surfaces, with some unique features, as also to fabricate new types of self-cleaning materials. In this chapter a brief introduction to essentials of plasma processing will be first presented, followed by examples of plasma assisted surface modification. This will include plasma cleaning, plasma etching, plasma polymerization/deposition, etc. Subsequently, various plasma assisted techniques to achieve a variety of self-cleaning surfaces will be highlighted. A unique combination of plasmabased approaches and sol–gel derived coating will also be discussed.

**Keywords:** self-cleaning, atmospheric-plasma, cold-plasma, superhydrophilic, superhydrophobic

#### **1. Introduction**

#### **1.1 What is plasma?**

*The term has been first used to describe ionized gas by Irving Langmuir in the year 1929. Plasma can be defined as a quasi-neutral gas of charged and neutral particles characterized by collective behavior.*

Depending on the energy of particle constituting matter, the matter can be broadly divided into 3 states – solid, liquid, and gases. However there exist energetically fourth state of matter apart from these three states, known as, *Plasma*. Generally, the transformation from one state to another is achieved in the presence of energy sufficient enough to overcome intermolecular forces. Likewise the transformation to fourth state of matter, plasma state, from gaseous state takes place when the gas molecules are heated to extremely high temperature or subject to high energy radiation [1]. The most common examples of species that exist in plasma state are interstellar matter such as sun and other stars. The primary benefit of plasma-assisted solid processing arises from the ability to conduct chemical reactions at substrate temperatures significantly lower than those achievable through standard reactions occurring at thermal equilibrium [2].

Plasma, with its fascinating characteristics, arises from the electromagnetic interactions among charged particles. It comprises electrons, ions, neutrals, electromagnetic radiation (photons), and electromagnetic fields, all contributing to its unique properties [3–9]. One notable attribute of plasma is its striking quasi-neutrality, where the positive and negative charges are intricately balanced, leading to an overall state of electrical neutrality [10].

Plasma can be broadly classified into two groups based on temperature and equilibrium conditions. The first group is thermal or equilibrium plasma, characterized by high ionization and thermal equilibrium among charged particles at temperatures around 104 K. The second group is non-thermal or non-equilibrium plasma, commonly known as cold plasma. In cold plasma, ionization is milder, and electrons do not reach thermal equilibrium with heavier particles. As a result, the gas temperature remains close to room temperature or slightly elevated, typically ranging from 40 to 150°C. Additionally, the electron density in cold plasma is typically below 1000 m−3 [11]. Non-thermal or cold plasmas can be generated under both low-pressure and atmospheric-pressure conditions. Generally, the temperature of each species within the plasma is determined by the average kinetic energy of the individual particles.

The fundamentals of cold, low-pressure plasmas has been discussed in an extensive detail in the book "*Cold Plasma in materials fabrication" by Alfred Grill, IBM Research Division, T.J Watson Research Center, New York* [10]*.* The behavior of plasma closely resembles the behavior of neutral gas, and it is possible to somewhat explain its property as described by *Kinetic Theory of Gases.* In contrast to neutral gas molecules, the movement of particles composing plasma results in localized concentrations of positive and negative charges. These concentrations of charge give rise to long-range coulombic fields that influence the trajectories of charged particles. Consequently, a charged particle in plasma tends to follow a path that, on average, aligns with the electric field present.

Plasma is broadly characterized by the following basic parameters,

$$\mathbf{T\_e = 1 eV, n\_e = 1010 \,\mathrm{cm}^{-3}, \lambda\_D = \sqrt{\frac{\varepsilon\_o kT\_e}{n\_e e^2}} = 74 \,\mathrm{\mu m} \tag{1}$$

• The presence of a plasma sheath near a surface is a result of electrons moving at significantly greater speeds than ions, causing them to depart from the plasma and create a positive charge in the surrounding area of the surface.

Near the surface, an electric field emerges that hinders the motion of electrons and accelerates ions. As a result, the surface acquires a negative potential, referred to as a negative self-bias, relative to plasma. In contrast, the plasma itself maintains a positive potential. The localized concentration of charged particles is confined within a volume equivalent to the Debye length (λD). It is important to highlight that beyond this volume, the charge density of ions matches the charge density of electrons, leading to the plasma's overall electrical neutrality. This property is the key reason why plasma is commonly regarded as a quasi-neutral gas.

#### **1.2 Cold-plasma generation**

To create and maintain cold plasma, gaseous molecules are typically exposed to direct current (DC), radio frequency (RF), or microwave (MW) power. In 1923, Irving Langmuir discovered the distinctive characteristics of DC glow discharge in a discharge tube and coined the term "plasma." It was observed that when voltage is applied to the gas in the discharge tube, the number of free electrons gradually increases and accelerates in response to the applied electric field, leading to a steady rise in the current (I). Once the applied voltage reaches a certain threshold value, known as the breakdown potential (Vb), an avalanche process occurs due to the interplay of three simultaneous processes:

When equilibrium is reached between rate of formation and recombination of electrons, the discharge becomes self-sustaining. This leads to a significant breakdown in the gas, causing it to emit a glowing effect. Consequently, there is an abrupt rise in current accompanied by a voltage drop. The following schematics (**Figure 1**) summarize what prevails in plasma reactors commonly used in most laboratories.

#### **Figure 1.** *Schematic depicting typical processes inside the plasma reactor.*

#### **Figure 2.**

*Variation of electron density with gas temperature for gas discharges of the typical atmospheric cold plasma. (Adapted from ref. [5].)*

In recent studies, cold plasmas have gained significant attention and are generated using methods such as corona discharges, dielectric barrier discharges, and plasma jets. These plasmas are created at atmospheric pressure by employing pulses with durations ranging from 10−6 to 10−9 seconds. In this type of plasma, known as 'Cold Atmospheric Plasma' (CAP), electrons with high energy levels are generated. CAP has shown great potential for a wide range of surface engineering application [12] (**Figure 2**).

#### **2. Plasma assisted surface modification**

The plasma-based surface modification majorly involves: *Plasma treatment*, *plasma etching*, and *plasma polymerization*. In case of plasma treatment of materials as well as plasma polymerization on a substrate, the plasma can be majorly divided into following three categories [13]:

Every year the substantial increase in the number of research papers and patents on the use of plasma technology for producing a variety of new materials and functional coatings is indicating the myriad of opportunities created by plasma technology [14]. Plasma-based methods are highly prevalent in surface treatment technologies and find extensive application in areas such as cleaning, surface activation, enhancing adhesion, corrosion resistance, biomedical applications, and the development of self-cleaning coatings [14–20].

*Plasma Based Approaches to Achieve Self-Cleaning Surfaces DOI: http://dx.doi.org/10.5772/intechopen.112378*

Plasma techniques are employed in various processes related to substrate treatment, including cleaning the substrate using gaseous plasma prior to coating, as well as modifying the surface after applying the desired coating material [21, 22]. Plasma etching also known as dry etching, has emerged as a crucial technique in microfabrication processes for microelectronic devices, aligning with advancements in the field of microelectronics [10]. However, so far plasma-based approaches were not extensively used to develop self-cleaning surfaces in commercial scale. Vacuum based plasma techniques impose major challenges with respect to cost-effective large area deposition in ambient conditions. Though, one can find in several published literatures the use of plasma deposition techniques for developing functional self-cleaning coatings.

#### **2.1 Plasma cleaning**

Plasma treatment is a highly effective method for preparing materials for subsequent processing, as it enhances their surface energy and removes contaminants introduced during the production process. This treatment significantly improves the quality, durability, and adhesive properties of coatings or printing applied to the surface. By subjecting materials to plasma treatment, their surface is modified in a way that optimizes their compatibility with secondary manufacturing applications [23]. The process involves a reactive treatment wherein positive and negative ions, electrons, and radicals interact and collide in the presence of an electric potential difference. Through this energetic interaction, plasma treatment primes the surface, creating an environment that enhances the acceptance and bonding of subsequent manufacturing processes. By increasing the surface energy and eliminating contaminants, plasma treatment ensures that adhere more effectively, resulting in superior quality and longer lifespan of the applied materials. *Roger L. Shannon* and *Roger B. Gillette* together filed a patent that claims a plasma cleaning device for cleaning contaminated surfaces present in a high vacuum [24]. Basic steps involved in plasma cleaning of a substrate have been presented in **Figure 3**.

#### **2.2 Plasma etching**

Extensive research has been conducted on plasma-assisted surface modification techniques, aimed at customizing various surface properties through the treatment of target substrates with gaseous plasma. The etching process facilitated by chemically reactive plasma involves a combination of physical and chemical mechanisms, which depend on the stability and volatility of the products formed through the interaction between the etching gas and the material [13]. Etching by chemically non-reactive plasma occurs by a physical deposition process such as magnetron sputtering [25, 26].

**Figure 3.** *Steps involved in typical plasma cleaning process.*

The plasma etching is carried using a gaseous discharge containing reactive species such as atoms and radicals that react with the surface to form volatile compounds that evaporate from the surface under proper reaction condition leaving an etched target substrate. It is also possible to create desired nanostructure on the substrate by varying the process parameters and underlying plasma chemistry of the reactive etchant gas [27, 28].

The reactive ion etching (RIE) is one of the most widely used dry etching techniques. The etching in this system is achieved by suitable combination of chemical reactivity of plasma species and the physical effects caused by ion bombardment [29]. A schematic depiction of a typical RIE system is given **Figure 4**.

It should be noted that in RIE systems both the chemical reaction on the surface of target substrate and the removal of the volatile products so formed on the etched surface are enhanced by ion bombardment of gaseous plasma. Basic steps involved in plasma cleaning of a substrate have been presented in **Figure 5**.

#### **2.3 Plasma polymerization**

Plasma polymerization is a process that involves the deposition of an organic polymer through plasma-enhanced chemical vapor deposition (PECVD). This process entails the decomposition of an organic monomer gas, followed by the deposition and polymerization of the excited species onto the substrate's surface. Films deposited through plasma polymerization can have thicknesses ranging from several

**Figure 4.** *Schematic depicting dry etching in a reactive ion etching system.*

**Figure 5.** *Steps involved in a typical plasma assisted etching.*

#### *Plasma Based Approaches to Achieve Self-Cleaning Surfaces DOI: http://dx.doi.org/10.5772/intechopen.112378*

tens to several thousands of angstroms. These deposited films, known as plasma polymers, exhibit chemical and physical properties that differ from conventional polymers. Starting materials for plasma polymerization can include hydrocarbons, fluorocarbons, and organic compounds containing nitrogen, oxygen, or silicon. Plasma polymers do not possess distinct repeating units like conventional polymers. Instead, their properties are primarily determined by the plasma parameters used during the deposition process. For example, it is not possible to identify a specific product as a plasma polymer of ethylene, as various products can be obtained from an ethylene plasma. Therefore, plasma polymers derived from ethylene are not equivalent to polyethylene. It's worth noting that the presence of functional groups, such as double bonds, is not a prerequisite for the monomer used in plasma polymerization. The process can occur even without such functional groups. Furthermore, the process of plasma polymerization yields a polymer coating through a rapid step growth reaction mechanism [13]. Plasma polymerization does not rely on a single species for its occurrence; instead, a diverse range of active species generated during the process actively participate in the polymerization reaction. The specific type and quantity of these activated species are contingent upon the reaction conditions employed during plasma polymerization [30]. Since both the kind of reactive species (monoradical and diradicals) are formed simultaneously, reaction between them is also very much likely to occur by cross cycle reactions, depending on the concentration of both the species.

Important factors that determines the deposition of films by plasma polymerization processes are type of monomer, carrier/reactive gas and it mass flow, pressure, temperature, input power, and reactor type [31]. Another important factor that ascertains the several important surface properties of plasma assisted modification is the frequency source used to generate plasma.

Srivatsa et al. developed a custom plasma polymerization deposition system (**Figure 6**) to enhance both surface wettability and hardness [24]. Subsequently, a four-layer antireflective coating (ARC) is applied in the same chamber on a polycarbonate (PC) substrate, consisting of alternating thin films of titanium dioxide and

*Typical plasma polymerization deposition system set up. (Reproduced from ref. [24].)*

silicon dioxide with varying thicknesses. This results in the creation of a robust and durable ARC on the PC substrate.

Jung *et al.* demonstrated effect of microwave and rf plasma on the photocatalytic properties and wetting behavior of TiO2 thin films deposited on glass substrate using dip-coating techniques [32]. Han *et al.* adopted the plasma polymerization processing method for obtaining a broadband transparent, superhydrophobic surfaces usingCH4, C4F8, and He gas mixture in an Rf plasma reactor at atmospheric pressure [33]. Ji et al. conducted a study where they presented the production of a superhydrophobic coating using a combination of atmospheric pressure middle frequency (Mf) and radio frequency (Rf) plasmas in an in-line process [34]. The coating was created by utilizing a mixture of Ar gas and HMDSO solution.

Barankin et al. developed a novel low-temperature atmospheric pressure plasma technique for polymerizing liquid fluoroalkylsilane (FAS) precursors, providing an effective method for producing hydrophobic coatings on glass and acrylic surfaces [35]. This innovative approach involves the atmospheric plasma curing of liquid FAS precursors, resulting in films with exceptional properties. The coatings exhibited a total surface energy of 11 dynes/cm, characterized by 10% polarity and average water contact angles ranging from 106° to 110°. To enhance adhesion between the FAS molecules and the surfaces, oxygen plasma activation was utilized, proving crucial in achieving strong bonding. The hydrophobic coatings generated through this method exhibit excellent transparency, stability, and long-lasting performance for over a year. This durability is attributed to the robust bonding between the fluoroalkyl ligands and the surfaces, preventing degradation and ensuring sustained hydrophobic characteristics.

#### **2.4 Atmospheric pressure plasma**

Various plasma sources (ion sources) which worked under vacuum conditions have long been used to tailor material properties [36]. In a major development in recent years their counter parts for processing under atmospheric conditions have started becoming available.

To enable affordable access to plasma techniques, researchers are now extensively exploring atmospheric pressure-based methods. There is a wide variety of atmospheric-pressure plasmas used in the processing of materials. Traditional sources of plasma, such as transferred arcs, plasma torches, corona discharges, and dielectric barrier discharges, are being employed. These sources generate plasmas with electron and neutral temperatures exceeding 5000°C, and the density of charged particles generally ranges from 1016 to 1019 cm−3 [37, 38]. They are primarily used in metallurgy applications due to their high-temperature nature. On the other hand, non-equilibrium plasmas created by corona discharges and dielectric barrier discharges have gas temperatures ranging from 50 to 400°C, and the density of charged particles falls within the range of 1010 to 1015 cm−3 [39, 40]. In order to create a dielectric barrier discharge (DBD) plasma, a high-frequency voltage is applied to a ceramic material, which leads to the generation of plasma within the ceramic's holes. Consequently, objects like an endless fabric tape can be passed over the ceramic, allowing for a significant surface area to be activated in a short amount of time. It is important to note that DBD plasma production results in the generation of ozone (O3) [41]. Therefore, it is crucial to utilize an extraction system when operating DBD for extended periods in enclosed spaces. Popular atmospheric plasma deposition has been presented in **Figure 7**.

*Plasma Based Approaches to Achieve Self-Cleaning Surfaces DOI: http://dx.doi.org/10.5772/intechopen.112378*

**Figure 7.**

*Various atmospheric plasma deposition processes.*

Among various atmospheric plasma processes, DBD plasma technique is highly advantageous due to its ability to generate high-energy electrons and facilitate rapid reactions. Moreover, it can be employed with various gases without causing electrode corrosion. As a result, this technique has found wide applications in diverse fields, including sterilization, material surface cleaning, semiconductor manufacturing, and surface treatment. Owing to its versatility and effectiveness, further research can explore its potential for optimizing deposition parameters and investigating the long-term stability and durability of plasma-treated surfaces in different environmental conditions. However, their non-uniform nature limits their use in materials processing.

In contrast, the atmospheric-pressure plasma jet shares similarities with a conventional low-pressure glow discharge [40, 42–44]. The plasma jet typically maintains a gas temperature between 25 and 200°C, with charged-particle densities at approximately 1011 cm−3. Additionally, there are high concentrations of reactive species, ranging from 10 to 100 ppm. An important advantage of this plasma source is that it can operate without requiring a vacuum chamber, which significantly broadens its range of potential applications. In summary, atmospheric-pressure plasma jets present a promising solution for a wide range of materials processing applications. They offer moderate gas temperatures, relatively high particle densities, and a rich presence of reactive species. The absence of a vacuum requirement enhances their versatility, allowing them to be effectively utilized in various material processing scenarios. Moreover, to address the constraint imposed by the reactor size, which limits the use of moderate-sized substrates for functionalization or modification, a recent technology called air plasma spray has emerged. This technology aims to enhance the adhesion, uniformity, mechanical hardness, and other surface properties of the target substrate.

In a recent study by Marchand et al., they showcased the preparation of long-lasting superhydrophobic films on various substrates under ambient conditions, eliminating the need for a vacuum chamber. They achieved this by utilizing a tetramethylsilane (TMS) precursor [45]. A superhydrophobic coating was successfully demonstrated by Jin et al. using an in-line process that incorporated atmospheric pressure middle frequency (Mf) and radio frequency (Rf) plasmas [34]. The process involved the utilization of a mixture of Ar gas and HMDSO solution. Han et al. introduced a singlestep processing technique for achieving broadband transparency (covering wavelengths from visible to near-infrared range) and super-hydrophobicity on surfaces. This was

accomplished using a mixture of helium (He), methane (CH4), and octafluorocyclobutane (C4F8), gases in a radio frequency (RF) plasma reactor operating at atmospheric pressure [33]. By employing atmospheric pressure, He/CH4 plasma, it is possible to achieve a chemically dominant hydrophobicity without the need for etching agents. The effects of plasma treatment on hydrophobicity and transparency were investigated and a large contact angle ~165° and low hysteresis of 4.9° was obtained.

Hossain et al. investigated tetramethylsilane (TMS) and 3-aminopropyl methyl silane (APDMES) as precursors to apply superhydrophobic coatings onto glass substrates. The deposition process involved a plasma jet that relied on dielectric barrier discharge [46]. The resulting films demonstrated exceptional water repellency properties, with a water contact angle of 163° and a sliding angle of only 5°. Additionally, the films exhibited a surface roughness of 50 nm, which further enhanced their superhydrophobic characteristics and self-cleaning abilities. The deposition process utilizing the dielectric barrier discharge plasma jet proved to be a straightforward and efficient technique for creating superhydrophobic coatings on various substrates.

In a separate study, Lin et al. utilized a CO/N2 DBD plasma along with acrylic acid as a precursor to alter the surface of a PTFE film [47]. This process led to creation of hydrophilic functional groups such as –COOH, C–O, C–C, and others on the PTFE surface. Through two reaction treatments, the water contact angle decreased to 140°, and after twenty reactions, it reached less than 5°, indicating a significant improvement in surface wettability.

Fakhouri et al. introduces a novel approach for depositing TiO2 coatings using atmospheric pressure plasma jet (APPJ) coupled with a liquid precursor spray (droplet size∼100 μm) [48]. The developed technique enables high deposition rates, and the resulting coatings exhibit a highly porous structure with pore sizes ranging from 100 to 1000 nm, leading to a large specific surface area. The coatings deposited by APPJ demonstrate several notable advantages over the RF-sputtered coatings. Firstly, they exhibit a significantly higher specific surface area, which is beneficial for enhanced photocatalytic activity. The increased surface area provides more active sites for reactions, leading to improved catalytic performance. Moreover, the porous nature of the APPJ coatings further contributes to their enhanced specific surface area and catalytic activity. In comparison to RF sputtering, the APPJ technique offers superior deposition properties, including higher deposition rates and improved coating characteristics. These findings highlight the effectiveness of the APPJ technique in depositing TiO2 coatings with high specific surface areas and improved photocatalytic activity compared to RF-sputtered coatings.

#### **3. Plasma assisted self-cleaning surface**

Plasma coatings represent an exciting field within plasma technology, with significant potential for enhancing the functionality and value of materials across a wide range of applications. These coatings provide two primary surface properties: complete repellency to liquids (such as water and oil) or complete wetting ability. The process involves the formation of a nanoscale polymer layer across the entire surface of an object when it is exposed to plasma. This coating process is rapid, typically taking only a few minutes to complete. It is a very thin (few nanometers to microns) permanent coating, firmly bound to the material surface on an atomic scale.

Plasma coating technology generates surfaces with exceptional hydrophobic properties, exhibiting a "lotus effect" where water is repelled. These surfaces also possess

*Plasma Based Approaches to Achieve Self-Cleaning Surfaces DOI: http://dx.doi.org/10.5772/intechopen.112378*

dirt-repellent characteristics, enabling self-cleaning without the need for manual cleaning methods. Promising applications for plasma coatings include the coating of automotive components like aluminum wheel rims, as well as window frontages and glass panes [49].

Song *et al.* reported a one-step nanostructure fabrication process using plasmaassisted reactive ion etching (RIE) processing [50]. Mazumder *et al.* reported a similar hierarchical nanostructured transparent antireflection glass material with superomniphobic properties. The two-tier roughness was found to consist of primary nanopillars (~100–200 nm) superimposed with nanoparticles (~10–30 nm). The nanopillars were fabricated by metal dewetting followed by RIE and the nanoparticles are deposited using combustion chemical vapor deposition (CCVD). They have been able to achieve contact angles ~170° and 160° for water and oil, respectively and an average transmission of 93.8% in the wavelength range 400 to 700 nm using CF4 and O2 gas mixture, exhibits very high optical transmission and antifogging effects [51].

#### **3.1 Versatility of low temperature plasma**

In recent times, the advancement of low temperature plasma technology has emerged as a flexible and efficient approach for manipulating the surface structure of materials [52]. One specific technique, known as low temperature plasma-enhanced chemical vapor deposition (PECVD), offers several notable advantages. These include direct surface pretreatment, modification at low temperatures, low organic content, and the absence of post-processing requirements [53, 54]. PECVD utilizes the energy and reactivity of electrons, ions, free atoms, and free radicals present in the plasma to induce physical and chemical transformations on the material surface. Consequently, this process enables the formation of new functional groups with desirable properties or the creation of film layers with unique structures, making it a crucial method in the treatment of superwetting surfaces.

The significance of plasma-based process to achieve surface superwettability is evident from the surge in research publications. In the year 2000, the number of papers published on this topic was less than 3500. However, by 2010, this number had increased to nearly 9000, highlighting the expanding interest in the field. As of the current year, approximately 20,000 papers have been published in the area of plasma control of surface superwettability, demonstrating its continued relevance and widespread research focus [55].

Two widely used approaches for surface modification are atmospheric pressure plasma treatment and low-pressure plasma treatment [56, 57]. Atmospheric pressure plasma treatment includes techniques like dielectric barrier discharge plasma and atmospheric pressure plasma jet treatment. On the other hand, low-pressure plasma treatment involves methods such as radiofrequency (Rf) discharge plasma treatment and glow discharge plasma treatment. These methods enable the application of plasma to the surface, leading to the creation of nano-scale roughness [58]. Alternatively, by introducing precursors that are chemically linked to diverse functional groups, the desired surface wettability can be achieved [59].

Zhang et al. explored the potential of an atmospheric pressure air plasma jet, generated using a dielectric barrier structure with hollow electrodes (HEDBS), as a means to spray TiO2 films inside tubular substrates [60]. The results obtained from the self-cleaning test indicate that the proposed HEDBS approach, utilizing air as the working gas, offers a practical and efficient method for synthesizing thin TiO2 nanofilms, while simultaneously reducing costs and saving time. This approach holds significant promise for various applications, particularly for coating tubular substrates with TiO2 films.

Chemin et al. presented innovative method for synthesizing anatase TiO2/SiO2 nanocomposite coatings at atmospheric pressure, offering transparent anti-fogging and self-cleaning properties on polymer substrates [61]. The concomitant injection of titania and silica precursors in a blown-arc discharge allows for the efficient production of durable coatings with excellent performance characteristics. The findings of this study pave the way for the development of advanced coatings applicable in various industries where transparency, anti-fogging, and self-cleaning properties are highly desirable. The use of anatase TiO2 nanoparticles in conjunction with SiO2 in the nanocomposite coatings contributes to their enhanced durability and desirable properties. The combination of these materials not only imparts super-hydrophilicity, which facilitates water spreading and evaporation, but also harnesses the photocatalytic nature of TiO2, enabling the degradation of organic pollutants and further promoting self-cleaning functionality.

This plasma-based surface pre-treatment using air plasma gun results in clean and highly activated surfaces that can be easily wetted by water, eliminating the need for volatile organic compounds (VOCs) and the associated processes of drying and wastewater disposal. This technology finds application in various areas such as enhancing structural adhesive bonds in the automotive industry, creating seals for electronics applications, or achieving quick and bubble-free wet labelling with strong initial adhesion in the packaging industry. Moreover, the use of plasma gun has been reported for depositing thin film coatings like organosilicate and ceria nanocomposites, which provide improved hardness, adhesion, and UV-absorbing properties [62, 63].

Deicing and self-cleaning processes have gained significant global attention due to their crucial importance in various industries. One major challenge faced by these industries is the formation of atmospheric ice caused by super-cooled dewdrops in icy rain. This phenomenon poses a significant threat to aircraft, wind turbines, electric and telecommunication wires, highways, bridges, and other outdoor equipment and systems. Plasma-treated superhydrophobic (SH) coatings have emerged as a highly relevant solution for both deicing and self-cleaning applications in aircraft and other systems. Subeshan et al. the use of plasma-treated superhydrophobic coatings provides an effective solution for both deicing and self-cleaning requirements in aviation industries [64]. This study contributes valuable insights into the characteristics and performance of plasma-treated SH coatings on the AA 2024-T3 alloy, showcasing their ability to repel water, prevent ice formation, and maintain a clean surface under diverse atmospheric conditions.

The utilization of an air plasma gun for plasma-based surface pre-treatment leads to the formation of clean and highly activated surfaces that readily wet with water. This eliminates the need for volatile organic compounds (VOCs), as well as the associated processes of drying and wastewater disposal. The technology has diverse applications, including enhancing adhesive bonds in the automotive industry, creating seals for electronics, and achieving efficient and bubble-free wet labelling with strong initial adhesion in the packaging industry. Additionally, the plasma gun has been proven effective for depositing thin film coatings like organosilicate and ceria nanocomposites, which exhibit improved hardness, adhesion, and UV-absorbing properties [54, 55]. Hovish et al. presented a technique for growing a superhydrophilic organotitanate thin film with excellent thermomechanical properties using atmospheric plasma deposition. This thin film demonstrated an effective antifogging property, with a contact angle (θ) less than 5° [65]. Asadollahi et al. described the

development of an anti-icing coating on Al-6061 sheets. This involved pre-treating the surface and depositing organo-silane (HMDSO) using an atmospheric pressure plasma jet (APPJ) [66].

Additionally, Plasma jet printing (PJP) is a highly versatile and scalable printing method that utilizes plasma to interact with aerosolized inks, offering precise control over material characteristics. PJP represents an innovative approach for high-quality material printing, as it is environmentally friendly and can utilize lower quality inks. The process involves transforming the material into aerosol form, introducing it into the print head, and generating plasma through an electromagnetic field. The reactive species within the plasma interact with the ink, allowing for dynamic in situ tailoring of electronic properties. PJP's gas-phase operation eliminates liquid waste and toxic by-products, while enabling the use of unprocessed, low-grade inks to produce highquality prints [51].

#### **3.2 Integration of sol–gel and gaseous plasma-based approaches**

The combination of sol–gel and plasma-based techniques has led to the development of coatings with unique optical and surface properties. Jung et al. conducted a study that specifically explored surface modification using plasma treatment on a sol–gel derived TiO2 film, which served as a photocatalyst [32]. The study investigated the effect of gaseous plasma treatment (Ar/O2) on the enhancement of wetting behavior, specifically the attainment of a superhydrophilic property. In one of our own research work, we investigated sol–gel derived V-TiO2:SiO2 coatings and applied subsequent O2 plasma treatment [59]. Different weight percentages of V-doped TiO2 in the SiO2 sol were tested to achieve the best combination of optical transparency and self-cleaning properties. The resulting coatings demonstrated antireflective properties, superhydrophilicity, and visible photo-catalytic activity. O2 plasma treatment significantly reduced the contact angle of all V-TiO2:SiO2 formulations, reaching 0°. In another study, we combined sol–gel self-assembly with a plasma-based approach to create highly transparent, self-ordered, superhydrophilic, and photoactive TiO2 thin film coatings [60]. The TiO2 sol contained a block copolymer that facilitated the formation of regular pores in the film, resulting in reduced refractive index values (~1.31) and improved transparency (4% antireflection gain). The mesoporous TiO2 coatings exhibited excellent photocatalytic activity for VOCs. Nitrogen plasma treatment enhanced mechanical stability and hydrophilicity without affecting optical transmission. Under optimized conditions, the coatings achieved superhydrophilicity with a water contact angle of less than 5°. Optimal conditions were determined by varying the Rf self-bias potential and duration of plasma treatment.

Noppakun Sampo conducted research on the utilization of sol–gel methods in the development of a multifunctional magnetic iron-based solution for thermal spray techniques [61]. The study focuses on preparing liquid feedstocks for the solution precursor plasma spray (SPPS) process. By exploring sol–gel methods in thermal spray applications, valuable insights were gained into the development and characterization of the magnetic iron-based solution. The research specifically investigates the effects of organic chelating agents on the topography, physical properties, and phase composition of cobalt ferrite splats. These findings contribute to a deeper understanding of the mechanisms involved and have significant potential for practical applications in various fields. Similarly by employing the SPPS technique, Cai et al. successfully developed ceramic superhydrophobic coatings with desirable properties [67]. The deposition process has been shown in **Figure 8**. The chosen rare earth oxide as the

#### **Figure 8.**

*(a) Schematic of the SPPS deposition system. (b) Water droplets of varying sizes on the coated surface (with permission, Copyright © 2016 Yuxuan Cai et al.).*

coating material, along with the optimized spraying conditions, resulted in coatings with excellent hydrophobicity, mimicking the natural superhydrophobic surfaces. The outcomes of this study have significant implications for various applications where enhanced water repellency and self-cleaning properties are desirable, such as anti-icing coatings, corrosion protection, and efficient fluid flow management.

The sol–gel process is a commonly used method for synthesizing powders, coatings, and bulk materials. However, as a wet chemical technique, it has certain limitations, especially when applied as a coating method using aqueous colloidal solutions. The commonly utilized techniques such as dip and spin coating face challenges when applied to complex substrates. To overcome these limitations, the aerosol gel deposition method provides a promising solution. Pietrzyk et al. reported a novel technique to deposit Al2O3 coating by plasma enhanced aerosol-gel method [68]. By combining the aerosol-gel deposition of thin films with low-temperature plasma treatment, an innovative approach is established, introducing a novel method known as plasma-enhanced aerosol-gel for coatings production. This combined approach brings forth numerous advantages. Firstly, it enables the application of coatings on more complex substrates, which are difficult to coat using traditional techniques. Secondly, the aerosol-gel deposition method provides a homogeneous and controlled deposition of thin films. Lastly, the subsequent low-temperature plasma treatment enhances the properties of the deposited coatings, such as adhesion, durability, and functionality. This integration of aerosol-gel deposition and low-temperature plasma treatment represents a significant advancement in the field of coatings production, providing a versatile and efficient approach for the synthesis of high-quality coatings with improved properties.

#### **4. Conclusion**

Gaseous plasma, also known as cold plasma, exhibits an ion/electron density of around 1000 cm−3 and an electron temperature of 1 eV. Different types of plasma

reactors, such as DC, Rf, VHf, and MW, with varying excitation frequencies, play a crucial role in film processing. Pulsing the discharge subtly affects the film's characteristics.

Cold plasma finds applications in surface cleaning, etching, deposition, and grafting during film growth. Plasma polymerization overcomes limitations imposed by plasma chemistry, as plasma polymers' properties depend on plasma parameters rather than the monomer used. Surface superwettability control using gaseous plasma treatment (e.g., Ar/O2) has been extensively explored, resulting in improved superhydrophilic properties.

Recent advancements allow the generation of cold plasma at atmospheric pressure using techniques like Corona, DBD, Gliding Arc discharge, and Plasma jet, overcoming the need for vacuum-based reactors. The atmospheric pressure plasma jet (APPJ) technique proves effective in depositing TiO2 coatings with high specific surface areas and improved photocatalytic activity compared to vacuum-based plasma reactors. These approaches enable surface bombardment with plasma, creating nano-scale roughness. By introducing functionalized precursors, desired surface wettability can be achieved. Open-air plasma deposition is used for various thin film coatings, offering enhanced hardness, adhesion, and UV-absorbing properties. Plasma-treated superhydrophobic (SH) coatings are highly relevant for de-icing and self-cleaning applications in aircraft and other systems. An innovative method demonstrates the synthesis of anatase TiO2/SiO2 nanocomposite coatings at atmospheric pressure, providing transparent anti-fogging and self-cleaning properties on polymer substrates. Additionally, an atmospheric pressure air plasma jet generated using a dielectric barrier structure with hollow electrodes (HEDBS) shows promise for spraying TiO2 films inside tubular substrates.

To overcome challenges with complex substrates, traditional techniques like dip and spin coating are supplemented by the aerosol gel deposition method. This approach integrates aerosol-gel deposition with low-temperature plasma treatment, enabling the novel plasma-enhanced aerosol-gel method for coatings production. Moreover, plasma jet printing offers a versatile and scalable printing method that leverages plasma's interaction with aerosolized inks, allowing precise control and customization of material characteristics.

These advancements in plasma technology offer exciting possibilities for selfcleaning applications. Plasma treatments can modify surface properties to achieve superhydrophilic or superhydrophobic behavior. The integration of plasma techniques with innovative deposition methods paves the way for improved coating performance and functionality on various substrates.

#### **Author details**

Deepanjana Adak1 \* and Raghunath Bhattacharyya<sup>2</sup>

1 Surface Engineering Division, CSIR-National Aerospace Laboratories, Bangalore, India

2 School of Advanced Materials, Green Energy and Sensor Systems, Indian Institute of Engineering Science and Technology, Shibpur, India

\*Address all correspondence to<EMAIL_ADDRESS>
© 2023 The Author(s). Licensee IntechOpen. This chapter is 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.

*Plasma Based Approaches to Achieve Self-Cleaning Surfaces DOI: http://dx.doi.org/10.5772/intechopen.112378*

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## **Chapter 6** Nanomachining

*K.S. Srin, J. Ramkumar and Ravi N. Bathe*

#### **Abstract**

LIPSS (LASER Induced Periodic Surface Structures) is a term used to describe ordered or periodically structured nanostructures. Until newly, this term was not extensively researched or optimized for use in superhydrophobic self-cleaning applications. This machining method is one of the most sophisticated and cutting-edge ways to create nanofeatures like grooves and bumps without using any chemicals. Here we are trying to explore with polarization, machining speed, number of pulses, LASER fluence, shielding gas, and other parameters dependence on surface nanostructures and features that varied in size and orientation when milled by a LASER. Mainly, the aspect ratio of nano features are varied with axial spacing and along the horizontal diameter of the LASER beam. When polarization employed on nanomachining, the increased number of threshold LASER pulses also causes the structures to become LASER Induced Irregular Surface Structures, and the orientation and size of these features (between 200 and 400 nm) also affected by external interacts of material surface. The existing knowledge describes the nano feature generation is by the interaction of LASER beam and surface wavers. In light of this, one of the newest topics to emerge for the development of nanofeatures using femtosecond LASER.

**Keywords:** LIPSS, LASER materials processing, self-cleaning surfaces, femtosecond LASER, surface plasmon polaritons, polarization

#### **1. Introduction**

The concepts of LIPSS and laser-induced irregular surface structures need to be thoroughly investigated in order to justify their application at all levels of the pertinent femtosecond laser field. During the femtosecond laser machining, the surface molecules of the material are producing various sorts of self-assembled features ranging on few microns (200 nm - 400 nm) either in LIPSS or laser-induced irregular surface structures. The laser fluence's transition from near to far ablation threshold typically changes the morphology from a periodic configuration to a complicated configuration. The increase in laser fluence deepens the grooves and intensifies the roughness of its produced feature.

When a linearly polarized femtosecond laser beam strikes a solid under ambient conditions, parallel and perpendicular LIPSS with a spatial frequency close to the laser's wavelength are produced.