Open access peer-reviewed chapter
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 self-cleaning 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 real-world 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.
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, 6, 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.
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.
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 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.
Figure 3.
Various types of metal oxides for self-cleaning coating.
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 self-cleaning 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.
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.
Figure 4.
Various polymer nanocomposites for self-cleaning coating.
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].
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 surface-to-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. 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.
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.
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.
Figure 6.
A simple overview of current challenges and future projections for enhancing properties of MO/nanocomposites.
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, 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 self-cleaning 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.
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