Self-cleaning Coatings Research Articles

Photocatalytic self-cleaning coatings to remove oleic acid, an organic pollutant, from cotton fabrics

Photocatalytic self-cleaning coatings to remove oleic acid, an organic pollutant, from cotton fabrics

Synthesis of the hydrophilic additive by grafting glycidyloxypropyl trimethoxysilane on hydrophilic nanosilica and its modification by using dimethyl propionic acid for self-cleaning coatings

In this study, hydrophilic additives were synthesised in a two-step reaction; the first step was the grafting of GPTMS on hydrophilic fumed silica. The epoxy ring-opening reaction was done in the second step by treating it with a secondary acid (Dimethyl propionic acid). These additives were added to the water-based acrylic system to check the hydrophilic character. The additive was characterised using Fourier-transform infrared spectroscopy (FTIR), solid-state silica- nuclear magnetic resonance (NMR), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) with selected area electron diffraction (SAED), Thermo Gravimetric Analysis (TGA), Particle Size and Zeta Potential Analysed by Dynamic Light Scattering (DLS). The additive incorporated coating was characterised with scanning electron microscopy (SEM), contact angle, and stain resistance test to evaluate additive dispersion, hydrophilic and self-cleaning property. The 0.5 phr additive incorporated coating shows the superior hydrophilic and self-cleaning property.

Antireflective Self-Cleaning TiO2 Coatings for Solar Energy Harvesting Applications

Titanium(IV) oxide (TiO2, titania) is well-known for its excellent photocatalytic properties, wide bandgap, chemical resistance, and photostability. Nanostructured TiO2 is extensively utilized in various electronic and energy-related applications such as resistive switching memory devices, flat panel displays, photodiodes, solar water-splitting, photocatalysis, and solar cells. This article presents recent advances in the design and nanostructuring of TiO2-containing antireflective self-cleaning coatings for solar cells. In particular, the energy harvesting efficiency of a solar cell is greatly diminished by the surface reflections and deposition of environmental contaminants over time. Nanostructured TiO2 coatings not only minimize reflection through the graded transition of the refractive index but simultaneously improve the device’s ability to self-clean and photocatalytically degrade the pollutants. Thus, novel approaches to achieve higher solar cell efficiency and stability with pristine TiO2 and TiO2-containing nanocomposite coatings are highlighted herein. The results are compared and discussed to emphasize the key research and development shortfalls and a commercialization perspective is considered to guide future research.

Tunable Adhesive Self-Cleaning Coating with Superhydrophobicity and Photocatalytic Activity.

Superhydrophobic coatings with intelligent properties have attracted much attention because of their wide application in many fields. However, there is a limited amount of literature on superhydrophobic coatings whose wettability and adhesion can be adjusted by UV irradiation and calcination at the same time. In this study, amorphous SiO2 microspheres (A-SiO2) and nano-TiO2 particles (N-TiO2) were used to fabricate A-SiO2/N-TiO2 composites by wet grinding, and then, they were modified with polydimethylsiloxane (PDMS) and sprayed onto substrate surfaces to obtain a tunable adhesive superhydrophobic A-SiO2/N-TiO2@PDMS coating. It is worth noting that the wettability and adhesion of the coating to water droplets could be adjusted by UV irradiation and calcination. The mechanisms of the aforementioned phenomena were studied. Moreover, methyl orange solution could be degraded by the coating due to its photocatalysis. The as-prepared coating had good adaptation to different substrates and outdoor environments. Moreover, the surfaces of these coatings exhibited the same liquid repellency towards different droplets. This research provides an environmental strategy to prepare advanced self-cleaning coatings.

Cover Image, Volume 138, Issue 34

Surface engineered self-cleaning property has captured wide attention in scientific, as well as industrial arenas recently. Better protection from environmental pollutants and dirt that might adversely affect its performance is a key requirement for coatings that offer self-cleaning. The cover designed by Unnikrishnan N V and colleagues shows the development of self-cleaning microsphere coatings with a prolonged lifetime using an sol-gel assisted electro-spraying approach. In addition, high quality selfcleaning surfaces are generated by altering the type of polymers used. And modulating surfaces by addition of tri-block co-polymer is found to provide swapping of superhydrophobic nature to superhydrophilic nature. Better antibacterial and photocatalytic properties are shown by the samples. DOI: 10.1002/app.50880

Role of Hafnium Doping on Wetting Transition Tuning the Wettability Properties of ZnO and Doped Thin Films: Self-Cleaning Coating for Solar Application.

Herein, we successfully synthesized high-quality Hf-ZnO thin films with various Hf contents (0, 3, 6, 9, 12, and 15 at. %), which showed both superhydrophilic (6% Hf-ZnO) and ultrahydrophobic (15% Hf-ZnO) wetting behavior. Different characterization methods were opted to recognize the structural (XRD, SEM, AFM) and defect properties (XPS) of the pristine and doped materials, to understand the mechanisms underlying the tuning of wetting behavior (contact angle). Hafnium doping plays a noteworthy role in tuning the morphology of the ZnO nanostructures, roughness of the material surface, generation of defects, Lewis acid-base interactions, and wettability properties. We achieved a superhydrophilic surface with 6% Hf-ZnO owing to a smooth surface, less basicity, and maximum concentration of oxygen vacancies, and also an ultrahydrophobic surface with 15% Hf-ZnO because of the rough surface, high basicity, and minimum concentration of oxygen vacancies. The as prepared Hf-ZnO samples showed stable performance (stability, wearability, weatherability, and antifouling) under real-life conditions marking them multifunctional and biosafe material to be effectively used in solar and building's window. A wetting mechanism was established to relate the wetting behavior of the samples to oxygen vacancies (active sites for water dissociation: resulted due to charge mismatch of host cation (Zn2+) by the doped cation (Hf4+)), roughness (smooth surface (Wenzel) with minimum Rrms (0.588) portraying hydrophilic property and rough caltropic surface (Cassie-Baxter) with maximum Rrms (2.522) portraying hydrophobic property), basicity (H2O: Lewis Base; ZnO: Lewis acid; HfO2: Lewis base) and morphology (tube-like structure (0-6% Hf-ZnO) and caltrop-like structure (12-15% Hf-ZnO)).

Ag-TiO2/PDMS nanocomposite protective coatings: Synthesis, characterization, and use as a self-cleaning and antimicrobial agent

The protection of our artifacts becomes a necessity due to several decay phenomena induced by pollution, microorganism colonization, and water. This research study has been focused on elaborating photo-activated nanocomposites to protect outdoors heritage buildings. Hence, novel Ag-TiO2/PDMS hydrophobic nanocomposites (with low doping rates) thin films with enhanced photo-response activity have been developed to be used as self-cleaning durable protective coatings in the field of monument conservation. The protective coatings elaborated in this work are intended to have multifunctional properties (water repellent features, self-cleaning activity, antimicrobial agent). To reach our objective, silver-doped titanium dioxide nanoparticles (Ag-TiO2 NPs) at different doping amounts (0.1, 1, 3, and 5 mol%) were firstly synthesized by the sol-gel method. As-prepared NPs were characterized and their performance was evaluated comparatively to find out the appropriate low doping rates of the dopant by testing photo-degradation as well as photo-killing activities. The second part of this research is focused on elaborating Ag-TiO2/PDMS hydrophobic nanocomposites (selected low doping rates) as self-cleaning protective coatings. For this purpose, Ag-doped TiO2 NPs were dispersed in a polymeric binder (polydimethylsiloxane, PDMS) and applied to the very porous stone substrate, i.e., Lecce stone (LS). The suitability of the resulting nanocomposites (optimal Ag doped TiO2 NPs/ binder ratio; optimal doping amount) on the stone substrate was evaluated by performing different experimental analyses: chromatic measurements, contact angle measurements, water capillary absorption test, permeability to water vapour, pencil and Vickers hardness tests, optical microscope analyses, and scanning electron microscopic analyses combined with energy-dispersive X-ray spectroscopy (SEM-EDS), microbiological experiments, and self-cleaning test. Moreover, the durability of the nanocomposite coatings was also assessed after exposing to different ageing cycles. Results suggested that the samples doped with low concentrations (≤1 mol%) of silver (Ag-doped TiO2) together with PDMS can be considered as efficient and durable protective coatings for the highly porous bio-calcarenite stone. Such doping amount is considered very low compared to the other research works that have been performed in the same field.

Enhancing the Oil-Fouling Resistance of Polymeric Membrane Ion-Selective Electrodes by Surface Modification of a Zwitterionic Polymer-Based Oleophobic Self-Cleaning Coating

Due to the frequent oil spill accidents and pollution of industrial oily wastewater, oil fouling has become a great challenge to polymeric membrane ion-selective electrodes (ISEs) for applications in oil-contaminated areas. Herein, a simple approach is proposed to enhance the oil-fouling resistance of polymeric membrane ISEs by surface modification of a zwitterionic polymer-based underwater oleophobic coating. As a proof-of-concept, a classical poly(vinyl chloride) membrane-based calcium ion-selective electrode (Ca2+-ISE) is chosen as a model sensor. The zwitterionic polymer-based coating can be readily modified on the sensor's surface by immersion of the electrode into a mixture solution of dopamine and a zwitterionic acrylate monomer (i.e., sulfobetaine methacrylate, SBMA). The formed poly(SBMA) (PSBMA) coating alters the oleophilic membrane surface to an oleophobic one, which endows the surface with excellent self-cleaning properties without loss of the sensor's analytical performance. Compared to the pristine Ca2+-ISE, the PSBMA-modified Ca2+-ISE exhibits an improved analytical stability when exposed to oil-containing wastewater. The proposed approach can be explored to enhance the oil-fouling resistance of other polymeric membrane-based electrochemical sensors for use in the oil-polluted environment.

Self-Cleaning Coatings and Surfaces of Modern Building Materials for the Removal of Some Air Pollutants.

Air quality is one of the most important problems of the modern world, as it determines human health and changes occurring in other elements of nature, including climate change. For this reason, actions are taken to reduce the amount of harmful substances in the air. One such action is the use of building materials with special properties achieved by the application of self-cleaning coatings and photocatalytic additives. This article presents achievements in the field of additives and modifiers for building materials, whose task is to improve air quality. Concrete, cement, paints, and facade coatings modified based on the achievements of nanotechnology have been analyzed in terms of new properties and the possibility of their application in the area of modern environmental requirements. Both positive aspects and doubts were described in the scope of the effective reduction of the amount of gases such as VOC, NOx, dust and microorganisms.

Oxide-water interaction and wetting property of ceria surfaces tuned by high-temperature thermal aging

The high transmittance, environmentally robust, and unique interaction with water make lanthanide series rare-earth oxides promising for various applications such as next-generation self-cleaning coatings. However, the underlying mechanism of the anomalous hydrophobicity in lanthanide oxides remains controversial due to the lack of molecular-level studies in a well-controlled environment, which greatly limits the possibility to engineer their wettability for broader technical applications. Here, we systematically studied the mechanism that governs the wetting behaviors of ceria films by in-situ X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and water contact angle measurements under the environmentally controlled conditions. It was found that the hydrophobicity of ceria was mainly contributed by the adsorbed hydrocarbon, and the wettability could be enhanced on cleaner ceria surfaces with more degree of hydroxylation. Based on the significant dependence of wetting property on surface contamination, we further developed an in-situ annealing method to significantly improve the water-splitting performance of ceria powders by removing hydrocarbon contamination. These results suggest that the removal of surface contamination by high-grade oxidation is the key factor to facilitate the water dissociation of ceria films, providing new insights into the control of their wetting property and design of high-performance devices based on ceria films.

TiO2 and TiO2–Ag powders and thin layer toward self-cleaning coatings for PV panel integrated with sound-absorbing screens: Technical approaches

This work aims at assessment of TiO2 as the main layer component for self-cleaning layers in photovoltaic panels. TiO2 (derived from titanium (IV) butoxide or titanium (IV) isopropoxide) without and with silver was examined to find titania suitable microstructure and optical properties. For this purpose silver amounts ranging from 0.1 to 1% were used for 3 separate chemical methods of modification. Microstructure of powders was characterized by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with X-ray energy dispersion spectroscopy. Three techniques: spin-coating, doctor blade, and spray-coating were used to deposit TiO2 and TiO2–Ag layers on glass and silicon solar cells. The photocatalytic activity of TiO2 and TiO2–Ag were investigated in the presence of methylene blue. Concentration of dye, amount of silver, type of TiO2 with Ag modification and stability over time were analysed towards the best photocatalytic properties. Finally, TiO2 layers which were used to coat a new type of photovoltaic modules had marginal influence on photovoltaic parameters.

Facile fabrication of crystallized superhydrophobic hybrid coatings via solid-state hydrolysis/polycondensation of n-octadecyltrimethoxysilane

Superhydrophobic surfaces that roust to harsh environments and fully repel liquids are remarkable for practical applications of self-cleaning coatings. Herein, crystallized superhydrophobic polysilsesquioxane hybrid coatings have been accessed via a single-step solid-state hydrolysis/polycondensation of bulk n-octadecyl- trimethoxysilane (C18TMS) under hydrogen chloride/water vapor atmosphere. The resulted coatings showed coral-like surface morphology that composed by bilayered C18H37SiOx–OxSiC18H37 crystalline. It exhibited high water repellency (θA = 152° ± 1) and low hysteresis (△θ = 2° ± 1.1), providing remarkable self-cleaning properties. More interestingly, the hybrid coatings retained their superhydrophobic properties even exposure to highly corrosive media of 1 M HCl or NaOH solution for 12 h, respectively.

Polyaniline/graphite carbon nitride composite coatings with outstanding photo-induced anodic antifouling and antibacterial properties under visible light

Self-cleaning antifouling coatings are needed for many applications, motivating the search for low cost coatings with good antifouling and antibacterial properties and minimal impact to the environment. Herein, polyaniline-graphitic carbon nitride (PANI/g-C3N4) composites prepared by a simple physical blending method, then added to an acrylic resin to obtain antifouling coating formulations. Under visible light irradiation (λ > 420 nm), the PANI/g-C3N4 coatings offered excellent photo-induced anodic antifouling/antibacterial properties, with the optimum weight ratio of PANI:g-C3N4 in the coatings determined to be 1:9. Antibacterial experiments demonstrated that the PANI/g-C3N4 coatings offered excellent antibacterial activity towards E. coli (96.5 % inhibition efficiency) and S. aureus (95.3 % inhibition efficiency) under visible light. Based on the antifouling experiments and detailed chemical/electrochemical characterization studies, a plausible mechanism for the photo-induced anodic antifouling/antibacterial activity of the composite coatings was proposed.

Hydrophilic Antireflection and Antidust Silica Coatings.

We report on the optical and morphological properties of silica thin layers deposited by reactive RF magnetron sputtering of a SiO2 target under different oxygen to total flow ratios [r(O2) = O2/Ar, ranging from 0 to 25%]. The refractive index (n), extinction coefficient, total transmission, and total reflectance were systematically investigated, while field-emission scanning electron microscopy, atomic force microscopy, and three-dimensional (3D) average roughness data construction measurements were carried out to probe the surface morphology. Contact angle measurements were performed to assess the hydrophilicity of our coatings as a function of the oxygen content. We performed a thorough numerical analysis using 1D-solar cell capacitance simulator (SCAPS-1D) based on the measured experimental optical properties to simulate the photovoltaic (PV) device performance, where a clear improvement in the photoconversion efficiency from 25 to 26.5% was clearly observed with respect to r(O2). Finally, a computational analysis using OptiLayer confirmed a minimum total reflectance of less than 0.4% by coupling a silica layer with n = 1.415 with another high-refractive-index (i.e., >2) oxide layer. These promising results pave the way for optimization of silica thin films as efficient antireflection and self-cleaning coatings to display better PV performance in a variety of locations including a desert environment.

Soiling Effect Mitigation Obtained by Applying Transparent Thin-Films on Solar Panels: Comparison of Different Types of Coatings.

Dust accumulation on the front cover of solar panels is closely linked to location and orientation of photovoltaic (PV) installation. Its build-up depends on the module tilt angle, frequency of precipitation, humidity, wind strength and velocity, as well as grain size. Additionally, soil composition is determined by solar farm surroundings such as local factories, agricultural crops, and traffic. Over time, molecules of atmospheric dust agglomerate on top of each other and cause gradual reduction in generated energy. Manual cleaning techniques are required to restore working conditions of PV installation to their original conditions; however, they are time consuming and may lead to damage of the glass coverage. Therefore, implementing a different approach by utilizing self-cleaning and anti-dust coatings on front covers of module surfaces is thought of as a competitive manner of cleansing. Based on the varying properties of such thin-films, a division was made into hydrophobic, hydrophilic, and anti-dust coatings. In this article, the authors would like to present a comprehensive review of those types of transparent films. Moreover, a few hydrophobic coatings available on the Polish market were analyzed by applying them on glass tiles and covering them with three types of dust.

Facile synthesis of fluorine-free, hydrophobic, and highly transparent coatings for self-cleaning applications

Non-fluorinated coatings have recently emerged as a new approach to avoid contamination on transparent surfaces. This research work concerns the synthesis of highly transparent, hydrophobic, durable, and self-cleaning coatings as an alternative to fluorine-based coatings used on glass. The developed coating is produced by a chemical route (sol–gel method) using two silica-based precursors, hexamethyldisilazane, and tetraethoxysilane (HMDS/TEOS). Then, the prepared Gel HMDS/TEOS was deposited on glass slides by the dip-coating technique. Afterward, the effects of mechanical durability, water-drop testing, thermal cycling testing, and aging times on the hydrophobicity of the gel were analyzed. The properties of the coating were characterized by optical microscopy, scanning electron microscopy, UV–VIS-NIR spectrophotometry, and contact angle measurement. The results show that the organic modification of the gel by trimethylsilyl (–Si–(CH3)3) groups leads to a homogeneous and a hydrophobic surface. Sufficient dilution of this gel with ethanol has been found to provide a highly transparent coating without losing its hydrophobic property. In addition, the robust and durable coating exhibits high self-cleaning performance suitable for various industrial applications.

Recent study on TiO2 based Self-Cleaning Coating

Coating is a way of protecting the surface of the materials as glass, metal, clay, wood, and cementitious from pollutant, dust. self-cleaning coating can be achieved by modifying the surface to have super-hydrophobic, super-hydrophilic, and photocatalytic properties. This paper summarizes the recent studies on TiO2 based self-cleaning coating. The scope of this article is in the mechanism and fabrication route of TiO2 based self-cleaning coating, application technique such as spin coating, spray coating dip coating a nd chemical vapour deposition (CVD) and characterization. Self-cleaning coating can be characterized using many methods in order to determine their quality and feasibility. The characterization of the surface applied by self-cleaning coating can be in the form of numerical data and morphological images such as X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Atomic Force Microscopy (AFM), contact angle, hardness, and scratch resistance are also analysed a nd discussed.

Review on the Superhydrophilic coating of Electric insulator

In recent years, new preparation technologies of ultra-hydrophilic TiO2 films have emerged in an endless stream, such as spray pyrolysis, electrostatic spinning, spray printing, micro-arc oxidation, pulsed laser deposition, atomic force deposition, ion beam deposition, etc. These new preparation technologies are able to control the surface chemical composition and surface structure of the thin film more precisely and to prepare the ideal superhydrophilic TiO2 coating. Among them, the rf sputtering method has important research significance for the repair and maintenance of TiO2/SiO2 composite coating. Nano TiO2 has excellent photocatalytic activity and superhydrophilicity, making it an important material for the preparation of high self-cleaning superhydrophilic coatings. However, the application of TiO2 photocatalysis requires UV light, and there is less than 5% UV light in sunlight, so the application of TiO2 photocatalytic performance is greatly limited. In addition, the surface hydrophilicity of TiO2 under light also has great problems. Therefore, improving the surface hydrophilic durability of TiO2 has become a key research point.

Fluorine-free anti-droplet surface modification by hexadecyltrimethoxysilane-modified silica nanoparticles-coated carbon nanofibers for self-cleaning applications

In daily life, many surfaces become contaminated owing to dust/dirt accumulation via air pollution. Self-cleaning surface modification is one of the best ways to address this problem. Therefore, ultra-hydrophobic coatings have garnered significant attention owing to their potential applications featuring water resistance and self-cleaning ability. In this study, a simple, fluorine-free, as well as eco-friendly technique was utilized to fabricate durable self-cleaning coatings. This coating material consists of fluorine-free hexadecyltrimethoxysilane-altered SiO2 nanoparticle (NPs)-coated carbon nanofibers (CNF/SiO2-HDTMS) and a commercial gelatin based adhesive emulsion. Owing to the presence of the hydroxyl (−OH) functional groups of CNFs, SiO2 NPs could accumulate on CNFs surface, hence creating hierarchical microstructures that generate air pockets for improved hydrophobicity. In this study, the developed coating was applied onto a polyethylene sheet, glass fiber membrane, and glass via dip coating. These surfaces were targeted due to over-use in day-to-day life as well as in industrial applications such as plastic tents, umbrellas, windshields of vehicles, window and door glasses, skyscrapers, membranes, fabrics, papers and the list is endless. Interestingly, after the introduction of the adhesive based CNF/SiO2-HDTMS coating, the superhydrophilic microfiber filter became highly hydrophobic, with a water contact angle of 125°. Similar effect can be seen in case of the modified glass, where the average contact angle was determined to be around 141°. The CNF/SiO2-HDTMS coated poly bag exhibited excellent anti-droplet behavior with water contact and rolling angles of 136° and 12°, respectively. The self-cleaning coatings maintained anti-droplet behavior even after tests such as sand impact abrasion, and finger touch. This study explores the possible industrial applications of self-cleaning coatings at ambient temperature to improve the feasibility of their usage.

Self-stratifying amphiphobic coating based on functional polyacrylates

Self-stratifying coating is a promising and interesting coating system containing multi-resinous components, which instinctively stratify after coating on the substrate. Here, we have developed a new type of self-stratifying coating based on functional polyacrylate system. In this case a terpolymer of butyl acrylate, acetoacetoxy ethyl methacrylate and hydroxyethyl methacrylate was used as the polymer matrix. The whole coating system consists of a blend of a functional acrylic terpolymer (bearing acetoacetoxy, n-butyl as well as hydroxy ethyl functional group as the pendant group), phenyl methoxy functional siloxanes, hollow glass spheres (HGS) and aerogel particles (AP). 3-aminopropyl triethoxysilane was used as a crosslinker. Hydrophilic hollow glass spheres (of 20 μm average particle size) and hydrophilic aerogel particles (of 5 μm average particle size) were used to generate desirable roughness on the surface. Low quantity of a non-silanated fluorinated surfactant was incorporated, which self-stratifies to the surface during coating film formation and imparts oleophobicity. An oil and water repellent (amphiphobic) surface was developed with high water as well as diiodomethane contact angle (∼130°) by varying the ratio of glass spheres and aerogel to each other and the polymer. Rolling effect of water and oil drops was also prominently visible. The surface showed excellent resistance to polar and non-polar solvents, good resistance to heat (150 °C) and sand impingement after which the water contact angle remains unchanged. The oil repellency was easily restored by spraying or dipping in a dilute solution of the non-silanated fluorinated compound. This coating can be applied by conventional techniques such as spray application and can have potential applications in anti-soiling and self-cleaning coatings.