Self-cleaning Glass Research Articles

Theoretical insights into the surface physics and chemistry of redox-active oxides

Redox-active oxides find use in many applications, including catalysts, photovoltaic devices, self-cleaning glasses, chemical sensors and electronic components. Their utility derives from their unique ability to access multiple metal-charge states within a finite energy window. However, this property also confounds our ability to study reducible oxides, because it leads to structural, compositional and electronic complexities that elude simplistic models of materials structure and function. Oxygen vacancies play a critical role in shaping the functional properties of such oxides; most notably, they lead to mobile-charge imbalances that impact surface processes at substantial distances from the originating defect. Atomistic simulations are inherently equipped to illuminate these phenomena at a fundamental level; however, reducible oxides pose great challenges, owing to the high level of electron correlation needed to correctly describe them. Understanding how defects form, couple, propagate, agglomerate or repel each other and influence the surface properties of reducible oxides is only now coming into the grasp of modern theory and simulation capabilities. This knowledge is also key to discovering and controlling emergent materials properties with tunable multifunctionalities at the nanometre scale and beyond. Reducible oxides are tunable, multifunctional materials used in many applications, particularly in catalysis; their attractive properties arise from their interacting charge carriers, complex electronic structure and propensity to form mobile defects. This Review surveys theoretical methods to model and understand reducible oxides, using TiO2 as a prototypical example.

Rapid and Scalable Wire-bar Strategy for Coating of TiO2 Thin-films: Effect of Post-Annealing Temperatures on Structures and Catalytic Dye-Degradation.

Titanium dioxide (TiO2) thin films were rapidly coated on Corning glass substrates from the precursor solution using the wire-bar technique at the room temperature and then post-annealed at 400, 500 and 600 °C for 1 h under atmospheric conditions. The structural, morphological, optical, wettability and photocatalytic properties of the films were studied. X-ray diffraction analysis confirmed the formation of an anatase TiO2 structure irrespective of the post-annealing temperatures. The optical transparency of the films in the visible range was measured to be > 70%. A water contact angle (WCA) of ~0° was observed for TiO2 thin-film, post-annealed at 400 °C and 500 °C. However, WCA of 40.3° was observed for post-annealed at 600 °C. The photocatalytic dye-degradation using post-annealed thin-film was investigated indicating a steady improvement in the dye-degradation percentage (from 24.3 to 29.4%) with the increase of post-annealing temperature. The demonstrated TiO2 thin-films deposited by wire-bar coating technique showed promises for the manufacturing of large-area cost-effective self-cleaning window glass.

Plasmonic gold-embedded TiO2 thin films as photocatalytic self-cleaning coatings

Transparent photocatalytic TiO2 thin films hold great potential in the development of self-cleaning glass surfaces, but suffer from a poor visible light response that hinders the application under actual sunlight. To alleviate this problem, the photocatalytic film can be modified with plasmonic nanoparticles that interact very effectively with visible light. Since the plasmonic effect is strongly concentrated in the near surroundings of the nanoparticle surface, an approach is presented to embed the plasmonic nanostructures in the TiO2 matrix itself, rather than deposit them loosely on the surface. This way the interaction interface is maximised and the plasmonic effect can be fully exploited. In this study, pre-fabricated gold nanoparticles are made compatible with the organic medium of a TiO2 sol-gel coating suspension, resulting in a one-pot coating suspension. After spin coating, homogeneous, smooth, highly transparent and photoactive gold-embedded anatase thin films are obtained.

Assessment of environmental applicability of TiO2 coated self-cleaning glass for photocatalytic degradation of estrone, 17β-estradiol and their byproducts

Optimization of photocatalytic degradation of two natural estrogenic compounds, estrone (E1) and 17β-estradiol (17β-E2) in aqueous medium was performed on TiO2 coated Pilkington ActivTM self-cleaning glass as a novel approach to eliminate free nano-TiO2 releasing to the intended environment after treatment. The active glass was characterized by Atomic Force Microscopy (AFM), X-ray diffraction (XRD), and Raman spectroscopy to characterize the TiO2 nanoparticles. The main purposes were mineralization of target compounds in the treated water during the photocatalytic reaction and also to investigate the oxidation by products. Response Surface Methodology (RSM) has been applied to optimize the photocatalytic degradation by changing time, pH, and light intensity as effective factors. According to the results, time was the more effective parameter. The maximum efficiency degradation was achieved in alkaline media. Due to interactive effects between variable factors, 1 mg/L aqueous solution of E1 and 17β-E2 in water was totally decomposed by TiO2 photocatalyzed reactions under UV-C irradiation of 10.08 W/m2 for 52.49 min at pH 9.42. Results of GC-MS analysis were introduced 17-deoxy Estrone and 2-Hydroxyestradiol as intermediate products for E1 and 17β-E2, respectively. All of the peaks finally disappeared after 170 min. Optimized conditions were applied for real sample of wastewater, presenting 30.40% and 56.84% in the efficiency degradation of E1 and 17β-E2, respectively.

Preparation of a self-cleaning glass using solutions of titanium fluor complexes

The object of research is silicate glass with a nanostructured coating of titanium oxide (in the anatase modification), which is noted for photocatalytic activity and, as a result, acquires the ability to self-clean under ultraviolet irradiation. The existing industrial method of applying such a coating is carried out by the pyrolytic method, but it is effective for large-scale production, and is carried out for large-sized products from sheet float glass. For the production of small-scale, or piece products, it is not justified. This applies to products of complex configuration, and especially hollow. In the case there are methods of coating from the liquid phase. First of all, the sol-gel method. The classical such method requires titanium alkoxides, which are of high cost, as precursors. Cheaper and more flexible is the method of coating from solutions of titanium fluorine complex compounds. Ammonium hexafluorotitanate is used as a precursor, but it is expensive. It is proposed to modify the chain of chemical transformations, namely: to use the cheaper and more affordable ones as primary precursors. It is proposed to obtain it artificially, using the bifluoride method. During the study, ammonium bifluoride NH 4 HF 2 and titanium oxide TiO 2 are used, which, according to the proposed synthesis method, form (NH 4 ) 2 TiF 6 , its appearance is confirmed by X-ray phase analysis. Fluorination of titanium oxide Ammonium bifluoride occurred at a temperature not exceeding 200 °C. Fluorination is accompanied by the release of only water vapor and ammonia. A photocatalytic coating is obtained on float glass samples by precipitation of the anatase crystalline phase from an aqueous solution of (NH 4 ) 2 TiF 6 . The presence of anatase is confirmed by X-ray phase analysis. The size of crystalline formations does not exceed 15-20 nm. The self-cleaning ability is evaluated by the hydrophilicity test of the glass and the spectral characteristics of the coating in the ultraviolet range. Due to this, it is possible to obtain a self-cleaning coating on glass, which, in comparison with similar known ones, is not inferior in quality and has the following advantages: low cost and availability, absence of harmful emissions, complies with the principles of «green chemistry».

Improving preparation method of TiO2-NiO composite materials for self-cleaning glass

TiO2-NiO Composite have been synthesized as self-cleaning material on glass. TiO2 was prepared using titanium tetraisopropoxide (TTIP) by sol gel method, while TiO2-NiO composite was prepared from TiO2 synthesized with additional Ni(NO3)2.6H2O precursor. TiO2 synthesized powder was added to Ni2+ concentrated solution in aquadest so it formed crystalline layer on TiO2 powder. Core shell composite of TiO2-NiO was obtained through the drying process at 110 °C and calcined at 700 °C. Identification and characterization of NiOTiO2 composite were carried out using X-Ray Diffraction (XRD), and X-Ray Fluoresecence (XRF). XRD diffractograms show the appearance of TiO2 and NiO peaks. Another analysis with XRF showed that composition of TiO2-NiO composite are 2:1; 1:1 and 1:2 (w/w). The electronic spectrum from TiO2-NiO core shell result showed an increasing response in visible light so it can be applied as coating material in manufacturing technology of self cleaning glass.

Improving method of TiO2-Fe2O3 composite materials for self-cleaning glass preparation

Fe2O3-TiO2 composite have been synthesized as a self-cleaning material on glass. TiO2 was prepared from titanium tetraisopropoxide (TTIP) by sol gel method. Meanwhile, Fe2O3-TiO2 composite was prepared from TiO2 synthesized with additional precursor Fe(NO3)3•9H2O. TiO2 synthesized powder was added to Fe3+ solution in aquadest so it formed crystalline layer on TiO2 powder. Core shell composite of Fe2O3-TiO2 was obtained after calcination on temperature 600 °C. Identification and characterization of composite by X-Ray Difraction (XRD) showed that there are difractogram peaks of TiO2 and Fe2O3. Another analysis by X-Ray Fluoroscence (XRF) showed that composition of Fe2O3-TiO2 composite are 1:1, 1:1.5, 1:2, and 1:3 (w/w). The morphology of composite shown by Scanning Electron Microscope (SEM) that explained coverage character of TiO2 with Fe2O3 formed core shell type. The electronic spectrum of Fe2O3-TiO2 core shell result showed an increasing response in visible light so it can be aplicated as a coating material in manufacturing technology of self cleaning glass.

Polymer-inorganic hybrid coating with biomimetic superhydrophobic property: preparation, characterization, and enhanced stability

To improve the adhesion of superhydrophobic coating to glass substrate, a novel superhydrophobic coating with biomimetic micro-nano structure was prepared by a simple and efficient method of in-situ polymerization, using styrene and methyl methacrylate as mixed monomer, Azo diisobutyl cyanide as the initiator, 2-(2ˊ-hydroxyl-5ˊ-methyl phenyl) benzotriazole as the ultraviolet light absorber, styrene-methyl methacrylate copolymer, namely P(St-MMA), was introduced onto the surface of glass by covalent grafting, and finally obtained a self-cleaning glass with superhydrophobic property. The morphology and microstructure of the as-prepared superhydrophobic coating were characterized by SEM, FT-IR, XPS and contact angles. The adhesion strength between the superhydrophobic coating and glass substrate was evaluated by scratch test. At the same time, the thermostability and acid resistance of the as-prepared superhydrophobic coating were also assessed in our work. The results demonstrated that the as-prepared coating had a superhydrophobic surface, and the maximum contact angle, which was prepared at the 9:1 ratio of styrene to methyl methacrylate monomer, was 154.7° and the slighting angle was 5.6°. SEM images indicated that the grafted polymer showed a biomimetic micro-nano structure, and the adhesion between the coating and the glass substrate reached the 5B level, the maximum heat resistance temperature of the hydrophobic coating was 110 °C, and the pH value of the maximum acid resistance was 1. The good hydrophobicity was attributed to the synergistic effect of the hydrophobic coating with low surface energy and the rough surface controlled by the preparing procedure. The as-prepared superhydrophobic coating endowed ordinary glass with good self-cleaning function. And the method could also be extended to the other materials surface, such as metal, ceramics and other polymer materials. It has potential applications in glass curtain wall, bathroom glass, windshield, biological engineering and other fields alternative to traditional glass.

Self-Cleaning Glass

Self-cleaning glass with a coated external surface exhibits both photocatalytic and photo-induced superhydrophilic properties. Such properties provide the conditions for the decomposition of organic contaminants and washing-off of inorganic ones by rainwater flowing down in the form of a thin film. The basic data on the mechanism of photocatalysis and photo-induced hydrophobicity on titanium dioxide coatings are provided. The data on the optical, structural, photocatalytic, and superhydrophilic properties of the commercial self-cleaning glass are summarized. It is demonstrated that a layer of organic contaminants of a thickness of up to 10 nm can be removed from the glass surface within 1-hour exposure to sunlight/daylight. The absence of standards for self-cleaning glass is underlined; the problems and results of the glass preparation are indicated. An express-analysis method for the photocatalytic activity of self-cleaning articles is described. The results of long-term in situ testing prove the efficiency of these articles.

Optical Transparent and Hydrophobic Properties of TEOS/OTES Hybrid Materials by Sol-Gel Processing

The objective of this project was to develop the hydrophobic film for self-cleaning glasses. The effects of octyltriethoxysilane (OTES) additions to hydrolysis of tetraethylorthosilicate (TEOS) on hydrophobic and optically transparent properties were studied. The film was prepared by sol-gel method from the precursors namely, TESO, OTES, isopropanol alcohol (IPA), and deionized water (DI). The sols for coating were obtained with TEOS/OTES ratio of 50:50 to 99:1. The sols were deposited on a commercial glass and dried at 60oC for an hour. After drying, the film properties were characterized by fourier transform infrared spectroscopy (FTIR), UV-VIS Spectrophotometer, x-Ray Diffractometer (XRD), atomic force microscope (AFM), optical microscopy and contact angle meter. It was found that contact angles of the hybrid films increased with the OTES addition, reaching a maximum at 10 wt.%, and the contact angle values were the same as for further addition. The light transmittance was rather stable with increasing amounts of OTES. For the optimized condition, the water contact angle of 108o and light transmittance of 91%, was obtained with TEOS/OTES ratio of 90:10.

Combined photocatalytic properties and energy efficiency via multifunctional glass

This study reports on the photocatalytic activity and energy efficiency of two self-cleaning glass benchmarks, namely Pilkington Activ™ Blue (PAB) and Pilkington Activ™ Clear (PAC). These self-cleaning glasses owe their unique properties to a thin layer of titanium oxide (TiO2) able to degrade organic matter when exposed to UV light. The glass samples demonstrated photocatalytic activity towards a model Volatile Organic Compound (VOC), i.e. 2-propanol, and a representative stain molecule, i.e. methylene blue (MB), analyzed at a GC-FID and a UV–vis spectrophotometer, respectively. PAC was more active than PAB towards 2-propanol oxidation under ultraviolet irradiation. Similarly, PAC showed the best reactivity in the degradation of pre-adsorbed MB under simulated solar irradiation. However, comparable performance was found for 2-propanol degradation under simulated solar irradiation.Moreover, using building energy modeling, the energy saving potential of different glazing configurations of PAB and PAC was simulated for buildings in different climatic conditions. The results were compared with those of the Pilkington Optifloat glass, which has no TiO2 coating. The best performance was obtained with the double-glazed PAB configuration.The present study is the first work on assessing the performance of self-cleaning glasses, both as regards energy efficiency and photocatalytic properties. These two types of functionality are equally important but usually evaluated separately, preventing the assessment of the performance as a whole. The clear picture of the overall performance provided in this work for two widespread commercial functionalized glasses may serve as framework for comparing new emerging self-cleaning glasses with existing benchmarks.

Hydrophobic properties of textile materials: robustness of hydrophobicity

Extremely hydrophobic surfaces have been receiving considerable interest, such as in the contexts of self-cleaning glass or clothes, antifouling paintings, and the reduction of friction drag. A large variety of treatments permits the obtainment of (super)hydrophobic textile surfaces. The point here is to investigate the role of different geometrical textile parameters on the hydrophobicity, and more particularly, on the robustness of this property. The influences of solid surface roughness on the wetting behavior are commonly studied for model solid textures while textile roughness is largely deformable. A laboratory test method is suggested to evaluate this robustness. Some hydrophobic fibrous structures were prepared (using classical woven fabrics and pile fabrics) to investigate the influence of textile structures on their static and forced wetting properties. Static contact angles, contact angle hysteresis, and the contact angle after compression were measured. The meso- and micro-structures appeared to influence either the static wetting or the robustness of the hydrophobicity after compression.

Hydrophobic–Hydrophilic Surfaces Exhibiting Dropwise Condensation for Anti-Soiling Applications

Soiling of solar cover glass is a significant challenge that increases the levelized cost of electricity of solar photovoltaic energy through loss of electrical output and/or increased operation and maintenance costs. Hydrophobic coatings can reduce the cost of cleaning, but an external source of cleaning water is still required. Dew, however, can be harnessed to create a self-cleaning glass surface. To efficiently use condensation (i.e., natural dew) to create a self-cleaning glass surface, we fabricated a hybrid hydrophobic–hydrophilic coating, with an array of isolated hydrophilic circular rings. Water roll-off collection rates were measured in a simulated dew environment. The effect of hydrophilic ring geometry (location, arrangement, diameter, distance, and count) was studied to determine the optimal cleaning efficiency. The hybrid surface increased water collection rates by 95% over an uncoated (bare) glass surface and 51% compared with uniformly coated hydrophobic low-iron glass.

Photodeposited Ag-wires on TiO2 films

Highly conducting films of Ag are photodeposited onto commercial self-cleaning glass (Pilkington, Activ™), as well as in-house prepared sol-gel TiO2 films, using an aqueous solution of AgNO3 containing a sacrificial electron donor, glycerol. Wire tracks are created by irradiating the photocatalytic surface through a stencil, promoting Ag-deposition only on the exposed/irradiated areas. The ability of the Ag-wires to conduct, and so heat up the underlying glass, is investigated initially for demisting purposes. The photodeposited Ag-wires on Activ™ glass are found to have a resistance of ca. 150 Ω after 2 h irradiation (I = 4 mW cm−2, 352 nm BLB), whereas the Ag-wires on sol-gel film, prepared under the same conditions, are ca. 3 times less resistive (55 Ω). Repeat heating-cooling cycles are achieved by applying a voltage, 12 V, across the Ag wires and demonstrate the robustness of the Ag-wires on Activ™ and sol-gel films, which produce a consistent rise in temperature above ambient room temperature of 20 °C, and 40 °C, respectively. A Scanning Electron Microscopy (SEM) study of Ag-particle growth on sol-gel TiO2 films demonstrates island growth of the Ag-particles, producing Ag ‘wires’ that are only able to conduct once the Ag ‘islands’ overlap; typically this is after ca. 9 min of irradiation (I = 4 mW cm−2), with R = 200 kΩ and Ag’ island’ particle size = ca. 100 nm. Upon further irradiation, the particles eventually grow sufficiently large that most Ag particles overlap and the resulting Ag wires are highly conducting (R = 55 Ω after 2 h).

Hydrophobic zinc-tellurite glass system as self-cleaning vehicle: Interplay amid SiO2 and TeO2

Cost-effective, environmental amiable and maintenance free glasses with improved hydrophobic activity are needed for diverse industrial applications. Pollutant and dirt depositions on glasses that cause the visual obscurity and damages of the cultural heritages require inhibition. The underlying mechanism of hydrophobic interactions assisted self-cleaning traits of glass is poorly understood. It has been shown that excellent hydrophobic glass with water contact angle (WCA) above 90o and very low surface wettability can be achieved by controlling the surface roughness (SR), where liquid droplets remain perfectly spherical on such surfaces (literally without touching) before being self-cleaned (rolls off). Moreover, selection and optimization of constituent materials composition as well as the preparation technique play a significant role towards such success. Most of the previous attempts for the self-cleaning glass preparation were made via coating strategy on glass surface. Yet, preparation of super-hydrophobic glass surfaces with self-cleaning attributes remains an open challenge. Driven by this idea, we prepared a new glass system of composition (80 x) TeO2-20ZnO-(x)SiO2 (x = 0, 0.03, 0.06, 0.09 and 0.12 mol%) by melt-quenching method, where the proportions of SiO2 and TeO2 were interplayed. As-prepared samples (thin pellet without coating) were characterized using atomic force microscopy (AFM) and video contact angle (VCA) measurements. The effects of SiO2 concentration on the glass SR, surface energy and hydrophobic properties were evaluated. Glass 0.06 mol% of SiO2 revealed the optimal WCA of 112.39º and SR of 7.806 nm. It was established that a trade-off between SiO2 and TeO2 contents in the studied glasses could produce super-hydrophobic surface (WCA over 90º), leading to great opportunities for diverse self-cleaning applications.

Antireflective and self-cleaning glass with robust moth-eye surface nanostructures for photovoltaic utilization

Omnidirectional and broadband antireflection (AR) self-cleaning coatings are crucial for the performance enhancement of photovoltaic (PV) modules. Herein, we demonstrate biomimetic moth-eye structures on glass through inductively coupled plasma (ICP) etching process using thermal dewetted copper nanoparticles as the masks. Moreover, the etched glasses are chemically treated to strengthen the nanostructures. The moth-eye glass exhibits excellent AR effects with a reflectance below 3% over the wavelength range from 570 to 950 nm. As the incident angles of the light increasing from 0o to 60o, the relative improvement of the power conversion efficiency (PCE) of PV module with moth-eye glass produces a dramatically increases from 4.6% to 9.9%, compared with that of PV modules with flat glass. Additionally, the moth-eye glass also possesses superhydrophilic self-cleaning and antifogging abilities, which are attractive for outdoor applications.

Spray-Deposition of an Organic/Inorganic Blend for Fabrication of a Superhydrophobic Surface: Effect of Admixing with Silica Aerogel and Modified Silica Nanoparticles

Superhydrophobic and self-cleaning glass slides were fabricated using a facile and low-cost method through spray-coating of four types of blends consisting of stearic acid, the mixture of stearic acid and SiO2 nanoparticles, the mixture of stearic acid and SiO2 nanoparticles modified with oleic acid, and the mixture of stearic acid and SiO2 aerogel onto the surface. The nanocoated surfaces were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), and water contact angle (WCA) measurements. The results have shown that the mixture of stearic acid and SiO2 nanoparticles modified with oleic acid coating possessed the highest contact angle of about 158.6° and a low sliding angle while the mixture of stearic acid and SiO2 aerogel had an almost similar WCA but with a more satisfactory durability. In contrast, the stearic acid coating alone had a hydrophobic property and the mixture of stearic acid and unmodified SiO2 nanoparticles showed superhydrophobic properties without any self-cleaning and durability features.

Recent Advances in Low-Temperature Deposition Methods of Transparent, Photocatalytic TiO2 Coatings on Polymers

In this paper, we present an overview as well as current advances in the low-temperature deposition of highly crystalline suspensions of titania nanoparticles on polymers for photocatalytic applications. The presence of preformed titania nanoparticles yields the possibility of producing photocatalytically active coatings at reduced temperatures. Transparent and photocatalytically active TiO2 coatings that degrade organic matter, have been widely applied to bestow self-cleaning properties onto surfaces. This low-temperature deposition method and its transition to polymers would open an entire array of possible self-cleaning applications. During this research, incorporation of a silica buffer layer was applied to improve the compatibility of the inorganic coating on a substrate, such as polymethylmethacrylate (PMMA) and polyphenylsulphone (PPSU). The photocatalytic activity of the obtained coating was analyzed for its photocatalytic abilities by evaluating the color removal of a dye solution (methylene blue, MB) under UV irradiation and compared with commercial Pilkington Activ® self-cleaning glass. Our results indicate that the titania-coated silica-polymer systems yield a higher photocatalytic activity towards the degradation of organic pollutants. This method proves that the deposition of highly crystalline anatase suspensions on silica buffer layers is a viable method to produce photocatalytic coatings on heat-sensitive substrates.

Materials and Mechanisms of Photo-Assisted Chemical Reactions under Light and Dark Conditions: Can Day-Night Photocatalysis Be Achieved?

The photoassisted catalytic reaction, conventionally known as photocatalysis, is expanding into the field of energy and environmental applications. It is widely known that the discovery of TiO2 -assisted photochemical reactions has led to several unique applications, such as degradation of pollutants in water and air, hydrogen production through water splitting, fuel conversion, cancer treatment, antibacterial activity, self-cleaning glasses, and concrete. These multifaceted applications of this phenomenon can be enriched and expanded further if this process is equipped with more tools and functions. The term "photoassisted" catalytic reactions clearly emphasizes that photons are required to activate the catalyst; this can be transcended even into the dark if electrons are stored in the material for the later use to continue the catalytic reactions in the absence of light. This can be achieved by equipping the photocatalyst with an electron-storage material to overcome current limitations in photoassisted catalytic reactions. In this context, this article sheds lights on the materials and mechanisms of photocatalytic reactions under light and dark conditions. The manifestation of such systems could be an unparalleled technology in the near future that could influence all spheres of the catalytic sciences.

Durable and robust transparent superhydrophobic glass surfaces fabricated by a femtosecond laser with exceptional water repellency and thermostability

Self-cleaning transparent glass surfaces with periodic micro–nano structures fabricated by a femtosecond laser exhibit exceptional water repellency and thermostability.