Self-cleaning Films Research Articles

Self-cleaning PDMS films with durable superhydrophobicity and photocatalytic capability based on TiO2-modified nanopillar array

Self-cleaning films with superhydrophobicity and photocatalytic capability have attracted considerable interest in recent years for the synthetical chemical and physical self-cleaning performance to overcome individual inherent limitations. In the present work, a dual-functional self-cleaning film is proposed through the design of polydimethylsiloxane (PDMS) nanopillar array, swelling absorption of anatase-typed TiO2 nanoparticles, and final low-surface-energy modification of 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (PFDTS). Benefiting from the hierarchically mountain-like structure constructed by the PDMS nanopillar array and swelling absorbed TiO2 nanoparticles, the self-cleaning film presents a robust superhydrophobicity to resist chemical and mechanical damages (e.g. solvent/acid/alkali immersion, stretching, water impact, and sand abrasion). Combining the photocatalytic activity with adequate loading of TiO2 nanoparticles, the self-cleaning film demonstrates a great degradation capability of organic pollutants with a degradation efficiency over 98 %. Importantly, owing to the superhydrophobicity repairability induced by the migration of fluorocarbon chains, the self-cleaning film can highly recover the water repellency with a water contact angle (WCA) of 155o even after long-term irradiation under UV light. The findings conceivably promote the development and application of the self-cleaning materials with superhydrophobic and photocatalytic properties.

Strong Interface Interaction of ZnO Nanosheets and MnSx Nanoparticles Triggered by Light over Wide Ranges of Wavelength to Enhance Their Removal of VOCs

The characteristics of the surface and interface of nanocomposites are important for exerting multi-functional properties and widening interdisciplinary applications. These properties are mainly depending on the electronic structures of materials. Some key factors, such as the surface, interface, grain boundaries, and defects take vital roles in the contribution of desired properties. Due to the excellent sensitivity of the QCM (quartz crystal microbalance) device, the surface and interface features of the nanocomposite were studied with the aid of the gas-response of the sensors (Sensor’s Gas-Sensitivity) in this work. To make full use of the visible light and part of NIR, a ZnO/MnSx nanocomposite was constructed using hydrothermal synthesis for narrowing the bandgap width of wide bandgap materials. The results indicated that the absorbance of the resulting nanocomposite was extended to part of the NIR range due to the introduction of impurity level or defect level, although ZnO and MnS belonged to wide bandgap semiconductor materials. To explore the physical mechanism of light activities, the photoconductive responses to weak visible light (650 nm, etc.) and NIR (near-infrared) (808 nm, 980 nm, and 1064 nm, etc.) were studied based on interdigital electrodes of Au on flexible PET (polyethylene terephthalate) film substrate with the casting method. The results showed that the on/off ratio of ZnO/MnSx nanocomposite to weak visible light and part of NIR light were changed by about one to five orders of magnitude, with changes varying with the amount of MnSx nanoparticle loading due to defect-assisted photoconductive behavior. It illustrated that the ZnO/MnSx nanocomposite easily produced photo-induced free charges, effectively avoiding the recombination of electrons/holes because of the formation of strong built-in electrical fields. To examine the surface and interface properties of nanocomposites, chemical prototype sensor arrays were constructed based on ZnO, ZnO/MnSx nanocomposite, and QCM arrays. The adsorption response behaviors of the sensor arrays to some typical volatile compounds were examined under a similar micro-environment. The results exhibited that in comparison to ZnO nanosheets, the ZnO nanosheets/MnSx nanocomposite increased adsorption properties to some typical organic volatile compounds significantly. It would have good potential applications in photo-catalysts, self-cleaning films, multi-functional coatings, and organic pollutants treatment (VOCs) of environmental fields for sustainable development. It provided some reference value to explore the physical mechanism of materials physics and photophysics for photo-active functional nanocomposites.

Polyimide/titanium dioxide self-cleaning nano-hybrid films with high photocatalytic properties

Self-cleaning films are considered an emerging paradigm of environmentally functional materials. Enhancing the photocatalytic activities of self-cleaning film is a viable strategy for improving its antifouling characteristics. In this work, a polyimide/titanium dioxide (TiO2) film (PI/TiO2) film was developed by casting a mixture of polyamic acid solution and TiO2 hydrosol with a subsequent thermal imidization. TiO2 with a concentration of 1, 3, and 5 ​wt% was successfully incorporated into the PI matrix and was equally distributed in nanometer size in the PI/TiO2 nano-hybrid films. The addition of TiO2 into the film significantly improved its catalytic performance as indicated by the photocatalytic degradation of methylene blue and resazurin solution, suggesting a good self-cleaning property for antifouling applications. In addition, the thermal and mechanical stability of the PI film were improved with the addition of TiO2. After eight cycles, the PI-TiO2 films also demonstrated excellent cycling stability. This study introduces a novel self-cleaning film family composed of PI/TiO2 nano-hybrid film and emphasizes the engineering potential of photocatalytic materials in the context of self-cleaning film.

Electro-spray deposited TiO2 bilayer films and their recyclable photocatalytic self-cleaning strategy

Recyclable titanium dioxide (TiO2)-based photocatalytic self-cleaning films (SCFs) having a bilayer structure were prepared and assessed. These SCFs comprised two layers of fibers fabricated using an electrospinning process. The self-cleaning layer was made of acrylonitrile–butadiene–styrene (ABS) fibers with embedded TiO2 while the substrate layer was composed of fibers made by simultaneously electrospinning poly (vinyl alcohol) (PVA) and ABS. This substrate improved the mechanical strength of the SCF and provided greater adhesion due to the presence of the PVA. The experimental results showed that the hydrophobicity (as assessed by the water contact angle), photocatalytic properties and self-cleaning efficiency of the SCF were all enhanced with increasing TiO2 content in the ABS/TiO2 fibers. In addition, the introduction of the substrate layer allowed the SCFs to be applied to various surfaces and then peeled off when desired. The ABS fibers effectively improved the strength of the overall film, while deterioration of the ABS upon exposure to UV light was alleviated by the addition of TiO2. These SCFs can potentially be recycled after use in various environments, and therefore have applications in the fields of environmental protection and medical science.

Photocatalyst Activity Indicator Inks, paiis, for Assessing Self-Cleaning Films

ConspectusSemiconductor photocatalysis, usually referred to simply as “photocatalysis” is a relatively new and yet still a growing subject area, which has led to a billion-dollar industry, mostly focused on the production of self-cleaning architectural materials, such as glass, paint, tiles, fabric, plastic and concrete. All such materials need to be tested for activity, but for self-cleaning activity there are few appropriate tests; the International Standards Organization, ISO, for example, have at present only two, both of which are slow and largely ineffective for most commercial self-cleaning products. The photocatalyst community in general, and the manufacturers of self-cleaning photocatalytic products in particular, need a fast, simple, easy to use method of assessing photocatalytic activity, which can also be used to demonstrate the efficacy of self-cleaning materials in minutes, i.e., a quantitative and qualitative test.In this account we highlight our work on photocatalyst activity indicator inks, i.e., paiis, which address the above need and work via the photocatalyzed reduction of a dye, usually resazurin, Rz, and concomitant oxidation of an organic, almost always glycerol. We describe our early work on the Rz ink which showed that, when used to assess photocatalytic film activity, the measured rate of change in color is directly proportional to the measured rate of photocatalyzed mineralization of the popular test pollutants stearic acid, SA, and methylene blue, MB, by O2. We show that photography, coupled with digital color analysis, is just as effective and much less expensive at measuring the rate of color change exhibited by paiis as UV/vis spectrophotometry and also can be used for testing clear and opaque photocatalytic samples in the laboratory and in situ. We demonstrate how the Rz ink can be used to assess the activities of both UV and Visible light absorbing photocatalytic powders and thin films, and highly colored samples. The kinetics of the photocatalyzed color change are outlined, which are zero-order with respect to the initial dye concentration and ink film thickness. We describe the basic features of the recently published Rz-ink based ISO for the qualitative and quantitative assessment of activity of nonporous, flat photocatalytic films.Although the Rz ink is best suited for assessing films of moderate photocatalytic activity, such as found in most examples of commercial self-cleaning glass, other paiis are described which can be used to assess commercial products that have much higher or lower, activities, such as photocatalytic paints and tiles, respectively. Examples of how paiis have been used in research over recent years are given, a highlight of which is their use in the search for a suitable, visible light absorbing photocatalyst alternative to TiO2, that can compete with the latter in terms of cost, durability, stability, and activity. Finally, the novel, photocatalyst-based inks that have been developed as a consequence of the works on paiis are discussed briefly, including ones for, photodepositing metal films and conducting wires on photocatalytic films, cleaning tarnished metals, and indicating UV levels and dose and O2 concentration.

Synthesis and Self-Cleaning Property of TiO₂ Thin Film Doping with Fe3+, Al3+, Ce3+ Ions.

Enhancing the response to visible light and inhibiting recombination of photogenerated electrons and holes is a key point for strongly improved self-cleaning performances of TiO₂ self-cleaning films. In this work, TiO₂ thin film doping with three different ions (Fe3+, Al3+, Ce3+) on glass substrate respectively by sol-gel method to explore the effect of ions on self-cleaning performance of TiO₂ thin films. All the prepared samples are characterized by XRD, SEM, UV-Vis and water contact angle tester. Moreover, the self-cleaning mechanism of doping TiO₂ thin film is discussed. The water contact angle of TiO₂ thin film doping with 9% Fe (molar ratio), 5% Ce and 5% Al are reach at 0°, respectively. In comparison with the water contact angle of pure TiO₂ thin film (2.5°), TiO₂ thin film doping with metal ions exhibit better self-cleaning property.

Supersonic Cold Spraying for Energy and Environmental Applications: One-Step Scalable Coating Technology for Advanced Micro- and Nanotextured Materials.

Supersonic cold spraying is an emerging technique for rapid deposition of films of materials including micrometer-size and sub-micrometer metal particles, nanoscale ceramic particles, clays, polymers, hybrid materials composed of polymers and particulates, reduced graphene oxide (rGO), and metal-organic frameworks. In this method, particles are accelerated to a high velocity and then impact a substrate at near ambient temperature, where dissipation of their kinetic energy produces strong adhesion. Here, recent progress in fundamentals and applications of cold spraying is reviewed. High-velocity impact with the substrate results in significant deformation, which not only produces adhesion, but can change the particles' internal structure. Cold-sprayed coatings can also exhibit micro- and nanotextured morphologies not achievable by other means. Suspending micro- or nanoparticles in a liquid and cold-spraying the suspension produces fine atomization and even deposition of materials that could not otherwise be processed. The scalability and low cost of this method and its compatibility with roll-to-roll processing make it promising for many applications, including ultrathin flexible materials, solar cells, touch-screen panels, nanotextured surfaces for enhanced heat transfer, thermal and electrical insulation films, transparent conductive films, materials for energy storage (e.g., Li-ion battery electrodes), heaters, sensors, photoelectrodes for water splitting, water purification membranes, and self-cleaning films.

Laponite-Based Surfaces with Holistic Self-Cleaning Functionality by Combining Antistatics and Omniphobicity.

In the present work, perfluoroalkylated laponite nanoparticles with a high degree of functionalization (60 wt %) have been prepared and a methodology to prepare transparent, antistatic, and omniphobic laponite-based films with holistic self-cleaning properties against liquids, solids and liquid-solid mixtures has been developed. The intrinsic electrical and ionic conductivities observed in unmodified laponite coatings are combined with perfluoroalkyl-modified laponite clays. As a result, films with improved self-cleaning functionality based on dust-repellency and omniphobic liquid-repellence (sheet resistance in the range of 107 Ω/□ and contact angles of 106° (H2O) and 93° (oil)) were obtained. These unique films, being capable to repel dust and liquids, were applied to a variety of substrates (i.e., glass and plastics) and tested against solids and liquids of different nature with excellent performance. Bending tests of these holistic self-cleaning films deposited over flexible substrates showed better mechanical performance than unmodified laponite films.

A cost- and time-saving strategy of spraying TiO2 self-cleaning coatings in tubular substrates by air cold plasma

In this study, using an atmospheric pressure air plasma jet generated by a dielectric barrier structure with hollow electrodes (HEDBS), we developed an ultrafast process for spraying TiO2 self-cleaning films inside tubular substrates. Importantly, SEM images showed that the TiO2 particles were dispersed evenly in the tubular substrates. Furthermore, Raman and XRD pattern indicated the anatase structure of the HEDBS-spayed TiO2 coating after heating at 270°C. Further results of the self cleaning test suggested that the proposed cost- and time-saving HEDBS approach with air working gas could provide a feasible way for synthesizing thin TiO2 nanofilms.

Organic Functionalization and Properties of ZnO Nanosheets with Polymer Containing N-Vinyl Carbazole

To make full use of the visible light more effectively, many studies are focuses on ZnO baesd nanocomposites. To modify the surface of ZnO with functional polymer is a very simple and effective approach. PVK (N-vinyl carbazole polymer) is one of typical organic functional materials, which is generally used as charge transfer materials for the applications of several organic electronic devices. Surface modification of ZnO nanosheets with polymer containing –COOH and N-vinyl carbazole group was performed with self-assembly process for improving the adsorption to visible light and properties of charge transfer in nanoscale. A series of characterizations were carried out by SEM (scanning electron microscopy), Fourier-Transform Infrared (FTIR) spectra, UV-Vis (Ultra-violet visible spectroscopy), et al. The adsorption of the nanocomposite was extended to the region of visible light. The photoconductivity response to weak visible light was studied based on interdigital electrodes of Au on flexible PET (polyethylene terephthalate) film substrate with casting method. The photocurrent of ZnO nanosheets modified with the polymer containing N-vinyl carbazole to weak visible light was changed greatly. The organic-inorganic nanocomposite showed good activities to visible light, with which it can be easily produced photo-induced charges, avoiding the recombination of charges produced by visible light. Photocatalytic efficiency was examined by selecting typical organic pollutants and some good results were obtained, showing much prospect in the fields of photocatalysts, nanoreactors, self-cleaning films, coatings, and organic pollutants treatment of environmental.

Switching Direction of Liquid Flow by Amplification of Wetting Properties of Partilly Roughened Prism Array

Superhydrophilic and superhydrophobic surfaces inspired from nature have been developed for anti-fogging, anti-reflecting, water-repelling, anti-icing and self-cleaning films. The amplification of the hydrophilicity and hydrophobicity originates from surface roughness, particularly from their multiscale geometry. In this paper, we present a strategy to exploit the enhancement of wetting properties on roughened surfaces for switching the direction of unidirectional liquid flow on an opened microchannel. We exploit a unidirectional liquid spreading on a hierarchical two-face prism array, which have the symmetry in microscale while having the asymmetry in nanoscale. Based on the switching the direction of the lower critical angle, the transition of the surface chemistry results in the change of flow directions in an open channel.

Smart, reusable labels for assessing self-cleaning films.

Novel, reversible (reusable) photocatalyst activity indicator labels, which undergo a rapid colour change when in contact with a photocatalytic film via the photoreduction of methylene blue contained within the label's adhesive, are explored as a method for assessing the activity of self-cleaning glass in situ and the laboratory, using digital photography.

A Simple Route to Morphology-Controlled Polydimethylsiloxane Films Based on Particle-Embedded Elastomeric Masters for Enhanced Superhydrophobicity

We present a simple route for controlling the surface morphology of polydimethylsiloxane (PDMS) films based on a standard replica molding technique incorporating a microparticle-embedded elastomeric master for enhancing surface wetting properties. The elastomeric masters are simply prepared by embedding microparticles (MPs) firmly into a surface of PDMS substrates using an abrasive air-jetting (AAJ) that can be potentially scaled up to large-area fabrication. The surface geometries of the PDMS masters can be easily controlled by using MPs with different shape and size in the AAJ process, resulting in easy control of the surface morphologies and resultant wetting and optical properties of the PDMS films after replicating. The PDMS masters are found to be highly durable, enabling repeated use to produce superhydrophobic PDMS films with similar characteristics. In addition, the fabricated PDMS films retain almost constant properties even under repetitive compressing and stretching deformations thanks to the mechanical robustness enabled by their all-elastomeric architectures. We show that the fabricated PDMS surfaces can be potentially employed as self-cleaning films in glass-based applications, even with complex surfaces, owing to their enhanced wetting properties, fairly good optical transparency, and superior mechanical stability.

Hydrophobization and characterization of internally crosslink-reinforced cellulose fibers

Transforming hydrophilic cellulose fibers into hydrophobic, non-hygroscopic fibers could potentially lead to a variety of new products, such as flexible packaging, self-cleaning films and strength-enhancing agents in polymer composites. To achieve this, softwood cellulose pulp was chemically modified with successive chemical treatments. First the C2 and C3 hydroxyl groups of the glucose units were selectively oxidized by periodate oxidation to reactive dialdehyde units on the cellulose chain, followed by a Schiff base reaction with 1,12-diaminododecane to crosslink the microfibrils within the fiber wall. This was done, because introducing high levels of alkylation resulted in fiber disintegration, which could be prevented by crosslinking. After internal crosslinking a second Schiff base reaction was performed with butylamine. This procedure yielded highly hydrophobic and low-hygroscopic cellulosic materials. The modified cellulose fibers were investigated by a variety of techniques, including Fourier transform infrared spectroscopy, nuclear magnetic resonance, field-emission scanning electron microscopy, thermogravimetric analysis, X-ray diffraction, moisture sorption and water contact angle measurements. The water uptake of the fibers after being modified reduced from 4 to around 1 %. Various reaction conditions were studied for optimum performance.

The Mechanisms on the Transmittance and Photo-Catalytic Activity of TiO<sub>2</sub> Film Doped with SiO<sub>2</sub>/PEG

TiO2sol was doped with SiO2and modified by adding PEG. Using the sol the self-cleaning antireflective films were prepared with dip-pulling method. Field emission scanning electron microscope (FESEM) test results showed that the TiO2-SiO2film surface distributed a large number of pores and grooves. The visible spectrophotometer testing found that the transmittance showed a decreasing trend with the increase of TBTT: TEOS ratio. Meanwhile, the photo-catalytic efficiency showed an increase firstly and then decreased with the TBTT: TEOS ratio increasing. When the ratio of TBTT: TEOS was 1:5, the films transmittance and the photo-catalytic efficiency can reach up to 97.1% and 13.5% respectively. The mechanisms on the transmittance and photo-catalytic activity of PEG-modified TiO2-SiO2films were studied. It was found that PEG-modified TiO2-SiO2films have higher porosity, transmittance and hydrophilicity than pure TiO2film. Furthermore, photo-catalytic activity of the TiO2-SiO2film was enhanced.

Metal oxide thin films and nanostructures for self-cleaning applications: current status and future prospects

Surfaces that exhibit reversible wettability toward water are extremely important for a variety of technological applications. In this context, the development of superhydrophobic and superhydrophilic surfaces for self-cleaning applications has been receiving a great deal of attention in the last few years. In this review, an overview of the current state-of-science and technology of self-cleaning surfaces is presented. The current understanding of physics of wetting leading to surfaces with predictive, controllable and reversible wettability is first presented. The review then focuses on materials, mainly metal oxides and their composites, employed for self-cleaning applications. It is shown that, although conventionally oxides and polymers are considered for self-cleaning applications, recent developments point toward the use of artificially engineered surfaces with hierarchical roughness. Applications of self-cleaning films in non-conventional areas such as protection of fabrics, solar cells and structures related to cultural heritage are discussed. The review ends with an outlook for the future in terms of science and technology of self-cleaning surfaces.

Superhydrophobic surfaces fabricated from nano- and microstructured cellulose stearoyl esters

Robust, superhydrophobic and self-cleaning films were fabricated using nano- or microstructured cellulose fatty acid esters, which were prepared via nanoprecipitation. The superhydrophobic films could be coated on diverse surfaces with non-uniform shapes by distinct coating techniques.

Photocatalysis Property of Titania-Based Thin Films with Covalent Grafting PANi as Sensitizer

Polyaniline sensitized titania thin film were fabricated by spin-coating from Ti-precursor containing PEG as surfactant via a sol-gel technique, in which titania-sol was synthesized by hydrolysis of tetrabutyl titanate in water-alcohol solution. The films were characterized by X-ray diffraction, high resolution transmission electron microscopy, field-emission scanning electron microscopy, thermal gravity-differential scanning calorimetry, and tested under self-made photocatalysis instrument. The results showed the synthesized porous films provided the active surfaces for self-polymerization of aniline. Compared with bare TiO2 thin films, the composite films displayed a enhanced photocatalytic activity and could be the potential photocatalyst for indoor air decontamination or self-cleaning films.

Photoinduced superhydrophilicity of TiO2 thin film with hierarchical Cu doping

Hydrophilic Cu–TiO2 thin films with a gradient in the Cu concentration were prepared on glass by layer-by-layer dip-coating from TiO2 precursors. The effects of the Cu doping on the structure and properties of TiO2 self-cleaning thin films are discussed. The Cu gradient markedly affects the hydrophilicity of the films, with the water contact angle significantly reduced compared with those of the pure or uniformly doped TiO2 thin films. This enhanced hydrophilicity is explained by the more efficient absorption of the solar light and by the reduced recombination of photoexcited electrons and holes in the TiO2 films containing a gradient of Cu dopants.

Solution-derived photocatalytic films for environmental cleaning applications

When photocatalytic water treatment is concerned, suspended catalyst in the aqueous phase is usually more efficient than immobilized on an inert support, but in the former case an undesirable separation/recycling step is needed. We have therefore concentrated on the preparation of immobilized catalysts in the form of films on glass and aluminium supports. The low-temperature sol-gel processing route to obtain transparent thin TiO2/SiO2 films for self-cleaning purposes and thicker TiO2/SiO2 coatings for efficient removal of pollutants in water and air are presented. The synthesis is based on a production of a nanocrystalline titania sol with a silica binder that after deposition does not require thermal treatment at high temperatures. Depending on the target application, some specific synthesis parameters and photocatalytic activity testing conditions are illustrated. For water-cleaning coatings fast kinetics is required, which was achieved by addition of a highly active titania powder into the sol. The same preparation procedure was used to prepare efficient air-cleaning coatings. On the other hand, self-cleaning films were thinner and transparent to keep the original appearance of the substrate and they solidified at ambient conditions. Advanced methodologies to evaluate photocatalytic activity of the films were applied.