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.

The thickness characterization of transparent protective coatings on functional, transparent materials is often problematic. In this paper, a toolbox to determine the thicknesses of a transparent coating on functional window films is presented. The toolbox consists of a combination of secondary ion mass spectrometry and profilometry and can be transferred to other transparent polymeric materials. A coating was deposited on designed model samples, which were characterized with cross-sectional views in transmission and in scanning/transmission electron microscopy and ellipsometry. The toolbox was then used to assess the thicknesses of the protective coatings on the pilot-scale window films. This coating was synthesized using straightforward sol-gel alkoxide chemistry. The kinetics of the condensation are studied in order to obtain a precursor that allows fast drying and complete condensation after simple heat treatment. The shelf life of this precursor solution was investigated in order to verify its accordance to industrial requirements. Deposition was performed successfully at low temperatures below 100 °C, which makes deposition on polymeric foils possible. By using roll-to-roll coating, the findings of this paper are easily transferrable to industrial scale. The coating was tested for scratch resistance and adhesion. Values for the emissivity (ε) of the films were recorded to justify the use of the films obtained as infrared reflective window films. In this work, it is shown that the toolbox measures similar thicknesses to those measured by electron microscopy and can be used to set a required thickness for protective coatings.

In recent years, functional window films have gained much interest. These windows films can be used for a wide variety of applications. Some common examples of these applications are UV blocking, window tinting, insulation and shatter resistance. Most of the window films are functionalized by thin layers on a flexible substrate. It is also known that these thin layers are not stable in air and prone to deterioration caused by moisture and abrasion. This drawback can be overcome by the deposition of an additional scratch resistant coating. In this research, a protective silica coating with an increased mechanical strength and flexibility is deposited on a Polyethyleneterephtalate (PET) film. The latter is also functionalized with a thin metal oxide layer through sputtering and is often used for functional window films. Incorporation of hybrid organic-inorganic species inside the silica precursor, results in an increase of mechanical strength and, simultaneously, an increased flexibility and durability of the silica coating. The precursor solution was deposited onto the substrate by reverse gravure roll-coating. The ratio between web speed and application speed was optimized to obtain sufficient wetting of the film. A temperature program for inline drying of the coating is set up. The temperature program is important to transfer the results to industrial setups where the time available for drying is limited. One must keep in mind that the temperature should be high enough to obtain complete drying of the precursor solution while it should be low enough to avoid melting or breakdown of the organic substrate.

In this work, we present preparation and stabilization methods for highly crystalline TiO2 nanoparticle suspensions for the successful deposition of transparent, photocatalytically active TiO2 thin films toward the degradation of organic pollutants by a low temperature deposition method. A proof-of-concept is provided wherein stable, aqueous TiO2 suspensions are deposited on glass substrates. Even if the processing temperature is lowered to 150-200 °C, the subsequent heat treatment provides transparent and photocatalytically active titania thin layers. Because all precursor solutions are water-based, this method provides an energy-efficient, sustainable, and environmentally friendly synthesis route. The high load in crystalline titania particles obtained after microwave heating opens up the possibility to produce thin coatings by low temperature processing, as a conventional crystallization procedure is in this case superfluous. The impact of the precursor chemistry in Ti(4+)-peroxo solutions, containing imino-diacetic acid as a complexing ligand and different bases to promote complexation was studied as a function of pH, reaction time and temperature. The nanocrystal formation was followed in terms of colloidal stability, crystallinity and particle size. Combined data from Raman and infrared spectroscopy, confirmed that stable titanium precursors could be obtained at pH levels ranging from 2 to 11. A maximum amount of 50.7% crystallinity was achieved, which is one of the highest reported amounts of anatase nanoparticles that are suspendable in stable aqueous titania suspensions. Decoloring of methylene blue solutions by precipitated nanosized powders from the TiO2 suspensions proves their photocatalytic properties toward degradation of organic materials, a key requisite for further processing. This synthesis method proves that the deposition of highly crystalline anatase suspensions is a valid route for the production of photocatalytically active, transparent films on heat-sensitive substrates such as polymers.

A thin film of crystalline titanium dioxide applied as a self-cleaning coating is a versatile system due to its properties of being transparent and colourless, chemically stable, non-toxic and relatively cheap. Most importantly it has the ability to become photocatalytically active and superhydrophilic upon UV-irradiation. These self-cleaning coatings can mitigate staining, fogging and the odour and deterioration caused by dirt. This results in the fact that self-cleaning coatings are an important topic in research, and are highly commercially relevant. In this work, we extend the use of these coatings to polymers, since this opens up a large market of self-cleaning coatings for applications such as (touch)-screens, visors, light-domes and noise barriers on highways. This transition to polymer substrates, however, poses some challenges. It is imperative for the photocatalytic properties that the TiO2 thin film is of the anatase crystal phase. In order to induce crystallinity to a film deposited from a precursor solution, these films need to be subjected to elevated temperatures (>400°C). This is incompatible with the use of polymers as substrates, since these high temperatures will cause deformation or deterioration of the polymer substrate. In order to circumvent this issue, films are deposited on the substrates from colloidal suspensions that already contain anatase nanocrystals. Other challenges are the wettability of the polymer surface by the colloidal suspension and the durability of the coating. Therefore, the use of chemical linkers to bind titania nanocrystals to the surface of a polymer substrate is explored. This covalent linking is envisioned to improve the durability of the final titania coating.

Diatom microalgae are used to fabricate silica–titania photocatalysts for air purification by means of a biological immobilization process into the diatom frustules.

A method to obtain photocatalytically active thin films of anatase nanocrystals on polymer substrates was explored. Anatase nanocrystals were synthesized by a fast hydrolysis synthesis in an apolar solvent and characterized with regard to their crystallinity, size, and dispersibility and the stability of the resulting suspensions. The stable titania nanocrystal suspensions were further processed for their use in polar solvents using ligand exchange. Oleic acid was exchanged for 3-aminopropyltriethoxysilane (APTES), resulting in aqueous suspensions of charge-stabilized nanocrystals. These were adapted for use as coating suspensions for surface-treated PMMA substrates in order to obtain thin films containing anatase nanocrystals covalently coupled to the surface of the PMMA substrates. Thereby, the ligand exchange was beneficial for increasing the compatibility and durability of the inorganic/organic composite, by the formation of a covalent amide bond between the silane ligands on the nanocrystals and the carboxylic acid groups on the polymer substrate. The surface morphology, transparency, and photocatalytic activity toward the degradation of organic pollutants of the coatings, obtained through dip-coating, were evaluated.

TiO2 coatings can be used to create transparent, photocatalytically active, self-cleaning surfaces and therefore have a very high industrial relevance. Functionality in this kind of TiO2 layers is only obtained after crystallization of the material at temperatures above 400 °C. This means that chemical deposition approaches do not allow deposition on heat-sensitive substrates such as polymers. Therefore, it is important to investigate innovative deposition processes that can significantly reduce the minimal temperature required for crystallization and full processing. In this work, we studied the synthesis of crystalline titania nanoparticles by microwave-assisted solvothermal methods. The use of microwaves allows to optimize the production and energy efficiency of the synthetic process. Once stable suspensions of anatase nanoparticles are obtained, these are optimized for use in ink-jet printing devices. The fact that the crystalline building blocks for the layer are now already present in the precursor inks, allows efficient deposition of transparent titania coatings at reduced temperatures. Weathering/durability tests and extensive characterization of the photocatalytic activity and hydrophilic behaviour of these layers was performed to analyze the performance and processing limits for this kind of titania coatings.

In this paper, we present a microwave-assisted, hydrothermal method for the synthesis of TiO2 suspensions. These were obtained from Ti4+ aqueous precursor solutions using titanium-isopropoxide with EDTA and triethanolamine or tetraethylammonium hydroxide by applying a microwave treatment at temperatures between 100 and 140 °C. The influence of the ligands, pH, reaction temperature and time on the particle size and crystallinity was investigated and discussed. A selection of the TiO2 suspensions was deposited on glass substrates using piezoelectric driven ink-jet printing. The rheological properties of the suspensions were evaluated against the relevant criteria for ink-jet printing and their jetting behavior was analyzed. The ink-jet printed layers were heated at temperatures between 150 and 500 °C to create transparent titanium oxide layers. The photocatalytic activity of the finally obtained layers was analyzed by following the decomposition of a methylene blue solution under UV illumination. The presence of preformed titania nanoparticles makes it possible to obtain photocatalytic active coatings at reduced heating temperatures. This can widen the application range of these functional layers to heat-sensitive substrates such as polymers. The influence of the heat treatment temperature on the film roughness, thickness and photocatalytic activity was also studied.