Photoactive Coatings Research Articles

Photoactive self-cleaning zinc oxychloride coatings

Zinc oxychloride cement (ZOC) is considered a promising quick-drying material for fabricating photoactive coatings composed of three components: zinc oxide (ZnO), zinc chloride (ZnCl2), and water. However, its massive application has not been implemented due to the lack of research about its stability and mechanical performance. Thus, this article aims to provide insights about the self-cleaning photoactivity of ZOC coatings incorporated with titanium dioxide (TiO2) nanoparticles to boost the photocatalytic performance. Two stoichiometries of ZnO:ZnCl:H2O (112 and 415) were studied to investigate the influence of the molar ratio of the components on the physical and chemical properties. According to these results, no significant differences were observed in the physical and chemical properties between the two stoichiometries studied; however, the mechanical properties were affected. The coatings crystallized as simonkolleite Zn5(OH)8Cl2·H2O with a laminar hexagonal morphology, show high surface area, and absorbs light in the UVA region. The self-cleaning activity was evaluated using four model pollutants: Rhodamine B, Methylene blue, Orange G, and Carbon under accelerated weathering conditions. All the coatings were photoactive to decompose the pollutants from their surface and the addition of TiO2 nanoparticles did not significantly enhance the self-cleaning efficiency, but it improves the mechanical performance of the coatings. These results could be associated with the fact that TiO2 works as filler for voids in the coatings without affecting its settings. The stability of the coatings was confirmed after the exposure to accelerated weathering at high humidity conditions, which places them as an attractive, low-cost, and easy to apply self-cleaning material.

Multi-target Pulsed Laser Deposition (PLD) Technique for the growth of μm-to-nm scale Photo-active Thin-film Coatings

There has been an unprecedented increase in the growth of photonic components over the last 25 years based on different photonic materials; each having structural/functional limitation in integrated devices. The challenge is that the semiconductors are grown inside MBE chambers, whereas the polymeric waveguides are fabricated by spin-coating. By comparison, glass and crystal-based materials are processed via sputtering and sol-gel techniques. None of these materials processing techniques, therefore, are compatible for a single-step device fabrication, due to the incompatibilities of chemical and physical properties of individual materials. A solution for overcoming the materials limitation is to develop a multi-materials deposition chamber which allows sequential/heterostructure growth on a substrate, without compromising the structural, spectroscopic, and device performances. The rare-earth-ion doped glass- and crystal-based devices are pumped with semiconductor lasers, suggesting that the glass-semiconductor devices might perform better when structurally integrated which may also help in reducing the pump-power for achieving efficient population inversion. We explain the applications of PLD for controlling the structure of thin-films grown on inorganic and metallic substrates for photonic device and photo-active coatings for biological applications, respectively. Examples of materials deposited on dissimilar substrates are discussed with applications such as photonic devices and photo-bioactive surfaces for sensing.

Photoactive coating based on waterborne polyurethane and carbon quantum dots as a prevention strategy for bacterial resistance

This study aims to develop photoactive coatings based on carbon quantum dots (CQDs) and waterborne polyurethane (WPU), capable of preventing the formation of bacterial biofilms. CQDs synthesized by microwave-assisted pyrolysis showed spherical morphology with an average size of 10.6 ± 3.1 nm, containing chemical groups with oxygen and nitrogen on the peripheral surface and photoluminescence at 460 nm and quantum yield of 63.2 % under excitation at 360 nm. Subsequently, CQDs were incorporated during WPU polymerization to form nanocomposites. The WPU/CQDs nanocomposites showed superior mechanical, rheological, and thermal properties concerning the pure WPU, due to the interaction of the nanoparticles with the polymeric matrix. The nanocomposites exhibited transparency and high photoluminescence, confirming the efficiency of the polymeric matrix in protecting the nanoparticles against photoluminescence extinction in the solid state. In addition, the WPU nanocomposite generates reactive oxygen species (ROS) after irradiation with blue light. The materials did not show cytotoxicity for BALB/3T3 murine fibroblasts after 24 h incubation. After illumination, the nanocomposite showed photodynamic activity against Pseudomonas aeruginosa (Gram-negative), thus validating its potential as a photoactive coating with sterilizing properties.

Photocatalytic TiO2 nanosheets-SiO2 coatings on concrete and limestone: An enhancement of de-polluting and self-cleaning properties by nanoparticle design

In this study, we develop TiO2-SiO2 nanocomposites that can be employed as photoactive coatings to create surfaces with antipollution and self-cleaning properties in urban buildings. In particular, anatase titanium dioxide with a high percentage of {001} facets, commonly called titania nanosheet (TNS), was used as active phase. This approach allows us to improve the antipollution properties of similar sols based on P25. The obtained TNS were intensively characterized by UV–Vis diffuse reflectance absorption, X-Ray diffraction and transmission electron microscopy to rationalize the sol synthesis and fully understand the photocatalytic performances. Our results clearly show that after treating the titania nanosheets with NaOH to remove adsorbed residual fluoride from the synthesis, they can be easily dispersed and the photocatalytic activity is dramatically improved. This material has been shown to effectively reduce concentrations of NOx, soot and organic dyes under simulated sun light radiation.

Singlet Oxygen Generation in Microcapillary Optical Elements with Photoactive Coatings

The spectral-luminescent properties of oxide nanocoatings containing ZnO and Ag nanoparticles and the processes of singlet-oxygen generation in these coatings are studied. A microchannel photoactive cell for water and air purification systems is developed based on the obtained experimental data. The developed microchannel photoactive cell is an optical element made of quartz glass with channels of given shape, size, and spatial position, which contain zinc and magnesium nanocrystals and silver nanoparticles. A high efficiency of singlet-oxygen generation by ZnO and ZnO–MgO–Ag films and microchannel cells under UV (370 nm) and violet light (405 nm) irradiation is demonstrated.

Cytotoxicity, Antioxidant, Antibacterial, and Photocatalytic Activities of ZnO-CdS Powders.

In this work, ZnO–CdS composite powders synthesized by a simple chemical precipitation method were thoroughly characterized. The morphological, structural, compositional, photocatalytical, and biological properties of the prepared composites were investigated in comparison with those of the pristine components and correlated with the CdS concentration. ZnO–CdS composites contain flower-like structures, their size being tuned by the CdS amount added during the chemical synthesis. The photocatalytic activity of the composites was analyzed under UV irradiation using powders impregnated with methylene blue; the tests confirming that the presence of CdS along the ZnO in composites can improve the dye discoloration. The biological properties such as antioxidant capacity, antibacterial activity, and cytotoxicity of the ZnO, CdS, and ZnO–CdS composites were evaluated. Thus, the obtained composites presented medium antioxidant effect, biocidal activity against Escherichia coli, and no toxicity (at concentrations less than 0.05 mg/mL for composites with a low CdS amount) for human fibroblast cells. Based on these results, such composites can be used as photocatalytic and/or biocidal additives for photoactive coatings, paints, or epoxy floors, which in their turn can provide a cleaner and healthier environment.

3D-Printed Photoactive Semiconducting Nanowire–Polymer Composites for Light Sensors

We demonstrated 3D-printed photoactive composites for light sensors. The composites consist of semiconducting polymer nanowires dispersed in an insulating polymer matrix through a thermo- or photocuring process. The nanowires formed percolated networks for charge-carrier collection at a very low nanowire volume concentration. Photodetectors had been fabricated with such composites and obtained a low leakage current density of 25 nA cm–2, a large conductivity change of 18025 times upon light irradiation (1 sun), and a high specific detectivity of 4.5 × 1010 Jones. The composite precursors before curing had been used as paints on different surfaces including wood, cardboard, and glass fiber cloth to create photoactive coatings on these surfaces. The precursors had also been used for 3D printing to create photoactive structures with a mechanical modulus of 4 GPa and an ultimate strength of 36 MPa.

Optimized photoactive coatings prepared with functionalized TiO2

Functionalization of TiO2 nanoparticles with silane coupling agents was investigated aimed at low-temperature photoelectrode manufacturing for solar driven water splitting application. Different silanes were grafted on the surface of TiO2 in toluene solvent under mild condition. The electrodes were prepared with spin coating by dispersing functionalized particles in DMAc onto FTO glass and dried under vacuum atmosphere at low temperature. UV–Vis spectroscopy of TiO2 powder and its electrodes was studied, and it was found that the spectrum of the modified TiO2 slightly shifted to higher wavelengths. The electrode prepared with functionalized TiO2 showed photocurrent density of up to 0.14 mA cm−2 compared to 0.04 mAcm−2 for pristine TiO2 at 1.23 V, in the water oxidation reaction. The increase in photocurrent density was due to better binding of the TiO2 particles to the substrate resulting in better charge collection observed under SEM. To enhance the photoelectrochemical efficiency, heat treatment was performed and 300 °C was found to be the best heat treatment temperature. The incident photon to current efficiency measurement exhibited an external quantum efficiency up to 4.9% for this heat-treated electrode. Mott-Schottky was plotted to examine the flat band potential. The result showed that the modification resulted in a decrease in the flat band potential suggesting that the charge recombination loss is lower compared to neat TiO2 electrode.

Antimicrobial surfaces with self-cleaning properties functionalized by photocatalytic ZnO electrosprayed coatings.

Photoactive coatings of sol-gel ZnO suspensions were electrosprayed on glass substrates to produce self-cleaning antimicrobial functionalized surfaces. ZnO-functionalized materials exhibited a uniform external surface consisting of a pattern of microspheres with diameters in the 100-300 nm range. Electrospray allowed surface densities up to 0.30 mg cm-2 that displayed considerable hydrophilicity. Water contact angle decreased with UV irradiation to values below 10°. Two different UV doses were tested by adjusting the irradiation time to simulate Summer-Spring and Winter-Fall conditions. The functionalized coatings showed excellent photocatalytic properties towards the photodegradation of Methylene blue. The electrosprayed surfaces also displayed antibacterial activity against Staphylococcus aureus, with >99.5% reduction in the number of culturable cells. The biocidal activity is attributed to the photogenerated reactive oxygen species on the surface of ZnO coatings and the bioavailable zinc ions produced from ZnO dissolution. The photoactive coatings kept surfaces free from bacterial colonization and biofilm formation.

TiO₂-SiO₂ Coatings with a Low Content of AuNPs for Producing Self-Cleaning Building Materials.

The high pollution levels in our cities are producing a significant increase of dust on buildings. An application of photoactive coatings on building materials can produce buildings with self-cleaning surfaces. In this study, we have developed a simple sol-gel route for producing Au-TiO2/SiO2 photocatalysts with application on buildings. The gold nanoparticles (AuNPs) improved the TiO2 photoactivity under solar radiation because they promoted absorption in the visible range. We varied the content of AuNPs in the sols under study, in order to investigate their effect on self-cleaning properties. The sols obtained were sprayed on a common building stone, producing coatings which adhere firmly to the stone and preserve their aesthetic qualities. We studied the decolourization efficiency of the photocatalysts under study against methylene blue and against soot (a real staining agent for buildings). Finally, we established that the coating with an intermediate Au content presented the best self-cleaning performance, due to the role played by its structure and texture on its photoactivity.

Photocatalytic activity of electrophoretically deposited (EPD) TiO2 coatings

This paper describes the application of electrophoretic deposition for air pollution removal using anatase as a photoactive coating. In this study, the anatase form of TiO2 has been applied to (1) fluorine-doped tin oxide (FTO)-coated glass; (2) 304L stainless steel; and (3) titanium substrates using isopropanol and acetylacetone-based solutions at 20, 40, 60 and 80 V. In order to increase the strength of the substrate–anatase interface without transforming the phase into rutile, samples were calcined at 450 °C for 2 h. The resulting coatings were characterised by Raman spectroscopy, X-ray diffraction, non-contact optical profilometry and scanning electron microscopy. The photocatalytic activity of the deposited coatings were evaluated in the gas phase for nitrogen dioxide (NO2) removal by electron ionisation mass spectrometry, whilst irradiated by light of wavelength 376–387 nm for 100 min. Anatase phase titania supported on a FTO-coated glass substrate showed the highest photoactivity for NO2 remediation. This was attributed to the formation of a three-dimensional nanostructure with properties determined by the deposition conditions. This work provides routes for the development of low-cost and large area photoactive coatings for pollution control.

Producing photoactive, transparent and hydrophobic SiO2-crystalline TiO2 nanocomposites at ambient conditions with application as self-cleaning coatings

Nowadays, the enhancement of atmospheric pollution is dramatically increasing the presence of soiling on buildings in every city of the world. Thus, the development of photocatalysts as self-cleaning coatings is a promising challenge. The first object of this work was to develop a simple synthesis route for obtaining SiO2-crystalline TiO2 nanocomposites at ambient temperature. Thus, it meets the requirements to produce photoactive coatings on buildings and, additionally, it can be used in other applications requiring low temperature. The second objective was to gain insights into the structure of these materials and to establish the relationship between their structure and their performance as photocatalysts.The synthesis process involves mixing titanium and silicon alkoxides in the presence of oxalic acid. An organic silica oligomer is also added to reduce surface energy and consequently, to give hydrophobic properties to the material. These products were applied as a coating on marble specimens in order to investigate their effectiveness. The results obtained highlight that oxalic acid acts a chelating agent of the titanium precursor, giving rise to a homogeneous TiO2–SiO2 material. In addition and most importantly, oxalic acid produces photoactive anatase crystals at ambient temperature. From our investigation on the structure of these materials, we conclude that the formation of independent domains of TiO2 inside of SiO2 matrix, and the anatase crystal size are key factors for improving the photoactivity of the coatings. We also conclude that the oxalic acid concentration plays a crucial role in the formation of this optimum structure.

Self-cleaning and depollution of fiber reinforced cement materials modified by neutral TiO2/SiO2 hydrosol photoactive coatings

Environmental pollution has an evidently adverse impact on the buildings that are constructed by the glass fiber reinforced cement (GRC) materials. In the present work, the stable, neutral TiO2/SiO2 hydrosols were prepared by using the Ti(SO4)2 as titanium source, HNO3 as peptizing agent, and SiO2 as stabilizer through a simple and low cost process. The morphologies and structures of TiO2/SiO2 hydrosol were further characterized by the TEM, SEM, XRD, and FTIR measurement. In the synthetic hydrosol, lots of nanoparticles with the diameters in the range of 10–20nm can be observed. TiOSi band were formed, as observed from the FTIR spectrum. The Na2O·SiO2 was detected from the SEM. After drying the TiO2/SiO2 hydrosol, the XRD shown that the TiO2 has an anatase structure and the SiO2 is amorphous. The TiO2/SiO2 hydrosol can be compactly coated on the GRC surface due to the existence of Na2O·SiO2 binder and exhibited high photocatalytic activity and stability in the degradation of Rhodamine B.

Low-Temperature Sol-Gel Synthesis, Spectroscopic Properties and Conductivity of the Thin Films of TiO2–CuO Nanoparticles

The highly photoactive conductive coatings of nanosized titania and a titania/copper(ii) oxide nanocomposite were prepared based on the low-temperature sol-gel transformations and interactions of multiphase colloidal system components.

Soft-chemistry synthesis of highly active TiO2-CuO heterostructures having high photoactivity and magnetic properties

TiO2 and CuO based heterostructures are of interest as film photocatalysts, spintronics materials, sensorics, etc. A new approach to obtaining titania nanosized crystallites in the anatase/brookite form and a titania/copper(II) oxide nanocomposite that allows the production of photoactive coatings has been proposed in the present work. The possibility of preparing well-crystallized materials at near-room temperature without the calcination stage is a feature of the proposed approach based on sol-gel transformations and the interaction of the multiphase colloidal system components. Using the method of dipping the polymer substrates into colloidal solutions, thin coatings are generated on the basis of the synthesized materials. The structural changes occurring upon the interaction of TiO2 and CuO nanoparticles during synthesis have been studied by dynamic light scattering and scanning probe microscopy methods. UV and visible absorption spectra of TiO2 and CuO films coated on a polymer substrate by a dipping method, as well as the spectral activity shift of TiO2/CuO heterostructure coatings, have been obtained. The magnetic force microscopy method was employed to investigate the magnetic regions of heterostructure films.

Production of nanocrystalline titanium dioxide photoactive coatings for decomposition of organic water pollutants in a flow reactor

An efficient and technologically simple technique has been proposed for the immobilization of nanocrystalline titanium dioxide on a quartz glass. It consists in deposition of the powder from its aqueous suspension with the addition of acetylacetone. The technique makes it possible to keep the phase composition and size of the initial titanium dioxide nanoparticles, the coating uniformity, and its stability under hydromechanical treatment. The photocatalytic efficiency of the coating in the reaction of the decomposition of a methyl orange dye in an aqueous solution in a flow reactor is shown.

Photoluminescent Epoxy/Gd2O3:Eu3+ UV‐cured Nanocomposites

AbstractUV‐cured photoluminescent coatings emitting in the visible spectral region are obtained by dispersing Gd2O3:4 mol% Eu3+ nanorods in three different commercially available photocurable epoxy resins. The effect of the concentration of the Gd2O3:4 mol% Eu3+ nanorods on the kinetics of the photocuring reaction is followed using real‐time FT‐IR. Thermal and the mechanical properties of the cured films are also investigated. Moreover, the effect of the matrix and nanorod concentration in the composites on the emission and excitation spectra and the 5D0(C2) decay time of Eu3+ is evaluated. The composite materials present original photoluminescence properties and demonstrate the potential of UV curing as a technique to develop smart photoactive coatings. magnified image

The production of anatase-rich photoactive coatings by plasma electrolytic oxidation

A promising approach to producing photoactive surfaces is to apply the plasma electrolytic oxidation (PEO) process to commercially pure titanium substrates, creating a highly porous surface with a high anatase content (>90% by volume). In this paper, work is outlined in which such PEO coatings have been produced under a variety of conditions, and their photo-catalytic efficiency assessed. Coating phase constitutions and surface microstructures have been investigated. Both treatment time and electrolyte concentration were varied, in an attempt to optimise the process for this application. Microstructures did, as expected, exhibit high anatase contents and porosity levels, but there were changes as the coatings became thicker and also a dependence on electrolyte composition. As an example of a photo-catalysed reaction, a study was made of the degradation of methylene blue dye under UV irradiation, in the presence of both untreated titanium substrates and PEO-treated surfaces. There are indications that both high anatase content and thicker coatings have beneficial effects on the catalytic efficiency, although a tendency was observed for the anatase content to fall as coating thickness was increased. There were also variations in the phosphorus contents of these coatings, although this did not appear to affect the photo-catalytic efficiency.

Titania nanocomposite polyethersulfone ultrafiltration membranes fabricated using a low temperature hydrothermal coating process

Thin mesoporous coatings of TiO 2 nanoparticles were developed for in-house and commercial PES membranes of varying pore sizes using a low temperature hydrothermal (LTH) approach. Titania sol–gel particles were dip-coated onto membrane substrates, followed by heat and UV treatments to extract the residual organic templates. Dip-coating parameters such as drying and holding time, dipping and withdrawal velocities, and the number of coating cycles were varied to optimise the microstructure and surface properties of the coating. Coated membranes exhibit a dual level hierarchical roughness and hydrophilicity which was maintained without continuous UV illumination. The organic templating agent (Pluronics F127) enhanced adhesion of the particles; however the heat treatment collapsed some pores in the tighter ultrafiltration membranes. Results showed that the coatings were mechanically robust and photoactive. Passive protein adsorption was reduced significantly on the TiO 2 coated surfaces. Filtration performance of coated and uncoated 500 kDa membranes was also investigated with humic acid as a model foulant, and an increase in flux recovery was observed during multiple fouling and cleaning cycles with the titania coated membrane. The LTH approach provides a platform for further surface functionalization of polymeric membranes to generate photoactive coatings, tuneable hydrophilicity, low fouling surfaces and novel surface architecture.

Titania and titania nanocomposites on cellulosic fibers: Synthesis, characterization and comparative study of photocatalytic activity

Titania and titania nanocomposites (doped by Ag, Zr, and Ag–Zr) were coated on cellulosic fibers via sol–gel dip-coating method. The resulted coated-fibers were characterized by X-ray diffraction, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS), and BET surface area measurement. Photocatalytic activity of the nanocomposites coated-fibers was determined by photomineralisation of methylene blue (MB) and eosin yellowish (EY) under UV–Vis light. The progress of photodegradation of dyes was monitored by diffuse reflectance spectroscopy. The XRD of all samples display anatase-type structure. All samples demonstrated photo-assisted self-cleaning properties when exposed of titania composites to UV–Vis irradiation. The Ag–Zr co-doped titania nanocomposite was found to be the most significant photoactive coatings in comparison with other samples. Our results showed that the synergistic action between the silver and the zirconium species in the Ag–Zr TiO 2 nanocomposite is due to both the structural and the electronic properties of the photoactive anatase phase. This report unequivocally indicates that modification of titania by co-doping is an effective method for increasing the photocatalytic activity.