1. Introduction
The photocatalytic layers are nowadays of great interest for water [
1] and air [
2] purification. Titanium dioxide (TiO
2) layers are particularly popular because of their relatively high efficiency, non-toxicity, and affordability. TiO
2 have been widely applied for contaminant remediation and microorganism destruction [
3,
4].
TiO
2 is often used as a coating on hard and durable materials such as glass. Recently, there has been an interest in depositing it on flexible substrates such as polyurethanes, polyesters, polyvinyl chlorides, and others, for photovoltaic, textile, and paper industries, and others. The main problem is the stabilisation of TiO
2 on the substrate in order not to release it from the photocatalytic layer into the environment [
4,
5,
6]. Generally, thermal methods of hundreds of degrees Celsius [
7] are used to increase the adhesion and photocatalytic activity. Another problem lies in the TiO
2 photocatalytic activity that disturbs the organic materials onto which TiO
2 is deposited. The main directions of the research are to protect the substrates from UV and photocatalytic degradation, to ensure sufficient penetration of the pollutants to the photocatalyst, to provide flexibility, and to develop the process of preparing such a photocatalytic system that could be used on an industrial scale [
6,
8].
One way to protect the substrate from degradation by thermal treatments and photocatalytic processes is to use a suitable binder as a matrix. The matrix serves as a mechanical support for TiO2 and as a protective layer for the substrate. However, the bonding of the binder itself may not be a sufficient condition for a functional photocatalytic layer. Other treatments are needed that may also be thermal treatments, but it would be possible to use the photocatalytic processes themselves in the layer using UV-irradiation or short-term exposure to plasma for low-temperature treatment.
Many areas extensively use mesoporous oxide thin films as functional and structural materials. They include protective and low-dielectric constant layers, selective gas permeation membranes, wettability layers, and gas sensors [
9,
10,
11].
In this work we developed and investigated polysiloxane as the TiO
2 anchor matrix and we obtain the titania-siloxane composition (TSC), which can be used for deposition on flexible substrates without the need for thermal treatments which damage the substrate. Siloxane (oligomer or polymer) can be obtained from organosilicon precursors, which can be further doped withTiO
2 particles. Siloxane serves as a matrix for anchoring TiO
2, and, at the same time, it can retain porosity to successfully adsorb the pollutants to the surface of the photocatalyst grains deposited deeper in the layer [
6,
12]. The siloxane used in this study contains a certain proportion of methyl moieties that provide solubility and prevent gelling. On the other hand, the methyl moieties reduce the transfer of electrons generated by the TiO
2 and significantly deteriorates the photocatalytic activity of TiO
2. There is also a certain proportion of hydroxyl groups in siloxanes. The hydroxyl group, as a residual group, induces moisture adsorption through hydrogen bonding when exposed to moisture [
13]. Moisture (H
2O) is an important part of the photocatalytic processes taking place in titanium dioxide. Moisterure-binding hydroxyl groups improve the wettability of the surface and the photocatalytic activity itself and is a better supply of pollutants in the aqueous medium [
14]. In order to remove the organic matter from the siloxane and improve the transfer of the electrons, siloxane must be completely mineralised towards amorphous silica surface. This step is traditionally performed by thermal annealing at hundreds of °C. On the other hand, this approach is not compatible if flexible and thermally-sensitive substrates such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are used. Low-cost substrates are favorable for future generation manufacture of emerging technologies, including flexible and printed electronics. Therefore, it is important to investigate novel low-temperature methods compatible with rapid and low-temperature post-treatment of siloxanes as replacements for traditional thermal annealing that is not compatible with flexible electronics.
In this work we also investigated two non-thermal methods for post-processing of titania-siloxane composition (TSC) layers: UV-irradiation and open-air plasma. UV-irradiation was already successfully tested for the fabrication of amorphous TiO
2 thin films. UV-irradiation at room temperature leads to a higher conduction band minimum level of the film and a smaller amount of hydroxyl group at the film surface, compared to the thermal-assisted (100–250 °C) UV-annealing or the thermal-only annealing (500 °C). [
15]. Plasma treatment can be used for calcination and removal of organic residues from sol–gel and generation of mesoporous films [
16,
17,
18,
19,
20]. The plasma technique is more attractive because it has many advantages, such as low processing temperature, short processing time and inexpensive equipment [
9].
We studied the properties of TSC in synergy with UV-irradiation and plasma treatment as the techniques for non-thermal curing of photocatalytic layers to improve photocatalytic activity. The main parameter for the mineralisation of siloxane was the study of the decrease of methyl groups by the Fourier-transform infrared spectroscopy (FT-IR) method. The photocatalytic activity was monitored by voltammetric measurements and photocatalytic degradation of AO7.
3. Materials and Methods
3.1. Synthesis of Siloxane
The starting substance for the synthesis of siloxane was methyltriethoxysilane (MTEOS) (Alfa Aesar, 98%, Haverhill, MA, USA). MTEOS was hydrolysed with acidic water. Ethanol was formed during the hydrolysis and subsequently distilled. Siloxane was extracted with diethyl ether (Penta, 99.7%), and it was dissolved in absolute ethanol (Penta, 99.8%, Prague, Czech Republic) after evaporation of the extracting agent. The siloxane solution in ethanol was stored at a temperature below 0 °C [
6]. The expected chemical reaction can be summarised by the following equation [
26]:
3.2. Preparation of TSC
Two series of samples were prepared for experiments, as shown in
Table 3. TiO
2 (P25, Sigma Aldrich, 99.7%, St. Louis, MO, USA, particle size ≤ 25 nm, SSA 45–55 m
2/g [
27]) was dispersed in dowanol and a siloxane 20% solution was added. The resulting suspension was further diluted with hexanol. The TSC were printed with a Dimatix (DMP-2800) material printer on soda-lime glass and FTO-coated glass.
3.3. Mineralisation of the Printed TSC on the Substrate
The siloxane/TiO
2 coatings were mineralised by two methods: UV-irradiation and plasma treatment. UV-irradiation was generated by a Sylvania UV lamp
Figure 9 (mercury, 125 W, Budapest, Hungary). The radiation intensity was set and held at 9 mW∙cm
−2. Samples were placed under 5 mm of distilled water and irradiated for 0, 15, 30, 60, 90, 150, and 210 min.
The plasma treatment was performed by RPS400 (Roplass s.r.o., Brno, Czech Republic) equipped by dielectric barrier discharge (DBD) with a coplanar arrangement of the electrode system: A diffuse coplanar surface barrier discharge (DCSBD) plasma unit. The DCSBD is capable of generating a thin surface plasma of very high-power density up to 100 W∙cm
−3 at very low temperature ~70 °C in open air environment at atmospheric pressure. The details of the DCSBD plasma can be found in Homola et al. [
28,
29]. The plasma exposure times of siloxane surfaces in this study were 0, 2, 4, 8, 16 and 32 s.
3.4. Characterisation of Siloxane
The properties of siloxane solutions in ethanol were studied by means of various methods. Since the ink-jet printing is compatible only to a certain range of viscosity (10–30 cP) and other rheologic parameters, it is important to determine the viscosity, surface tension, and size of agglomerates, in order to verify if the compositions have the desired properties and do not suffer complications during printing.
The concentration range of siloxane (10, 20, 30, 40, 50%) was used for viscosity measurements carried out on a rotating rheometer AR-G2 (TA Instruments) with a roller sensor in a cylinder at 25 °C. The surface tension was measured by tensiometer KSV Sigma 701 using the du Noüy ring method with a platinum ring.
The SEC-MALS-dVI-dRI system was used for GPC measurements. Chromatographic part (Agilent Technologies, Santa Clara, CA, USA): consisting of isocratic pump, degasser, autosampler, column and UV-VIS detector. Detectors (Wyatt Technology, Dernbach, Germany): DAWN HELEOS II: multi-angle light scattering (MALS), VISCOSTAR II: differential viscometer (dVI), OPTILAB T-REX: differential refractometer (dRI). The 10% siloxane solution was further diluted with absolute ethanol 1:1.
The thermal gravimetric analysis was performed with Instrument SDT Q600. The siloxane solution was dried to 100 °C and the solid was exposed to a temperature of up to 1300 °C in the air with a step 10 °C/min. Thermal analysis was also performed to give us insight into changes during the thermal mineralisation of siloxane.
3.5. Characterisation of TSC and Printed Layers on Glass
Brunauer–Emmett–Teller (BET) method was employed to determine the specific surface area (SSA) of TSC. BET was performed on Autosorb iQ. Prior to BET analysis, the samples were dried at 100 °C.
FT-IR Nicolet iS5 was used in order to measure the decrease the methyl groups after non-thermal curing by UV-irradiation and plasma treatment. The TSC samples on soda-lime glass were measured on FT-IR using the Omnic program. The spectral region characteristic of the 2950 cm
−1 and 2895–2840 cm
−1 methyl groups was observed [
30].
The photo-electrochemical characterization of TSC on the FTO-coated glass was performed by linear sweep voltammetry at room temperature using a two-electrode setup with the 1 cm
2 titania patches. The printed FTO slide was scratched with a diamond knife and thus two isolated FTO strips were created. One strip with the printed titania patch, served as the working electrode and the opposite naked FTO strip as the counter electrode. This setup was fitted into a custom build quartz cuvette. The cuvette was filled with 0.1 M perchloric acid (conductivity 36 mS∙cm
−1) and fitted onto an optical bench equipped with a fluorescent UV-A lamp emitting a broad peak centered at 365 nm (Sylvania Lynx-L 11 W). A magnetic stirrer was placed beneath the cuvette and a magnetic flea inside the cuvette provided efficient electrolyte mixing. The lamp emission was monitored by Gigahertz Optic X97 Irradiance Meter with a UV-3701 probe and the irradiance was set to 2 mW∙cm
−2 by adjusting the lamp-to-cuvette distance. Measurements of generated photocurrents were performer with an electrometer build on the basis of National Instruments Labview platform and supplying a linear voltage gradient of 10 mV/s from −0.5 to 2 V and measuring the generated currents down to submicroampere range [
24].
Photocatalytic degradation test of TSC on glass with 12 cm2 E38-10AD patches was performed by degradation recrystallization AO7 (Centre for Organic Chemistry) solution in water. The quartz cuvette was filled with 2 mg/L aqueous solution of AO7 and fitted onto an optical bench equipped with a fluorescent UV-A lamp emitting a broad peak centered at 365 nm (Sylvania Lynx-L 11 W). The absorbance of the resulting solution at a 4 cm optical path was 0.5 at a wavelength of 480–490 nm. A magnetic stirrer was placed beneath the cuvette and a magnetic flea inside the cuvette provided efficient solution mixing. The lamp emission was monitored by Gigahertz Optic X97 Irradiance Meter with a UV-3701 probe and the irradiance was set to 2 mWcm−2 Measurements of decrease in absorbance AO7 were performer with a spectrometer Red Tide USB650 (Ocean Optics, Prague, Czech Republic) and an Ocean View program.
The TSC samples printed on FTO-coated glass were also examined by the TESCAN Mira3 XMH scanning electron microscope. Coatings were printed on FTO, in order to prevent charging and deposition of conductive layer onto TSC.
4. Conclusions
This contribution presents the non-thermal curing of titania siloxane coatings by two atmospheric methods. Titania siloxane coatings are promising alternative for other photo-catalytically active layers on substrates that cannot withstand processing at temperatures >150 °C, e.g., thermally sensitive polymers and paper. This work demonstrates and compares slow and efficient UV-irradiation, and extremely rapid plasma treatment, as methods for low-temperature sintering of titania siloxane photo-catalytically active coatings. It was also investigated that the main mechanism of the non-thermal curing is the removal of the −CH3 methyl groups from the siloxane chain while the bulk structure of the titania content remained unaffected. Although, UV-irradiation showed more efficient and led to the removal of most of the detectable methyls from the siloxane surface, the plasma treatment resulted in much faster processing times in the order of tens of seconds. This, on the other hand, is promising for the implementation of the method into fast roll-to-roll production line. Since both methods, UV and plasma, are scalable to several meters in width, the employment of any of them or even the combination of UV and plasma treatment could be of interest development of new large scale thin-film photocatalysts.