The structural studies of Ag containing TiO2–SiO2 gels and thin films deposited on steel
Graphical abstract
Introduction
TiO2–SiO2 thin films find an application in many branches because of their unique and valuable properties. Titania-silica thin films exhibit deodorizing or self-cleaning effects under ultraviolet light [1], [2], [3] so they act as self-cleaning materials, also for outdoor applications. They possess good mechanical properties [4], the low thermal expansion coefficient, the high thermal stability, the photo-induced hydrophilicity [5] and the good chemical durability. Titania-silica materials seem to be attractive for photocatalytic and display applications [6], these materials are also applied as anticorrosive protective layers [7] or antireflection films [8].
TiO2–SiO2 materials prove themselves to possess good bioactivity what cause their wide application in medicine, as protective or bioactive coatings on implants, dental materials and others. The addition of silver remaining in nanoscale in the structure gives TiO2–SiO2 thin films another antibacterial properties what is especially important from the medical point of view. Human beings are often infected or destroyed by different microorganisms as bacterium or virus so creating anticorrosive and antibacterial layers gives double benefit: a protection of the basement material and more important, the protection of human health [9], [10], [11].
There is a wide spectrum of methods allowing to obtain titania-silica layers on different type basements like spray pyrolysis, chemical vapor deposition (CVD), sol–gel method [12], [13], [14], [15] and liquid phase deposition (LPD). In this work, the sol–gel method and the dip-coating technique were chosen to synthesize titania-silica bulk samples and thin films on steel.
The main aim of this work was to obtain gels and thin films from the system TiO2– SiO2 and TiO2–SiO2/Ag and to study their structure. Because it is usually difficult to evaluate structure and properties of thin films that is why corresponding samples were obtained in two forms: as thin films and bulk samples. Furthermore, thin films were deposited in two ways: from two component sol TiO2–SiO2 and as “a sandwich” structure, covering the basement with one component SiO2 or TiO2 sol. Coatings were doped with Ag by the addition of its nanoparticles into the SiO2 sol. The second objective was to examine the influence of Ag dopants on the SiO2 matrix structure of synthesized samples in comparison with Ag free ones as was reported [3]. Additionally, the study of bioactivity properties of Ag containing titania-silica coatings were performed.
The following precursors were applied to prepare the initial sols: titanium tetraisopropanolane: Ti(C3H7O)4, tetraethoxysilane TEOS: Si(C2H5O)4 whereas silver nitrate AgNO3 was used to incorporate Ag into the silica sol.
To prepare 7% (weight) SiO2 sol, two initial suspensions were prepared, one containing TEOS disoluted in half amount of ethanol, and second, containing the remaining volume of ethanol C2H5OH, HCl and distilled water. Both solutions were mixed and aged for 2 h separately. After that, the final solution was prepared and homogenized for another 30 min.
To synthesize the pure titania sol, the similar procedure as in case of the silica sol was applied with ethanol used as a solvent with addition of CH3COOH acid as the catalyst. At the beginning, the suspension of Ti(C3H7O)4 and half amount of alcohol was obtained and in parallel, the second solution of the remained CH2H5OH and CH3COOH was prepared. Next, the both suspensions were mixed very slowly and then homogenized also for 30 min.
The two component TiO2–SiO2 sol was synthesized by preparing the separate solutions of titanium tetraisopropanolane and the tetraethoxysilane with ethanol, and then mixing them in appropriate SiO2:TiO2 = 1:1 M ratio. Finally, the suspension containing the remained part of C2H5OH, HCl and distilled water was added to TiO2–SiO2 mixture and the sol prepared was once more thoroughly homogenized.
SiO2/Ag sol was synthesized in a similar way as a pure silica one. The first solution containing TEOS and a half amount of ethanol was thoroughly mixed with the second suspension of the proper amount of AgNO3 disoluted in ethanol with addition of HNO3 and distilled water. Ag concentration in ssynthesized amples varied then between 1.0 and 2.0% mass.
The surface of the steel basement has to be cleansed before the coating deposition. In case of stainless steel type 1H18N9 of composition (wt %): Fe <68%, 17–19% Cr, 8–10% Ni, 2% Mn, Si 0.8%, C 0.12%, the purification included the chemical degreasing and the etching which influenced on the adhesion, the tightness and the durability of synthesized thin films.
The samples were obtained in a form of bulk ones and coatings. All bulk samples were prepared by the annealing (in air) of SiO2 gel, SiO2/Ag gel, TiO2–SiO2 gel and TiO2–SiO2/Ag one at different temperatures, from 500 °C, through 800 °C up to 1000 °C, for half of hour at each temperature.
Coatings on steel were deposited by a dip-coating method, samples were pulled out with the stable speed 20 cm/min. Thin films were synthesized in two different ways: as single layers obtained one by one from one-component SiO2, then TiO2 sols (the type of “sandwich” structure), or deposited as multi component coatings from two component TiO2–SiO2 sol. The addition of Ag was fulfilled by applying an additional SiO2/Ag layer. All coatings were annealed in air at 500 °C, after the deposition of each two layers to improve their hardness and adhesion.
FTIR spectra were obtained with Digilab Division FTS-60V spectrometer by BIO-RAD and Vertex 70v by Bruker. They were measured for KBr pellets (bulk samples) and as transmission spectra and as reflection ones (for thin films). All spectra were obtained in the range of 400–4000 cm−1, with 4 cm−1 resolution. X-ray diffraction patterns were obtained in Panalytical X'Pert diffractometer, applying CuKα radiation and standard Brag-Brentano configuration. SEM images were observed with NOVA SEM 200 FEI microscope. AFM surface images were obtained in Bruker Multimode 8 Atomic Force Microscope, using Peak Force Tapping mode and SNL-10 probes.
Section snippets
FTIR studies of TiO2 bulk samples (gels)
In the spectra of the annealed TiO2 gel (Fig. 1) one can distinguish a group of bands typical of stretching vibrations of Ti–O bonds, in the range of 543–616 cm−1. Additionally, as in the case of most gels, bands due to the vibrations of OH− groups at about 3430 cm−1 and of molecular water at 1630 cm−1 are observed.
FTIR studies of SiO2 and SiO2/Ag bulk samples (gels)
In the spectra of samples heated up to 1000 °C (Fig. 2), vibrations typical of Si–O connections are observed: at 473–467 cm−1 due to O–Si–O bending vibration, in the range of
Conclusions
- 1.
FTIR spectra were measured as the absorption, transmission and reflection ones and all types of measurements gave very similar results concerning bands positions. Primarily the bands typical for Si–O connections were observed in all spectra containing silica so the structure was based on the skeleton built of Si and O atoms, with other cations/atoms incorporated in it. The bands connected with the presence of non-tetrahedral atom (Ag) in the structure of gels and thin films were observed at
Acknowledgments
The financial support of the AGH University of Science and Technology grant no 11. 11. 160. 767.
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