Functional behaviour of TiO2 films doped with noble metals
Summary (2 min read)
- Titanium dioxide (TiO2) is a transparent semiconductor material, with a high bandgap, varying between 3.0 and 3.4 eV 1-4 .
- Concerning its behaviour, TiO2 is known for its biocompatibility, non- toxicity, chemical stability, high hardness and high optical transmittance, combined with a high refractive index, between 2.4 and 2.9 5-10 .
- Among these phases, anatase is known for its excellent photocatalytic activity 12 and it is kinetically stable at low temperatures.
- Brittleness is an important feature for any kind of thin films system, which may restrict its use in some applications that require flexible substrates 15-17 .
- At the same time, the dispersion of such noble metals (Au, Ag) throughout the TiO2 matrix can create thin films with metallic nanoparticles (NPs), responsible for the so-called localised surface plasmon resonance (LSPR) 23 , which gives rise to a set of unique properties that enables the film system to be used in some appliactions that were fristly impossible when ussing pure oxide-type films.
2. Experimental details
- The deposition chamber is composed by two vertically aligned rectangular magnetrons, in a closed field configuration.
- The primary vacuum of the deposition chamber (with pressures of ~0.3 Pa) is achieved using a rotary vane vacuum pump, a Balzers Duo 012A.
- Three detectors were employed in the chamber; one located at a 140º scattering angle and two pin-diode detectors located symmetrically to each other, both at a 165º scattering angle respective to the beam direction.
- The morphology of the films was analysed by Scanning Electron Microscopy, SEM, in an Ultra-high resolution Field Emission Gun Microscope, NOVA 200 Nano SEM from FEI Company.
- Normalised wear rates were calculated after measurement of the worn track section for each sample, using a Taylor Hobson profilometer.
3. Results and discussion
- 3.1. Chemical, Morphological and Structural characterisation RBS analysis revealed an O/Ti atomic ratio very close to 2, suggesting the presence of a roughly stoichiometric TiO2 matrix.
- From now on, the films will be mentioned according to the names used in Table 2 (1 st column), corresponding to the noble metal content expressed in mole % (Au,Ag).
- From this point, both systems follow different tendencies concerning their microstructure when the noble metal (Au or Ag) is added to the TiO2 growing matrix.
- According to the set of results obtained by XRD one can clearly identify two different regions in both systems: an amorphous zone for lower Ag or Au concentrations (< 50 mol %), where the existing noble atoms/clusters did not aggregate to form crystalline domains, followed by a crystalline zone where the concentration of noble metal in the film is more important.
- Another important feature that is worth mention is the apparent contradictory results between SEM and XRD.
3.2. Mechanical Properties
- Figure 4 depicts the hardness (H) and reduced Young’s modulus (E*) as a function of the noble metal mol % in each identified zone (amorphous and crystalline, accordingly with the structural analysis).
- From Figure 4 one can observe that for the film with the smallest Au concentrations, the hardness and reduced Young’s modulus was higher than the TiO2 film.
- In the crystalline zone, the critical loads decrease to values even lower than those of the reference critical load values of TiO2 film.
- Nevertheless, the sample with the highest Another plausible explanation for the high wear rate of this sample is that the Ag particles may agglomerate and cause abrasive wear.
- The structural and morphological analysis of the films showed significant differences in the structural evolution during film growth.
- Regarding adhesion, the distinction between the two systems is not clear, only the sample with the smallest content of gold exhibits a much higher adhesion.
- A major important result is that the sample with the lower Au content (Au = 14 mol %) revealed the best functional performance under the performed tests, being the only one that improved the mechanical properties of the host matrix (TiO2).
- This research was sponsored by FEDER funds through the COMPETE program (Programa Operacional Factores de Competitividade) and by FCT (Fundação para a Ciência e a Tecnologia), under the projects PEST-C/FIS/UI607/2013 and PEst-C/EME/UI0285/2013.
- The authors also acknowledge the financial support by the project Nano4color - – Design and develop a new generation of color PVD coatings for decorative applications (FP7 EC R4SME Project No. 315286).
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Cites result from "Functional behaviour of TiO2 films ..."
...This can be attributed to the increase of the amount of Ag atoms reaching the film during the early stages of its growth, as also previously reported and discussed in other related works [34,44]....
...On the other hand, the Au/TiO2 system presents a dense and compact morphology, with very low surface roughness, regardless the gold pellets area, once again in accordance with some previous results [3,44]....
"Functional behaviour of TiO2 films ..." refers background in this paper
...Furthermore, the absorption bands in the visible spectrum range are the main feature associated with the presence of Au and Ag NPs.23 This effect can produce a palette of colours9 if one can tune the LSPR position, the bandwidth and peak height through changes in the size, distribution and shape of the NPs, as well as in the host dielectric matrix (such as TiO2)....
...Furthermore, the absorption bands in the visible spectrum range are the main feature associated with the presence of Au and Ag NPs.(23) This effect can produce a palette of colours(9) if one can tune the LSPR position, the bandwidth and peak height through changes in the size, distribution and shape of the NPs, as well as in the host dielectric matrix (such as TiO2)....
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