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Journal ArticleDOI

Functional behaviour of TiO2 films doped with noble metals

22 Feb 2016-Surface Engineering (Taylor & Francis)-Vol. 32, Iss: 8, pp 554-561

Abstract: To evaluate the effects of different concentrations of noble metal in a TiO2 matrix, different films of both Ag/TiO2 and Au/TiO2 systems were prepared. Mechanical and tribological characterisation was carried out to evaluate the coating response as a function of the noble metal composition and (micro)structure of the films. The overall set of results indicates that the amorphous films reveal better results than the crystalline ones. For the amorphous samples, the reduced Young's modulus and the adhesion critical loads followed similar tendencies in both sets of films. Wear rates were similar for all samples except for the one with the highest silver content. To improve brittleness of TiO2 films, the results seem to indicate that a slight metal doping is preferred, and Au proved to be a better choice than Ag. In fact, the sample with the lowest Au content revealed a better mechanical behaviour than the pure TiO2 film.
Topics: Amorphous solid (55%), Noble metal (54%)

Summary (2 min read)

1. Introduction

  • 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.

4. Conclusions

  • 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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1
Functional behaviour of TiO
2
films doped with noble metals
M.S. Rodrigues
1,2 *
, J. Borges
1,3
, C. Gabor
4
, D. Munteanu
4
, M. Apreutesei
5
, P. Steyer
5
, C. Lopes
1,2
,
P. Pedrosa
1,3,6
, E. Alves
7
, N.P. Barradas
8
, L. Cunha
1
, D. Martínez-Martínez
1
, F. Vaz
1,3
1
Centro/Dep. de Física, Universidade do Minho, Gualtar, 4710 - 057 Braga, Portugal
2
Instituto Pedro Nunes, Lab. de Ensaios, Desgaste e Materiais, Rua Pedro Nunes, 3030-199
Coimbra, Portugal
3
SEG-CEMUC, Mechanical Engineering Dep., Univ. of Coimbra, 3030-788 Coimbra, Portugal
4
Materials Science Department, Materials Science and Engineering Faculty, Transilvania
University of Brasov - Romania, 500036, 29 Eroilor Blvd.
5
INSA de Lyon, MATEIS Laboratory, Eq. CorrIS/SNMS, Bât. B. Pascal, 7 Av Jean Capelle,
69621-Villeurbanne, France
6
Universidade do Porto, Faculdade de Engenharia, Departamento de Engenharia Metalúrgica e de
Materiais, Rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal
7
Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Av.
Rovisco Pais, 1049-001 Lisbon, Portugal
8
Centro de Ciências e Tecn. Nucleares, Instituto Sup. Técnico, Univ. de Lisboa, E.N. 10 (km
139.7), 2695-066 Bobadela, Portugal
* Corresponding author mprodrigues@ipn.pt ; marcopsr@gmail.com; +351 966 458 876
Abstract
To evaluate the effects of different concentrations of noble metal in a TiO
2
matrix, different films
of both Ag:TiO
2
and Au:TiO
2
systems were prepared. Mechanical and tribological
characterization was carried out to evaluate the coatings response as a function of the noble metals
composition and (micro)structure of the films. The overall set of results indicates that the
amorphous films reveal better results than the crystalline ones. For the amorphous samples, the

2
reduced Young’s modulus and the adhesion critical loads followed similar tendencies in both sets
of films. Wear rates were similar for all samples except for the one with the highest silver content.
To improve brittleness of TiO
2
films, the results seem to indicate that a slight metal doping is
preferred, and Au showed to be a better choice than Ag. In fact, the sample with the lowest Au
content revealed a better mechanical behaviour than the pure TiO
2
film.
Keywords: Magnetron sputtering; Thin films; Brittleness; Noble metal doping; Titanium dioxide
Nomenclatures / abbreviations:
Mole %

Poisson ratio
Ag
Silver
Ar
Argon
at.%
Atomic %
Au
Gold
DC
Direct Current
E*
Reduced Young's modulus
ECG
Electrocardiography
EEG
Electroencephalography
EMG
Electromyography
fcc
Face-centered cubic
H
Hardness
LSPR
Localised Surface Plasmon Resonance
n
Number of mole
NPs
nanoparticles
PVD
Physical Vapor Deposition
RBS
Rutherford Backscattering Spectrometry
SEM
Scanning Electron Microscopy
SERS
Surface-Enhanced Raman Spectroscopy
Si
Silicon
TiO
2
Titanium dioxide
XRD
X-Ray Diffraction
1. Introduction
Titanium dioxide (TiO
2
) is a transparent semiconductor material, with a high bandgap, varying
between 3.0 and 3.4 eV
1-4
. Concerning its behaviour, TiO
2
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
. Due to these characteristics, TiO
2
is widely used in

3
several optical devices, biomedical applications, dye sensitised solar cells, photo-electrolysis,
photocatalysis, as a coating for anti-fogging and evn as a self-cleaning coating material for glasses
5, 6, 11
. In basic terms, TiO
2
exists in both amorphous and crystalline forms. Specifically, the two
most important crystalline forms are anatase and rutile, both showing a tetragonal-like structure
lattice
12
. Among these phases, anatase is known for its excellent photocatalytic activity
12
and it is
kinetically stable at low temperatures. In the rutile form, TiO
2
has good structural stability at high
temperatures, together with a higher refractive index
13
. On the other way around, in the
amorphous form,TiO
2
has high blood compatibility and thus it is often used in several types of
biomedical applications
14
.
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
. The case of thin film systems designed for
polymeric base electrodes or sensors to be used in several types of biomedical applications,
including electroencephalography, EEG, and electrocardiography ECG
18
and electromyography,
EMG
15, 19, 20
, as well as some kinds of biological sensors
21
are particularly noticeable examples,
that are being develop in the group for some time. To overcome this drawback and improve
flexibility of the coated devices, the tailoring of the elastic modulus
22
of the thin film systems by
adding silver (Ag) or gold (Au) to such oxide material, is one of the most promising routines that
can be optimized. At the same time, the dispersion of such noble metals (Au, Ag) throughout the
TiO
2
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.
Furthermore, the absorption bands in the visible spectra range are the main feature associated with
the presence of Au and Ag nanoparticles
23
. This effect can produce a palette of colours
9
if one
can tune the LSPR position, the bandwidth and peak height through changes on the size,
distribution and shape of the NPs, as well as on the host dielectric matrix (such as TiO
2
).
Consequently, the tailoring of the optical properties of the nanocomposites is possible
24-26
.

4
Beyond colouring that was used for several centuries in the windows of the medieval cathedrals
and ancient Roman glass cups, advanced applications of such plasmonic nanocomposite materials
include: solar cells, optoelectronic devices, biosensors, gas sensors, magnetic storage, energy
conversion, optical filters, photocatalysis and surface-enhanced Raman spectroscopy (SERS)
27-35
.
As mentioned before, to overcome the limitation of experimental conditions, LSPR thin films
have to be developed with higher robustness and flexibility in order to support mechanical stress.
Taking this into account, and in order to evaluate the effects of different concentrations of Au and
Ag in a TiO
2
matrix, several thin films of Ag:TiO
2
and Au:TiO
2
systems were produced by
reactive magnetron sputtering. This physical vapour deposition technique was chosen since it is
known to be an environmentally friendly coating process that provides durable materials with low-
cost production, when compared to the traditional preparation methods
42
. The determination of
hardness, reduced Young’s modulus, wear rate and critical loads
36-38
was performed in order to
evaluate the brittleness of the coatings and to correlate the mechanical behaviour with the
composition and (micro)structure of the films
17, 39-44
.
2. Experimental details
The Au:TiO
2
and Ag:TiO
2
thin films were prepared by reactive DC magnetron sputtering
45
, on Si
(Boron doped p-type, <100> orientation, thickness of 525 µm), in a custom-made deposition
system
45
. The system is composed of a cylindrical deposition chamber (~40 dm
3
), a pre-chamber,
a vacuum system, a gas flow controller, an electrical system and a control unit. The deposition
chamber is composed by two vertically aligned rectangular magnetrons, in a closed field
configuration. To produce the films, only one magnetron was used, powered by a Hüttinger PFG
7500 DC (maximum output of 7.5 kW). 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. The
secondary vacuum (with pressures of ~10
4
Pa) is obtained using a TurboMolecular vacuum
pump, model PTM 5400 (400 L.s1) from Alcatel. To measure the gas pressure, the system is
controlled by a Leybold Penningvac PTR225 (10
7
10 Pa) and a Leybold Sky-Pirani Gauge

5
TR090 (10
2
10
5
Pa). The films were prepared using a substrate holder positioned at 70 mm from
the target, in a rotation mode-type (9 rpm). A titanium target (200×100×6 mm
3
, 99.8% purity)
containing different amounts of Au, or Ag, pellets (1 mm thick and 4.5 mm in diameter) incrusted
in the erosion track of the Ti target was used. The number of Au or Ag pellets was changed to
vary the flux of Au or Ag atoms towards the substrate and thus obtain films with different noble
metal concentrations. The power supply connected to the target was set to operate in the current
regulating mode, using a current density of 100 A.m
-2
on the Ti-Au, or Ti-Ag, target. The films
were prepared using an atmosphere composed of Ar and O
2
leading to a total pressure of about
4×10
-1
Pa. To promote and enhance the adhesion of the films, the Si substrates were treated using
an in situ etching process in Ar (pressure of about 5×10
-1
Pa) under a pulsed DC current of 0.5 A
with a duty cycle of 30%, during 1200 s. Find the etching and deposition parameters summarized
in Table 1.
Table 1 Etching and deposition parameters to produce the coatings
The
dep
osit
ion
par
ame
ters,
such as the target potential and current, gas pressure, argon flow and reactive gas flows, were
monitored before and during the deposition, using a Data Acquisition/Switch Unit Agilent
34970A, equiped with a multifunction module (334907A), where the cables (from analog outputs
Parameter
Etching
Deposition
Power Source
Pulsed DC Current (T
on
= 1536 ns,
f = 200kHz)
DC Current
Ar (sccm)
70
60
O
2
(sccm)
-
7.5
Time (s)
1200
3600
Current (A/m
2
)
25
100
T (ºC)
100
100
Bias
-
Grounded
P
Work
(Pa)
510
-1
410
-1
P
Base
(Pa)
~10
-5
10
-5

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