Download Tailoring crystalline structure in N-doped TiO2 thin films: application to photocatalytic and biological reactions.

Tailoring crystalline structure in N-doped TiO2 thin films: application to
photo-catalytic and biological reactions.
Rod Boswell, Christian Sarra-Bournet and Christine Charles
Space Plasma Power and Propulsion Group
PRL/PSPE, Australian National University
Canberra, ACT, Australia
e-mail: [email protected]
Abstract:
Photocatalytic reactions at the surface of titanium dioxide (TiO2) have been
attracting much attention in view of their practical use in environmental, energy
and biomedical applications [1]. However, the bandgap of TiO2 corresponds to
maximal wavelength absorptions in the UV. Thus, the development of photocatalysts exhibiting high absorption under visible light should allow a more
efficient use of the solar spectrum. The objective of this study was to obtain TiO2
doped with nitrogen (N-doped) by plasma sputtering in a helicon reactor.
Changing the impinging ion energy resulted in changes in the crystalline
nanocrystals from anatase to rutile. Moreover, introduction of nitrogen also
resulted in changes in the crystalline structure. Results will be presented on the
photon absorbance and interaction with blood simulants.
Keywords: bio-active thin films, photo catalysis, plasma deposition
1. Introduction
Titanium dioxide (TiO2) is one of the most studied
coating materials due to its many outstanding
properties. This material is hard with a high thermal
and chemical stability. Moreover, it presents a wide
electron energy band gap, transparency throughout
the visible spectrum and a high refractive index from
UV to far infrared. These properties led to the use of
TiO2 for a wide range of applications such as in
electronic devices, optical and hard coatings,
photocatalysts, and biomedical applications.
Titanium dioxide is found naturally in three
crystalline phases: anatase, rutile and brookite, as
well as in an amorphous phase. The amorphous
phase is sought after for good quality optical films
while anatase phase is known to have a higher
photocatalytic activity than the other crystal types of
TiO2, due to its larger bandgap energy (3.2 eV vs.
3.0 eV) and rutile is characterized by higher
refractive index and hardness.
Plasma sputtering deposition techniques allow the
substrate to be kept at low temperature (< 200°C)
during the process, which allows low melting point
materials to be used as substrates, such as plastics. It
also allows control and modification of the structure
and properties of the TiO2 films via the process
parameters like power, pressure, gas mixtures,
partial pressures, bias, temperature, etc.
The present paper studies the potential of using lowfield helicon plasma for a sputtering process of
titanium dioxide. The influence of the ion densities
on the characteristics of the TiO2 coatings, including
the density, refractive index, crystallinity, crystallite
size and surface elemental composition have been
investigated using a number of analytical techniques.
The overall objective of this study was to develop an
efficient one-step plasma deposition process to
obtain crystalline TiO2.
2 Experiment
The Helicon-Assisted Reactive Sputtering (HARES)
experimental setup is shown in Figure 1. The
diffusion chamber (180 mm High, 340 mm i.d.
aluminium) is equipped with vacuum gauges, optical
ports and holders for target and substrate (10 cm
diameter). The bottom chamber (350 mm High, 400
mm i.d. stainless steel) houses the pumping bench.
The two chambers are connected by a glass tube
(150 mm long and 150 mm diameter). A doublesaddle field type helicon RF antenna is located
around the tube, which is supplied with RF power
between 50-1000W at 13.56 MHz coupled to the
plasma via a RF matchbox. High-purity oxygen
(99.9 %) and argon (99.997 %) was injected in the
diffusion chamber by two 100-sccm mass flow
controller. The base pressure in the diffusion
chamber before deposition was 1x10-3 Pa.
Figure 1.Schematic of the HARES setup.
The substrate and target holders are situated 5 cm
above the exit of the source tube (z = 5 cm) and can
be easily moved in the radial direction. For the
experiment presented here, the target was set in
place directly in the centre of the diffusion chamber
(z = 5 cm, r = 0 cm) while the substrate holder was
moved to different target-to-substrate distance
(TSD) (z = 5cm, r = 3-11 cm). This configuration
allows a constant ion density on the target, thus a
constant sputtering rate, while changing the ion
density impinging on the substrate during the sputter
deposition process. A DC bias voltage was applied
to the target while the substrate was placed on an
electrical-floated substrate holder with no cooling.
3. Surface characterization techniques
As-deposited samples were analyzed while some
samples were also annealed after the plasma
deposition process in an oven furnace in an air
atmosphere at 300 °C for three hours. This
temperature was chosen as it is low enough as to
favour crystallisation of the metastable anatase phase
and not the rutile phase. The film refractive index
and thickness was measured by a spectroscopic
reflectometer using a Cauchy model. Validation of
the thickness measurements with the ellipsometer
was performed with a mechanical profiler. Density
of the deposited coatings was calculated by
measuring the mass difference of the sample before
and after the plasma deposition and coating volume
was integrated from a thickness mapping using the
spectroscopic reflectometer. The crystalline structure
of the TiOx samples was probed by x-ray diffraction
(XRD) using a conventional x-ray diffractometer in
a 2θ scan. Measurements were obtained at a 0.5degree angle, 40kV, 30mA with a Co Anode (Kα =
0.17889 nm) with detections scans between 20-100
degrees. To determine the stochiometry of the TiOx
films, Rutherford Backscattering Spectroscopy
(RBS) measurements were performed. For the TiOx
samples, A 2-MeV He ion beam was produced by
the accelerator and directed onto the coating. The
back-scattered beam was collected and analyzed.
3. Plasma Characterisation
Using an electron temperature of 4 eV, typical for
this type of discharge, a threefold increase in plasma
density can be found over a narrow range of
magnetic field values (0.8 mT < B0 < 5 mT) (Figure
2). This peak in density is thought to be due to a
helicon wave being launched in the system. In argon,
using the helicon dispersion relation for a magnetic
field of 2 mT and a plasma density of 1.5 x 1017 m-3,
the calculated wavelength of the helicon wave is ∼
27 cm, twice the antenna length (13 cm), which is a
well known matching condition for efficient helicon
wave excitations. This suggests a direct transition
from capacitive (E) to helicon wave (W) mode. The
transition back to E mode at higher fields is not so
well understood, but appears to be related to a
decrease in radiation resistance from the antenna,
and thus a lower power transfer efficiency. It can
also be observed that the plasma density obtained in
the 10% O2 / 90% Ar discharge are relatively similar
to the pure argon case while a pure oxygen discharge
sees a modest peak and a decrease in density for
higher magnetic field.
Figure 2. Langmuir probe ion densities
measurements at the exit of the source (z = 0, r = 0)
as a function of the magnetic field in the source for
various gases ( 100% Ar, 10% O2, 90% Ar, 
100% O2)
To operate in the low magnetic field helicon mode it
was elected to work at a magnetic field in the source
of 3.2mT. The coil in the diffusion chamber was also
used to confine the plasma and enhances the
uniformity
of
the
process.
Figure 3. Ion density (m-3) mapping in the diffusion
chamber for the optimum sputtering conditions
(10% O2, 90% Ar, 3.2 mT in the source)
A plasma density mapping of the diffusion chamber
is drawn in Figure 5. As it can be observed, the
plasma density is roughly uniform in the axial
direction with a decreasing profile in the radial
direction.
4. Thin film properties
Thin films of TiO2 were sputtered in various
configurations of plasma density and the results of
RBS and XPS will be presented in the talk. When
the positive ion bombardment on the substrate is
high, TiO2 rutile crystalline films can be obtained
directly from a one-step low temperature plasma
sputtering process, thus no post high-temperature
treatment step is required. Since those films are
already highly crystallized, they are thermally stable
and conversion into anatase phase is highly
unfavourable. TiO2 amorphous coatings can also be
obtained by lowering the ion density over the
substrate. Flowing nitrogen through the system lead
to changes in the band gap and improvements in the
photo-catalytic properties of the films.
The results demonstrate that helicon-assisted
reactive sputtering using a low-field helicon mode is
a convenient technique for the deposition of TiO2
films which enables tailoring of the crystalline
structure by controlling the deposition parameters in
a wide range to application demands.