Spray Deposition of Au/TiO2 Composite Thin
Films Using Preformed Nanoparticles
W. Wang, K. Cassar, S.J. Sheard, P.J. Dobson, P. Bishop, I.P. Parkin,
and S. Hurst1
Abstract. A single-step process to deposit a composite film of Au nanoparticles
in a TiO2 matrix has been investigated by a spray deposition technique. A preformed gold colloid was used as a precursor along with the titanium precursor to
deposit gold/titanium composite films onto glass. The composite films were deposited onto Pilkington float glass using spray coating. The deposition temperature was varied from 200 °C up to 550 °C. UV-vis spectra of the films showed
that the surface plasmon absorption maximum was red-shifted from 544 nm to
600 nm with increasing substrate temperatures, corresponding to a colour transition from red to blue in transmission. This process, based on a spray technique,
offers a simple, rapid and low-cost approach to large-area deposition. The single-step route leads to a homogeneous composite film and controllable properties
of the spray solution.
1 Introduction
There has been intense research interest in nanocomposites of metal nanoparticles
in a dielectric matrix [1-3]. Noble metal nanoparticles, e.g. Au, Ag, generally
exhibit a strong absorption peak in the visible range of the spectrum, due to the
surface plasmon resonance effects, which has led to application in optical devices
[4, 5]. The tuneability of the plasmon resonances can be achieved by careful
W. Wang and S.J. Sheard
Department of Engineering Science, Oxford University, Oxford, United Kingdom
K. Cassar and P. Bishop
Johnson Matthey Technology Centre, Reading, United Kingdom
P.J. Dobson
Oxford University Begbroke Science Park, Oxford, United Kingdom
I.P. Parkin
Department of Chemistry, University College of London, United Kingdom
S. Hurst
Pilkington Technology Centre, Lathom, United Kingdom
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design of the nanostructure such as its size, shape and composition. Dielectric
materials, especially transition metal oxides, have been extensively investigated
due to their functional properties, such as photocatalysis, electrochromism and solar control [6-9]. Nanocomposites comprising the matrix and embedded metal
particles therefore bear the functionalities of both phases and could be carefully
designed for giving the desired material properties. Such kind of nanocomposites
has shown potential application to glass surfaces to modify the optical properties
for window glass products used for solar control in buildings [10]. Here we
describe an approach for the deposition of a Au-nanoparticle/TiO2 composite
thin film using preformed gold nanoparticles. The method is based on the spray
deposition technique which is simple, flexible and capable of scaling up for large
area deposition with potentially a low processing cost. Au nanoparticles can be
deposited alone, or together with a matrix precursor such as TiO2 for a composite
thin film.
2 Experimentals
Apparatus set-up. The set-up for the laboratory-scale spray deposition comprises
three major parts: the syringe pump as the injecting system, the spray head and the
hotplate. Basically, the precursor liquid is first injected by the syringe pump and
projected from the needle which is underneath the nozzle. The atomisation of the
liquid droplets happens at the meeting point of the needle tip and the nozzle which
ends up with an aerosol that is sprayed and impacts onto the substrate heated by a
hotplate. The hotplate is integrated with an x-y table for fast, large-area uniform
deposition. By varying and optimising the reaction conditions such as the injected
liquid flow rate, compressed air pressure, hotplate temperature, and raster scanning speed, it is possible to fabricate a uniform thin film on a large area substrate.
Materials and experimental procedure. Gold nanoparticles were synthesised
and dispersed in water. A one-pot dispersion for depositing the composites of Au
nanoparticles in TiO2 was prepared by mixing 0.1 M titanium precursor with gold
colloids in water at a molar ratio of 10:1 to 20:1.
Typically, the precursor solutions were injected by the syringe pump under a
constant flow rate (1-3 ml/min) and spread out by the compressed air at a pressure
of 20-30 psi (138-206 kPa). The atomised droplets were sprayed onto the Pilkington float glass substrates (4mm thick, with SiO2 barrier layer on top) which were
heated at temperatures in the range of 150-550 °C. The coated glass samples received further heat treatment in most cases by placing them in a furnace at temperatures in the range of 200-600 °C for 30-60 minutes to remove any remaining
organic ligands and further enhance the decomposition of the precursor.
Characterisation. UV-vis spectra were obtained using a Varian Cary 5000 UVvisible-NIR spectrometer. Thickness measurements were carried out using a
Veeco Dektak 6M stylus profilometer. In order to obtain information about the
morphology of the particles and their size and distribution in the nanocomposite
Spray Deposition of Au/TiO2 Composite Thin Films
397
films, transmission electron microscopy (JEOL 2000FX, JEOL 4000HR) and
scanning electron microscopy (JSM 6300, JSM 840F) micrographs were taken.
3 Results and Discussion
The initial gold colloids were transparent and a deep red colour. The UV-vis
spectra of the gold colloid showed a surface plasmon resonance peak at 533 nm.
TEM imaging of the colloid on a holey carbon film showed the gold nanoparticles
Fig. 1 TEM images of gold nanoparticles of a mean size of 20 nm and UV-vis spectrum of
spray deposited thin film of gold nanoparticle on Pilkington float glass
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have a mean diameter of 20 nm with a narrow size distribution. Gold nanoparticle
thin films were spray deposited onto glass using this solution. The thin films were
deposited at 180 °C and without further annealing. The films were red in appearance when viewed in transmission and an intense surface plasmon absorption peak
at 542 nm was displayed in the UV-visible spectrum (Fig. 1).
The one-pot solution containing gold colloids and the titanium precursor in water was used for spray deposition of Au/TiO2 nanocomposite thin films. The solution shows a clear deep red colour and the stability was maintained for over two
weeks. Coating deposition was first carried out at 200 °C on Pilkington float
glass, followed by annealing at 400 °C or 600 °C respectively. Colour changes
from red to purple and to blue in transmission was observed. Optical spectroscopy
shows an absorption peak shift from 543 nm to 572 nm and then to 599 nm, respectively (Fig. 2). The film thickness decreases from 728 nm to 320 nm, 240 nm
respectively. This is probably due to the effects of annealing which removes organic ligands as well as consolidation of the TiO2 matrix, which is assumed to result in a higher refractive index of the matrix material.
In order to compare the “spray followed by annealing” process, a direct spray
deposition without annealing was conducted using the same solution but at different
Fig. 2 Optical absorbance spectra of gold nanoparticle/TiO2 matrix thin film on Pilkington
float glass before and after annealing. The small feature at 380-400nm is believed to be associated with the tin on the float glass
Spray Deposition of Au/TiO2 Composite Thin Films
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deposition temperatures ranging from 200 °C to 550 °C. The composite films were
successfully deposited at all these temperatures with strong adherence to the glass
substrates. Surface plasmon absorption peaks obtained from the UV-vis absorbance
spectra show a red-shift from 544 nm to 600 nm with increasing substrate temperature (Fig. 3). We attribute this to the formation and densification of TiO2 anatase in
the matrix material with the increasing substrate temperature which gives an increase of the refractive index of the matrix and hence a red shift for the surface
plasmon condition [9].
Fig. 3 Measured normalised absorbance spectra of gold nanoparticles/TiO2 thin films deposited with substrate temperatures from 200 °C to 550 °C which shows a shift of the surface plasmon peak resonance with increasing substrate temperature (see inset). The peaks
show a red-shift from 545 nm to 600 nm
The Au/TiO2 composite film deposited at 400 °C was characterised and analysed for comparison. The UV-vis spectrum shows a surface plasmon absorption
peak at 589 nm, compared to the one that was sprayed at 180 °C and annealed at
400 °C showing a peak at 572 nm. This is probably because better consolidation
of TiO2 can be achieved when sprayed onto a hotter substrate. The film is transparent and blue in transmitted light. It is also robust under washing and has good
adherence to the glass substrate. The gold nanoparticles could not be removed
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from the TiO2 matrix either, indicating the gold nanoparticles are strongly embedded in the film. SEM micrographs of the Au/TiO2 thin film suggest that the gold
nanoparticles are evenly distributed in the TiO2 matrix. However, there is also
evidence of the film cracking probably due the thermal expansion coefficient
mismatch between the film and the substrate. Nevertheless, this experiment has
demonstrated the possibility of a single process of using a stable dispersion containing both gold nanoparticles and matrix precursors and spraying at a high temperature which is compatible and favourable in an industrial on-line process.
4 Conclusions
A single step process for spray deposition has been demonstrated using a stable
dispersion containing both gold nanoparticles and matrix precursors which can be
deposited at a high temperature. The optical properties of the composite films can
be tailored by varying the properties of metal nanoparticles and matrix precursors
as well as the deposition temperature. This process, based on a spray technique,
offers a simple, rapid and low cost approach to large area deposition. Deposition
using preformed nanoparticles allows the design of nanocomposite by tailoring the
properties of nanoparticles such as size and structure. The single-step route leads
to more homogeneous composite film and controllable properties of precursors.
This film deposition method can easily be scaled up and is compatible with current industrial on-line processes.
Acknowledgments. The work is funded by Technology Strategy Board of UK. We thank
Guillermo Benito, Troy Manning and Paolo Melgari for helpful discussion as well as Professor Patrick Grant for courtesy of providing the spray equipment.
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