Aerosol assisted chemical vapor deposition using nanoparticle precursors: A route to nanocomposite thin films

Article abstract of DOI:10.1021/ja055563v

Gold nanoparticle and gold/semiconductor nanocomposite thin films have been deposited using aerosol assisted chemical vapor deposition (CVD). A preformed gold colloid in toluene was used as a precursor to deposit gold films onto silica glass. These nanoparticle films showed the characteristic plasmon absorption of Au nanoparticles at 537 nm, and scanning electron microscopic (SEM) imaging confirmed the presence of individual gold particles. Nanocomposite films were deposited from the colloid concurrently with conventional CVD precursors. A film of gold particles in a host tungsten oxide matrix resulted from co-deposition with [W(OPh)₆], while gold particles in a host titania matrix resulted from co-deposition with [Ti(OPr)₄]. The density of Au nanoparticles within the film could be varied by changing the Au colloid concentration in the original precursor solution. Titania/gold composite films were intensely colored and showed dichromism: blue in transmitted light and red in reflected light. They showed metal-like reflection spectra and plasmon absorption. X-ray photoelectron spectroscopy and energy-dispersive X-ray analysis confirmed the presence of metallic gold, and SEM imaging showed individual Au nanoparticles embedded in the films. X-ray diffraction detected crystalline gold in the composite films. This CVD technique can be readily extended to produce other nanocomposite films by varying the colloids and precursors used, and it offers a rapid, convenient route to nanoparticle and nanocomposite thin films.

Full text of DOI:10.1021/ja055563v

Published on Web 01/13/2006
Aerosol Assisted Chemical Vapor Deposition Using
Nanoparticle Precursors: A Route to Nanocomposite Thin
Films
Robert G. Palgrave and Ivan P. Parkin*
Contribution from the Department of Chemistry, UniVersity College London, 20 Gordon Street,
London, WC1H 0AJ, United Kingdom
Received August 26, 2005; E-mail: i.p.parkin@ucl.ac.uk
Abstract: Gold nanoparticle and gold/semiconductor nanocomposite thin films have been deposited using
aerosol assisted chemical vapor deposition (CVD). A preformed gold colloid in toluene was used as a
precursor to deposit gold films onto silica glass. These nanoparticle films showed the characteristic plasmon
absorption of Au nanoparticles at 537 nm, and scanning electron microscopic (SEM) imaging confirmed
the presence of individual gold particles. Nanocomposite films were deposited from the colloid concurrently
with conventional CVD precursors. A film of gold particles in a host tungsten oxide matrix resulted from
co-deposition with [W(OPh)6], while gold particles in a host titania matrix resulted from co-deposition with
[Ti(OiPr)4]. The density of Au nanoparticles within the film could be varied by changing the Au colloid
concentration in the original precursor solution. Titania/gold composite films were intensely colored and
showed dichromism: blue in transmitted light and red in reflected light. They showed metal-like reflection
spectra and plasmon absorption. X-ray photoelectron spectroscopy and energy-dispersive X-ray analysis
confirmed the presence of metallic gold, and SEM imaging showed individual Au nanoparticles embedded
in the films. X-ray diffraction detected crystalline gold in the composite films. This CVD technique can be
readily extended to produce other nanocomposite films by varying the colloids and precursors used, and
it offers a rapid, convenient route to nanoparticle and nanocomposite thin films.
Introduction
Several strategies for the production of semiconductor/metal
composites have been developed. As shown in Scheme 1, they
Nanoparticles are a major research focus worldwide, and a
variety of different shapes and compositions can now be
produced.1,2 Many technological applications of these particles
will depend on their incorporation into a host matrix; the result
is a nanocomposite, which serves both to immobilize the
particles and to render them chemically inert. In addition, the
incorporation of nanoparticles into a matrix can change the
properties of both the particle and the host. A popular topic for
investigation is semiconductor/metal nanocomposites, especially
noble metal nanoparticles incorporated into a semiconductor
matrix.3-10 The presence of nanoparticles can alter the Fermi
level of the semiconducting host material and assist in photo-
generated charge separation, and this has been exploited in
improved photocatalysts,3,11,12 photoanodes,13 and photochromic
and electrochromic films.14,15
can be broadly divided into four categories:
Route 1 is the synthesis of the semiconductor matrix followed
by addition of preformed nanoparticles in a second step.
Examples of this strategy include spin coating or dip coating a
semiconductor film with a nanoparticle solution.3,14-16 The
nanoparticles are chemically bound to the surface or become
trapped within pores in the semiconductor.
Route 2 is the synthesis of the semiconductor matrix followed
by application of metal ions in a second step, leading to
formation of metal particles in situ, within the film. High energy
ion implantation6,17,18 and spin coating with a solution of metal
ions followed by photocatalytic reduction or heat treatment8,19
are examples of this strategy.
Route 3 is the synthesis of the semiconductor matrix and
metal particles in a single step. Examples include sol-gel using
(1) The Royal Society and The Royal Academy of Engineering. Nanoscience
and Nanotechnologies; The Royal Society of Chemistry: London, 2004.
(2) Nanoparticlessfrom theory to applications; Schmid, G., Ed.; Wiley: New
York, 2004.
(3) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc. 2004, 126,
4943.
(4) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Colloids Surf., A: Physicochem.
Eng. Aspects 2002, 202, 119.
(5) Xu, X.; Stevens, M.; Cortie, M. B. Chem. Mater. 2004, 16, 2259.
(6) Liu, Z.; Wang, H.; Li, H.; Wang, X. Appl. Phys. Lett. 1998, 72, 1823.
(7) Mokari, T.; Sertchook, H.; Aharoni, A.; Ebenstein, Y.; Avnir, D.; Banin,
U. Chem. Mater. 2005, 17, 258.
(11) Lahiri, D.; Subramanian, V.; Shibata, T.; Wolf, E. E.; Bunker, B. A.; Kamat,
P. V. J. Appl. Phys. 2003, 93, 2575.
(12) Schimpf, S.; Lucas, M.; Mohr, C.; Rodemerck, U.; Bruckne, A.; Radnik,
J.; Hofmeister, H.; Claus, P. Catal. Today 2002, 72, 63.
(13) Hida, Y.; Kozuka, H. Thin Solid Films 2005, 476, 264.
(14) He, T.; Ma, Y.; Cao, Y.; Yang, W.; Yao, J. Phys. Chem. Chem. Phys.
2002, 4, 1637.
(15) He, T.; Ma, Y.; Cao, Y.; Yang, W.; Yao, J. J. Electroanal. Chem. 2001,
514, 129.
(16) Tian, Y.; Tatsuma, T. Chem. Commun. 2004, 1810.
(17) Stepanov, A. L.; Popok, V. N. Tech. Phys. Lett. 2003, 29, 977.
(18) Fukumi, K.; Chayahara, A.; Kadono, K.; Sakaguchi, T.; Horino, Y. J. Appl.
Phys. 1994, 75, 3075.
(8) Naoi, K.; Ohko, Y.; Tatsuma, T. J. Am. Chem. Soc. 2004, 126, 3664.
(9) Yang, C.; Kalwei, M.; Schuth, F.; Chao, K. Appl. Catal., A: Gen. 2003,
254, 289.
(10) Lee, M.; Chae, L.; Lee, K. C. Nanostruct. Mater. 1999, 11, 195.
(19) Stathatos, E.; Lianos, P.; Falaras, P.; Siokou, A. Langmuir 2000, 16, 2398.
9
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J. AM. CHEM. SOC. 2006, 128, 1587-1597
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A R T I C L E S
Palgrave and Parkin
Scheme 1. Routes to Metal/Semiconductor Nanocomposite Materials
both a semiconductor and nanoparticle precursor,13,20,21 the
related technique of liquid-phase deposition,22 and multitarget
magnetron sputtering deposition.23 Also, chemical vapor deposi-
tion using a separate precursor for each phase falls into this
category.24-26
Route 4 is the layer-by-layer deposition of metal particles
and semiconductor material, for example, laser ablation using
alternate metal and semiconductor targets.27
In routes 2-4, particles are formed in situ or concurrently
with the matrix. There are therefore limits on the type and
complexity of nanoparticle that can be incorporated, imposed
by the deposition conditions. The use of preformed nanoparticles
in route 1 is advantageous because a wider range of nano-
particles can be used. As the semiconductor deposition and
nanoparticle formation steps are independent, each can be
individually optimized. This strategy seems best for the creation
of nanocomposites that include more complex nano shapes and
structures.
Here we describe a method for the deposition of nanoparticle/
semiconductor composite thin films, which is shown as route 5
in Scheme 1. The method is based on aerosol assisted chemical
vapor deposition (AACVD), which uses a liquid-gas aerosol
to transport soluble precursors to a heated substrate. The method
has traditionally been used when a conventional atmospheric
pressure CVD precursor proves involatile or thermally unstable.
However, by designing precursors specifically for AACVD, the
restrictions of volatility and thermal stability are lifted, and new
precursors and films can be investigated. Ionic precursors and
metal oxide clusters have been used in aerosol assisted deposi-
tions as alternative routes to thin films.28,29 In this paper we
describe AACVD using gold nanoparticle solutions in toluene
as precursors. Noble metal colloids may appear to be the
antithesis of a CVD precursor, which normally requires volatile
molecules, but here we show that preformed gold nanoparticles
can be transported in an aerosol generated ultrasonically and
can therefore be deposited by AACVD. Nanoparticles can be
deposited either alone, yielding nanoparticle films in what is
strictly a physical process (no chemical reaction occurs), or
together with a conventional CVD precursor, yielding nano-
composite films of gold nanoparticles in a host matrix. Titanium
dioxide/gold and tungsten oxide/gold composite films have been
produced in this way. Gold nanoparticles were chosen as they
are well studied and their synthesis is relatively simple. They
were combined with titania and tungsten oxide as these are
important technological materials.13-15 There are two main
benefits to this technique over others. First, it is flexible: many
preformed nanoparticle solutions could be used, and combined
with any chemically compatible conventional precursor, to
produce a large range of nanocomposite films. Second, CVD
is a widely used industrial technique in fields such as micro-
electronics and window glass coating, and it has a number of
well-known advantages, not least the deposition of adherent,
conformal films. Gold/semiconductor nanocomposite films with
these qualities can now be easily and inexpensively produced
by the method presented here. The method uses preformed
particles and a single-step deposition. The Au/TiO2 films formed
in this process have unique optical properties, appearing blue
in transmitted light and red to reflected light. Furthermore, the
(20) Yang, Y.; Shi, J.; Huang, W.; Dai, S.; Wang, L. J. Mater. Sci. 2003, 38,
1243.
(21) Epifani, M.; Giannini, C.; Tapfer, L.; Vasanelli, L. J. Am. Ceram. Soc.
2000, 83, 2385.
(22) Deki, S.; Ko, H. Y. Y.; Fujita, T.; Akamatsu, K.; Mizuhata, M.; Kajinami,
A. Eur. Phys. J. D 2001, 16, 325.
(23) Qian, W.; Lin, L.; Deng, Y. L.; Xia, Z. J.; Zou, Y. H.; Wong, G. K. L. J.
Appl. Phys. 2000, 87, 1.
(24) Vepfek, S.; Reiprich, S. Thin Solid Films 1995, 268, 64.
(25) Flahaut, E.; Agnoli, F.; Sloan, J.; O’Connor, C.; Green, M. L. H. Chem.
Mater. 2002, 14, 2553.
(28) Cross, W. B.; Parkin, I. P.; O’Neill, S. A.; Williams, D. A.; Mahon, M. F.;
Molloy, K. C. Chem. Mater. 2003, 15, 2786.
(26) Li, D.; Choi, C. J.; Kim, B. K.; Zhang, Z. D. J. Magn. Magn. Mater. 2004,
277, 64.
(27) Wang, W.; Qu, L.; Yang, G.; Chen, Z. Appl. Surf. Sci. 2003, 218, 24.
(29) Binions, R.; Carmalt, C. J.; Parkin, I. P. Thin Solid Films 2004, 469-470,
416.
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Aerosol Assisted CVD Using Nanoparticle Precursors
A R T I C L E S
Table 1. Parameters Used To Deposit Nanoparticles and Nanocomposite Thin Films on Glass Using Aerosol Assisted CVDa
1
film no.
vol of Au colloid used/mL
conventional precursor, amt
molar ratio Au:precursor
solvent, amt
N
2 flow rate/L min-
1
2
3
4
5
6
5.0
5.0
0
1.0
2.0
4.0
none
toluene, 20 mL
toluene, 20 mL
toluene, 50 mL
toluene, 50 mL
toluene, 50 mL
toluene, 50 mL
1.0
1.0
2.0
2.0
2.0
2.0
W(OPh)6, 1 mmol
Ti(OiPr)4, 1 mmol
Ti(OiPr)4, 1 mmol
Ti(OiPr)4, 1 mmol
Ti(OiPr)4, 1 mmol
2:1
1:2
1:1
2:1
a All depositions were carried out at 450 °C.
S150B sputter coater, operating at 1.5 kV and 20 mA. Transmission
electron microscopy (TEM) was performed on gold nanoparticle
solutions, and was carried out on a JEOL JEM 100CX II. TEM samples
were prepared by evaporating a single drop of the sample onto a
conducting copper mesh. Powder X-ray diffraction (XRD) patterns were
obtained on a Bruker AXS D8 instrument using Cu KR radiation. To
record diffraction peaks from the thin films, a fixed incidence angle of
5° was used. Diffraction patterns were recorded with an area detector.
Color data, reflectance/transmittance spectra, and haze measurements
were recorded on a HunterLab UltraScan Pro instrument. Color analysis
was performed at 2° viewing angle using D65 artificial daylight, which
was also used for haze measurements.
high reflectance demonstrated by these films around 1100 nm
means they have great potential for solar control applications.
Experimental Section
Precursor Synthesis. All chemicals were purchased from Aldrich
Chemical Co. and used as received without further purification. Gold
colloids were synthesized in toluene using the Brust two-phase chemical
reduction method.30 HAuCl4‚3H2O (99%, 0.17 g) was dissolved in
deionized water (15 mL). Tetraoctylammonium bromide (TOAB) (99%,
1.04 g) was dissolved in toluene (40 mL), and the two solutions stirred
together. A solution of NaBH4 (0.19 g) in deionized water (25 mL)
was added dropwise with rapid stirring over a 30 min period, yielding
a dark red organic layer. This was separated, washed with portions of
dilute aqueous H2SO4 and water, dried over Na2SO4, and diluted to
100 mL volume with toluene, yielding a dark red solution. The
concentration of gold was 4.3 mmol of atoms L-1, 0.85 g L-1. To avoid
contamination of films deposited from these solutions, thiol capping
groups were not used. As a result, the colloids degraded over time,
changing from deep red on synthesis to pale purple or colorless within
three weeks when stored at 4 °C. To minimize the effect of colloid
degradation on the resulting films, all depositions were carried out using
colloids made on the same day. Tungsten phenoxide was synthesized
as we have previously described, through the reaction of WCl6 with
phenol in toluene.28 [Ti(OiPr)4] (97%) was purchased from Aldrich and
used as supplied.
Aerosol Assisted CVD. Depositions were carried out in a cold-
wall horizontal-bed CVD reactor. A substrate and top plate were used,
both of SiO2 barrier glass of dimensions 145 × 45 × 5 mm supplied
by Pilkington Plc. Deposition was carried out on the SiO2 barrier layer
to prevent migration of ions into the film from the glass bulk. The
substrate rested on a carbon heating block powered by a Whatmann
cartridge heater; the temperature was monitored by Pt-Rh thermo-
couples. A top plate was positioned parallel to the substrate and 8 mm
above it, and the whole assembly was contained within a quartz tube.
An aerosol was generated from the precursor solution in a glass flask
using a Pifco ultrasonic humidifier with an operating frequency of 40
kHz. The aerosol was directed to the reactor by nitrogen gas through
PTFE and glass tubing, entering the reactor between the top plate and
substrate; reactor waste left via an exhaust port. The gas flow was
continued until all the precursor mix had passed through the reactor,
typically taking 20-30 min depending on the gas flow rate. Films were
cooled in situ under a flow of nitrogen gas, and subsequently were
handled and stored in air.
Results
The initial stock gold nanoparticle solutions used were deep
red in color. Their UV/vis spectra showed a plasmon resonance
peak with a maximum at 533 nm. TEM revealed a mean
diameter of 10 nm and narrow particle size distribution.
Depositions of thin films on glass were carried out as shown
in Table 1. Three types of film were deposited: gold nano-
particle films from the gold nanoparticle solution alone (film
1), WO3/Au composite films from [W(OPh)6] and gold nano-
particle solution (film 2), and TiO2/Au composite films from
[Ti(OiPr)4] and gold nanoparticle solution (films 4, 5, and 6).
A series of titania films with different concentrations of gold
nanoparticles were deposited. The flow rates and volumes of
solvent used were selected to give the most extensive deposition
over the substrate.
Gold Nanoparticle Films. The deposition of gold nanopar-
ticles from gold colloid solutions occurred on both the substrate
and top plate, which is the glass plate that rests 8 mm above
the surface of the substrate. Films deposited on the substrate
were used in the analysis described below. Gold nanoparticle
films appeared red to transmitted light and yellow to reflected
light. The films were stored and handled in air with no apparent
degradation; however, the films were nonadherent and could
be easily removed by slight mechanical abrasion. The glass
could be wiped completely clean with a tissue, indicating that
the gold particles were weakly adsorbed on the glass surface,
rather than strongly bound to it or absorbed within it. UV/visible
spectroscopy revealed the characteristic gold plasmon resonance
peak, with a maximum at 538 nm (Figure 1). This was very
similar in position and shape to the plasmon peak observed in
the initial gold nanoparticle solution, which had a maximum at
533 nm, suggesting that the mean size and shape of the particles
are roughly similar to those of the precursor colloid. Au 4f 7/2
and 5/2 peaks at binding energies of 87.5 and 83.7 eV,
respectively, corresponding to metallic gold, were observed by
XPS.41 SEM confirmed the presence of individual nanoparticles
Analysis. X-ray photoelectron spectroscopy (XPS) measurements
were carried out on a VG ESCALAB 220i XL instrument using
monochromatic Al KR radiation. Binding energies were referenced to
surface elemental carbon 1s peak with binding energy 284.6 eV. UV/
vis spectra were obtained using a Thermo Helios-R spectrometer. Where
a solution spectra was taken, quartz cuvettes with a path length of 1
cm were used. Scanning electron microscopy (SEM) and energy
dispersive X-ray (EDX) analysis were carried out using a JEOL 6301F
instrument using voltages between 6 and 15 kV, at 8 µA. Before SEM
analysis, samples were sputter coated with gold using an Edwards
(30) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem.
Soc., Chem. Commun. 1994, 801.
(31) O’Neill, S. A.; Clark, R. J. H.; Parkin, I. P.; Elliott, N.; Mills, A. Chem.
Mater. 2003, 15, 46.
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A R T I C L E S
Palgrave and Parkin
Figure 1. UV/visible spectra of gold nanoparticles in toluene (A) and gold nanoparticle film on glass (1) deposited by AACVD, both showing a plasmon
resonance peak.
The apparent increase in particle size could be due to
agglomeration of particles, either in the gas phase or on the
substrate surface. In either case, it is significant that the
deposition yields a nanoparticulate film rather than a continuous
metal film or highly agglomerated particles, which might be
expected due to the associated energetically favorable reduction
of surface area and the relatively high temperature of deposition.
No nitrogen or bromine peaks were observed in the XPS
spectrum, indicating that atoms from the tetraoctylammonium
bromide used in the synthesis, which prevented particle ag-
glomeration in solution, are no longer present. This suggests
that the gold particle mobility on the surface is low, even at the
deposition temperature, preventing aggregation and continuous
Figure 2. SEM image of gold nanoparticle film on glass (film 1, Table 1),
produced by aerosol assisted CVD. The scale bar measures 1 µm.
film formation.
Titania/Gold Nanocomposite Films. Titania films were
deposited with 1, 2, and 4 mL of 4.3 mM gold nanoparticle
on the surface of the film (Figure 2). The particles appear
randomly distributed, roughly spherical, and around 50 nm in
diameter. Some particles seem to be agglomerates of two, three,
or more smaller particles. In other places, two or three particles
can be observed in close proximity but not conjoined. Bare glass
is visible between the particles, showing that less than one
monolayer has been deposited. Assuming the particles to be
spherical, this places the maximum film thickness at 50-100
nm, the diameter of a single particle.
solution (Table 1). The molar ratios of Ti:Au used in each
experiment were approximately 1:2, 1:1, and 2:1, respectively.
All films were deposited at a substrate temperature of 450 °C.
The films differed in thickness throughout the substrate, but
samples taken for analysis were approximately 300 nm in
thickness, determined by the pattern of interference fringes.
Undoped titania films produced by CVD are typically colorless
or pale yellow.31 Titania/gold composite films appeared pale
blue to transmitted light, with the intensity of the blue color
increasing with increasing gold nanoparticle content. All titania
composite films were strongly adherent to the glass, such that
they could not be removed by vigorous rubbing with tissue paper
and were undamaged in routine handling. Application of
pressure with a stainless steel stylus caused small portions of
the film to chip away. Optical haze, being the percentage of
D65 artificial daylight scattered by the films, was measured to
be 0.40%, 0.55%, and 0.75% for films 4, 5, and 6, respectively.
All these values are comparable to commercial window coatings,
which typically have below 0.5% optical haze.
(32) Link, S.; El-Sayed, M. A. In Semiconductor and Metal Nanocrystals;
Klimnov, V. I. Ed.; Marcel Dekker: New York, 2004; p 45.
(33) For more information on the CIELAB color space system refer to
http://www.cie.co.at/cie (the Web page of the CIE) or http://www.
colourware.co.uk/cpfaq/q3-21.htm.
(34) Dzhurinskii, B. F.; Gati, D.; Sergushin, N. P.; Nefedov, V. I.; Salyn, Y. A.
V. Russ. J. Inorg. Chem. 1975, 20, 2307.
(35) Talbot, L.; Cheng, R. K.; Schefer, R. W.; Willis, D. R. J. Fluid Mech.
1980, 101, 737.
(36) Palgrave, R. G.; Parkin, I. P. J. Mater. Chem. 2004, 14, 2864.
(37) Kielbassa, S.; Kinne, M.; Behm, R. J. J. Phys. Chem. B 2004, 108, 19184.
(38) Klar, T.; Perner, M.; Grosse, S.; von Plessen, G.; Spirkl, W.; Feldmann, J.
Phys. ReV. Lett. 1998, 80, 4249.
(39) Barreras, F.; Amaveda, H.; Lozano, A. Exp. Fluids 2002, 33, 405.
(40) Choy, K. L. Prog. Mater. Sci. 2003, 48, 57.
UV/vis spectroscopy of the TiO2/Au composite films showed
absorption peaks in the region of 580 nm which are assigned
(41) Brooks, A. R.; Clayton, C. R.; Doss, K.; Lu, Y. C. J. Electrochem. Soc.
1986, 2459.
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Aerosol Assisted CVD Using Nanoparticle Precursors
A R T I C L E S
Figure 3. UV/visible spectra of (A) gold colloid suspended in toluene used as a precursor solution and (4, 5, 6) TiO2/Au composite films of increasing gold
content (see Table 1) deposited on glass using aerosol assisted CVD.
to the red-shifted and broadened plasmon resonance of gold
nanoparticles (Figure 3). The gold nanoparticles could not be
removed from the TiO2/Au composite film by immersion in
common organic solvents or water, or by abrasion, as indicated
by the persistence of the plasmon absorption peak after these
treatments. Indeed, the gold particles could not be removed by
any physical method that did not also remove the titania film,
showing that the nanoparticles are either strongly bound to the
film or firmly contained within it. The red shift of the plasmon
absorption peak has been observed in similar nanocomposites,20
and can be accounted for in a number of ways. As the particles
are incorporated in titania, a matrix of high dielectric constant,
light will be absorbed at a longer wavelength. Red shifting of
the plasmon peak also occurs with an increase in particle size,32
or a reduction in particle spacing.4 All three factors probably
contribute to the change in position of the plasmon peak in these
nanocomposite films, and are difficult to deconvolute. The latter
two factors also cause broadening of the peak,4,32 and in addition,
a wider size and shape distribution is also expected to cause
peak broadening.
The intensity of the plasmon absorption in the TiO2/Au films
increased with increasing gold concentration. The reflectance/
transmission spectrum of the highest concentration composite
film was measured in the visible and IR regions (Figure 4).
The plasmon peak can be seen as a local minimum in the
transmission spectrum at 580 nm, and the titania band edge is
present at 390 nm. Other than these two features, the transmis-
sion spectrum is qualitatively that of the float glass substrate.
The reflectance spectrum shows a highly unusual broad peak
in the red and near-infrared regions with a maximum at 805
nm, where 35% of incident light is reflected. This feature is
not characteristic of titania or glass, so it must arise from the
gold particles. Indeed, related peaks have been observed in
matrixes of Au@SiO2 core-shell nanoparticles, where the
position and intensity of the reflection maximum was found to
be strongly dependent on the particle spacing.4 It is also
predicted by the Maxwell-Garnett model, which can be used
to predict the optical properties of dense assemblies of interact-
ing nanoparticles.4 The optical spectra will be considered in
more detail below.
One of the most immediately striking properties of the
titania-gold composite films is their color. The color was
quantified using the CIELAB color coordinates, which are used
to express perceived color and are an industry standard (Figure
5).33 Two parameters, a* and b*, define the color: positive a*
values correspond to red, negative a* values to green. Positive
b* values correspond to yellow, negative b* values to blue.
Figure 5 shows the color of the high concentration TiO2/Au
composite thin film to both transmitted and reflected light.
Transmitted light is blue-green, while reflected light is red,
which is consistent with the visible spectra (Figures 3 and 4).
For comparison, data are given for 5 mm thick sheets of
transition metal doped (body tinted) glass. Untreated glass has
close to no color (a* and b* are close to zero), while Fe and
Nd doped glasses are strongly colored. Significantly, the
intensity of transmitted color imparted by the composite film
is comparable to commercial body tinted glass, despite the film
being around 4 orders of magnitude thinner; the extinction
coefficients of gold particles are known to be greater than typical
organic or transition metal dyes.32 These films might therefore
be used as colored coatings with a coloration of similar
magnitude to that of traditional body tinted glass. Furthermore,
the near symmetric opposite coloration of these films in reflected
and transmitted light is unusual. This type of dichromism has
been seen in bulk materials that contain a dispersion of metal
nanoparticles. An example is the Lycurgus Cup, a Roman
artifact made of glass containing embedded Au, Ag, and Cu
nanoparticles.42
SEM imaging of the TiO2/Au films showed bright particles
on a darker textured background, interpreted as gold nanopar-
(42) Barber, D. J.; Freestone, I. C. Archaeometry 1990, 32, 33.
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A R T I C L E S
Palgrave and Parkin
Figure 4. Reflectance (%R), transmission (%T), and absorption (%A) spectra of titanium dioxide/gold nanocomposite film (6) on glass in visible and IR
regions.
Figure 5. Color space representation of the color of light reflected and transmitted by titanium dioxide/gold nanocomposite film (6). For comparison, the
color of light transmitted through 5 mm of untreated glass and 5 mm of glass treated with various concentrations of transition metal ions, known as body
tinted glass. The a* axis quantifies red and green, and the b* axis quantifies yellow and blue.
ticles embedded in a titania matrix. The particles appeared
randomly distributed and oriented. Some particles had formed
larger agglomerations, but the majority were less than 100 nm
in diameter. Figure 6 shows SEM images of titania films with
different concentrations of gold particles. No particles could
be seen in the film of lowest concentration, but in films with
higher concentrations particles were visible. The particle number
density increases with increasing precursor Au concentration,
but the particle size distribution is roughly constant. XRD
detected crystalline gold in the composite films (Figure 7); peaks
at 2θ values of 38.2° and 44.5° (Cu KR radiation) arise from
the (111) and (200) planes in the Au cubic lattice.20 The intensity
of those peaks increased with increasing gold incorporation,
although the width remained approximately constant. The
Scherrer equation was used to estimate the gold crystallite
diameter using the (111) peak. In the two films of highest gold
concentration, this was found to be 10 nm. This diameter is
much smaller than the particles observed by SEM, suggesting
that each particle is composed of several crystals, most probably
several of the original precursor nanoparticles. It appears,
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absorption maximum shifted to longer wavelengths, became
broader, and increased in intensity, as shown in Figure 8. The
greatest change to the optical spectra was seen after 20 min of
annealing, and no further change was observed after 80 min
(Figure 8). These specific changes to the plasmon peak are
discussed in the following section. A similar shift has been
observed in composite films of gold and barium titanate.20 In
the data presented here, the breadth of the peaks was calculated
using the peak half-width at three-fourths maximum on the low-
energy (red) side of the peak.
Figure 6. SEM images of titania/gold nanocomposite films. Gold particles
are visible as bright spots against a darker background, due to their higher
electron density. Images A, B, and C are taken from films 4, 5, and 6,
respectively (Table 1). A greater number of particles can be seen in films
made from higher concentrations of gold colloid precursor.
therefore, that there is some degree of particle agglomeration
at some point during the deposition, either in the gas phase or
on the surface of the substrate. However, this agglomeration
does not change the intrinsic crystallite size of the Au particless
which are unchanged from the initial Au colloid solution. No
TiO2 peaks were observed in the X-ray diffraction pattern,
indicating that the deposited titania matrix had low crystallinity.
XPS was performed on the titania film with the highest
concentration of gold. This confirmed the presence of gold and
titanium in the film; a single gold environment was observed
with Au 4f 7/2 and 5/2 peaks at binding energies of 83.4 and
87.2 eV respectively, corresponding to Au metal. This shows
that the nanoparticles do not undergo a chemical reaction in
the CVD process, for example oxidation, and they remain
metallic. Ti 2p 1/2 and 3/2 binding energies were observed at
458.8 and 464.9 eV, corresponding to Ti4+ ions in TiO2.34
Quantification using peak areas gave a gold-to-titania atomic
ratio of around 1:100 at the surface. In the bulk, the Au:Ti ratio
rose to 1:25, as determined by EDX. Both these values are much
lower than the concentration in the original precursor solution
(Table 1), and this may be due to the deposition of Au particles
on the cold walls and top plate of the CVD apparatus, a result
of the thermophoretic effect.35
XRD (Figure 9) showed that the gold peaks became slightly
narrower on annealing: the full width at half-maximum (fwhm)
of the more intense (111) peak fell by 0.1° for the two films
with highest concentrations of gold after 20 min at 550 °C,
indicating a small increase in crystallite size. In contrast, the
titania cystallinity increased markedly. Peaks corresponding to
anatase phase titania appeared after annealing at 2θ values of
25.9°, 48.7°, and 54.6°, as shown in Figure 9. The higher the
concentration of gold in the film, the more intense these
emergent titania peaks were, although the peak width was
similar. The gold particles might act as nucleation sites for titania
crystal growth; hence films with a higher number of gold
particles show larger amounts of crystalline titania.
Tungsten Oxide/Gold Nanocomposite Films. To assess
whether the method could be extended to other matrix materials,
depositions using gold colloid and tungsten hexaphenoxide were
carried out (Table 1). Tungsten oxide/gold composite films were
dark blue on deposition, due to the presence of reduced tungsten
states in a WO3-x stoichiometry. Dark blue substoichiometric
tungsten oxide has been previously deposited by AACVD, and
is not due to the presence of nanoparticles.28,36 The films were
adherent and continuous, and could not be removed from the
glass by vigorous rubbing with a tissue. Application of pressure
with a metal stylus caused small portions of the film to chip
away. No gold plasmon peak was directly observed in the UV/
The titania/gold composite films were annealed in air at 550
°C for 20 min periods. Annealed films appeared a lighter shade
of blue than before annealing, and correspondingly the UV/vis
Figure 7. Glancing-angle powder XRD patterns of TiO2/Au composite films, showing cubic gold peaks (O). From bottom, films 4, 5, and 6 (Table 1). The
intensity of the gold peaks increases with increasing gold concentration.
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Palgrave and Parkin
Figure 8. Left: UV/visible spectra of TiO2/Au composite films after annealing at 550 °C for 20 min. From bottom, films 4, 5, and 6 (Table 1). The intensity
of the plasmon peak increases with increasing gold concentration. Right: Changes in optical absorption of film 6 (high Au concentration) on extended
annealing at 550 °C. The plasmon peak shifts to higher wavelengths and becomes broader on annealing.
Figure 9. Glancing-angle powder XRD patterns of TiO2/Au composite films after annealing at 550 °C for 20 min. Cubic gold peaks (O) and anatase TiO2
peaks (b) are present. From bottom, films 4, 5, and 6 (Table 1). The films with higher concentrations of gold show more intense anatase peaks, showing
that gold particles promote crystallization of the host matrix.
vis spectrum, although it was probably masked by the strong
visible absorption of the tungsten oxide. An XRD pattern, shown
in Figure 10, showed cubic gold peaks at 2θ values of 38.8°
and 45.1° assigned to (111) and (200) planes. Two strong peaks
at 2θ values of 23.9° and 48.5° are assigned to the (020) and
(040) reflections of monoclinic WO3, with strong preferred
orientation in the (020) direction, as previously observed in WO3
films deposited from [W(OPh)6].28 The presence of crystalline
gold XRD peaks is strong evidence that gold nanoparticles are
present in the film as deposited despite the lack of a visible
plasmon peak. In addition, SEM imaging (Figure 11) shows
randomly distributed particles embedded in the tungsten oxide
matrix.
The tungsten oxide/gold films were annealed at 450 °C for
20 min. On annealing, the films became pale yellow, corre-
sponding to fully oxidized WO3. After annealing UV/vis
spectroscopy showed a plasmon peak at 604 nm. As with the
titania composite films, no method could be found to remove
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Figure 10. Glancing-angle powder XRD patterns of WO3/Au composite films before (bottom) and after (top) annealing at 450 °C for 20 min. Before
annealing, cubic gold (O) and the (020) and (040) peaks of monoclinic WO3 (b) are present, showing strong preferred orientation of the semiconductor film.
After annealing, peaks from other monoclinic WO3 planes are present (9), indicating some randomization of the orientation of crystals in the film.
nanoparticles or nanostructures in general (e.g., rods, wires,
tubes), the nanoobjects form in the CVD reactor itself from the
reaction of one or more precursors. Examples include the works
of Verprek et al.,24 Flahaut et al.,25 Choi et al.,26 and many
others. In all these cases, the deposition conditions must be
controlled to ensure the correct size and shape of nanoobject is
formed. The aerosol assisted method presented here shows that
CVD using preformed nanoparticles as precursors is possible.
As such, the nanoparticle synthesis is distinct from the deposition
Figure 11. SEM image of tungsten oxide/gold nanocomposite film, 2
step, so that deposition parameters do not need to be tailored
(Table 1). The scale bar measures 1 µm. Gold particles are visible as bright
to accommodate nanoparticle formation. Rather, parameters can
be chosen to suit the conventional precursor and substrate,
although the transport of nanoparticles to the substrate must
still be considered. In addition, it is expected that nanoparticles
of complex compositions might be used, such as core-shell
particles or alloys, which might be difficult to produce in situ
in a CVD reactor.
spots against a darker background, due to their higher electron density.
the nanoparticles without also removing the tungsten oxide
coating. XRD showed the emergence of monoclinic tungsten
oxide peaks absent in the preannealed film, although the
intensity was lower than the (020) and (040) peaks. No change
in the tungsten oxide (020) and (040) peaks was observed. This
can be interpreted as the partial randomization of the crystallite
orientation after annealing, although strong preferred orientation
remains.28 As in the titania composite films, the gold peaks did
not significantly change on annealing. XPS confirmed the
presence of tungsten oxide and gold in the annealed films,
showing Au 4f peaks at 84.3 and 88.0 eV. These binding
energies were slightly higher than seen in the TiO2/Au com-
posite, although within the range expected for Au metal
nanoparticles.37 Tungsten was observed in a single environment
with W 4f peaks at 35.9 and 38.1 eV, indicating W6+ ions in
WO3. The lack of W4+ and W5+ environments confirms the
full oxidation of the tungsten oxide to stoichiometric WO3 by
the annealing process.
The size of the toluene aerosol droplets produced by the
ultrasonic humidifier can be estimated as 20 µm using Lang’s
method, which relates the vibration frequency (in this case 40
kHz), surface tension, and density of the liquid to aerosol droplet
size.39,40 The density and surface tension of pure toluene were
used for this calculation. Assuming that the concentration of
nanoparticles in the aerosol droplets is the same as in the bulk
precursor solution, and that the particles are 10 nm spheres, as
shown by TEM, the number of gold nanoparticles in a single
aerosol droplet can be calculated. In the highest concentration
solution, used to deposit gold nanoparticle films and tungsten
oxide/gold nanocomposite films (films 1 and 2), there are around
1.4 × 104 nanoparticles per droplet. Titania/gold composite films
(4, 5, and 6) were deposited with concentrations of 1.2 × 104,
5.8 × 103, and 2.9 × 103 particles per droplet, respectively.
The gold particles observed in the composite and nanoparticle
films are larger than those in the precursor solution, indicating
Discussion
Nanoparticles in Chemical Vapor Deposition. To the best
of our knowledge, in current CVD processes that deposit
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that agglomeration has taken place. However, it is clear that it
is not the entire contents of an aerosol droplet that agglomerates.
spectra of the Au@SiO2 core-shell films showed peaks in
reflectivity in the red and near-IR regions, which were dependent
on particle separation. Mulvaney et al. successfully explained
this using the MG model of dipole interaction between the
particles.4 That this peak also appears in the reflectance spectrum
of the titania/gold nanocomposite films is strong evidence that
the gold particles therein are close enough to interact. As
mentioned above, the gold particles might acquire a thin titania
coating during the deposition process. If so, then this titania
layer would prevent gold particles fusing while allowing them
to come close enough to interact as described by the MG model.
The behavior of nanoparticles in the CVD reactor differs from
that of a conventional molecular precursor due to the relatively
large size of the nanoparticle. Gas-phase particles are subject
to a thermophoretic force when exposed to a temperature
gradient.35 This force is directed away from the hot surface, so
particles act as if repelled from a hot surface and attracted toward
a cold surface. This is usually of benefit in CVD as it prevents
particles (formed by gas-phase reaction of precursors or
otherwise present) from being incorporated into the growing
film. Since the flow of gas in the reactor is laminar rather than
turbulent, thermophoresis is usually the dominant force deter-
mining the deposition location of particles. It is therefore
unsurprising that when deposited alone, without an additional
precursor, the majority of gold nanoparticles are found on the
top plate. However, when deposited with an additional precursor,
gold particles are present throughout the substrate coating,
indicating that the presence of an additional precursor somehow
overcomes the thermophoretic effect. One possibility is that gas-
phase reaction of the precursor on the surface of the gold
nanoparticles produces coated gold particles, which are less
affected by the asymmetric bombardment of gas molecules than
smaller, uncoated particles. They can therefore find their way
to the substrate more easily. Nonetheless, the reduction in Au:
Ti atomic ratio from around 2:1 to around 1:25 in film 6 (as
quantified by EDX analysis) indicates that a large proportion
of the gold is lost during deposition. This indicates that the
majority of gold particles are deposited on the cold walls of
the reactor and the top plate, or pass from the reactor in the
exhaust, rather than deposit on the substrate.
In addition, the MG model predicts red shifting of the
plasmon peak with reduced particle separation, which might
explain the change in plasmon peak on deposition (Figure 3).
However, Mie theory provides an alternative explanation, that
the particles may fuse, creating larger particles, which also show
a red-shifted plasmon peak. Of course it may be that case that
both mechanisms are at work, and both contribute to the plasmon
red shift. Nonetheless, the further red shift on annealing is
probably better explained by the MG model. In the absence of
any gold ions in the sample (as shown by XPS), this must be
caused by either an agglomeration of particles or a reduction
in separation of dipole interacting particles. Annealing is known
to increase film density by eliminating pores and voids, and
thus would be expected to reduce particle separation. The MG
model can thus explain the red shifting of the plasmon peak on
annealing of the films. The absence of any further red shift after
80 min of annealing represents the elimination of all the voids
and the limit of the mobility of the particles within the film.
Potential Applications. A wide range of applications has
been suggested for semiconductor/metal nanocomposites in
general, as outlined in the Introduction. In addition, several other
possibilities present themselves. The reflection properties of the
titania/gold films may lend themselves to heat mirror applica-
tions, since they reflect infrared light and transmit visible light.
Heat mirrors are used in solar control applications. In this case
the higher reflectivity in the near-IR would mean that a window
incorporating these particles would reflect away much of the
heat portion of solar radiation (which is most intense between
ca. 800 and 1500 nm). This would enable a reduction in solar
gain and a reduction in air conditioning costs. The optical clarity
of the films makes them suitable for use as window coatings
where tinted glass is required. This would be a valuable
alternative to body tintingsthe current practice of coloring the
whole pane of glasssas it is an expensive process to implement
on a glass float line. A coating that gave the same intensity in
color, and whose color could be varied by a simple change in
precursor concentration, may make tinted glass much less
expensive to produce. However, we believe that the main
application of this technique lies in the variety of films that
can be produced by varying the precursors and nanoparticles
used. One obstacle that must be overcome is the relatively low
efficiency of incorporation of nanoparticles into the films.
Film Color and Optical Properties. The color and optical
spectra are perhaps the most interesting features of the composite
films produced in this study, and these characteristics are
dominated by the plasmon resonance of the gold nanoparticles
incorporated within the films. Three observations are important
here: the red shifting of the plasmon peak on deposition, the
further red shifting on annealing of the films, and the reflection
peak in the red and near-IR regions. Mie theory is frequently
used to model the plasmon peak in gold and other metallic
nanoparticles. Mie theory is applicable to low particle concen-
trations, including isolated single nanoparticles,38 but breaks
down in very dense colloids, where significant dipole interaction
between the particles occurs.5,6 In this intermediate regime,
between nanoparticulate and bulk phases, the Maxwell-Garnett
(MG) model has been used to predict the plasmon resonance.5
We have considered which model best fits the observations cited
here, and whether the particles in the films behave in isolation
or interact.
Mulvaney et al. studied films of Au@SiO2 core-shell
particles on glass, the gold cores of which showed dipole
interaction.4 In their work, the MG model was used to
successfully predict the optical properties of dense arrays of
gold particles that were kept physically separate by their
surrounding layers of silica. This structure bears a resemblance
to the nanocomposite materials produced in this work, where
gold particles are also separated by a dielectric material. This
is especially true if, as discussed above, the gold particles acquire
a shell of TiO2 during the CVD process. The optical spectra
for both sets of films show similar features. The reflection
Conclusion
Metallic gold nanoparticles can be deposited as thin films
with little change in their size or shape, and without chemical
reaction, under AACVD conditions. They can also be deposited
in conjunction with conventional CVD precursors, resulting in
the formation of continuous, durable nanocomposite films
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containing randomly distributed nanoparticles, as demonstrated
by TiO2/Au and WO3/Au depositions. The gold particles cannot
be removed from the composite films without also removing
the metal oxide coating. The concentration of nanoparticles in
the film can be altered simply by changing the concentration
of gold colloid in the precursor mix. No reaction takes place
between the film and the gold particles, although the gold
plasmon resonance peak shifts to longer wavelengths on
incorporation, due to a variety of factors including the high
refractive indices of the metal oxide films, dipole interaction
between particles, and an increase in mean particle size. On
annealing, the plasmon peak shifts to higher wavelengths in
titania/gold composite films, which is best explained by a
reduction in particle separation. The gold particles show cubic
Au peaks in the XRD pattern of the film. The presence of gold
nanoparticles can enhance the crystallization of titania: films
with higher proportions of gold particles showed a greater
increase in anatase X-ray diffraction peaks on annealing,
compared to films with lower concentrations of gold.
The technique described here could be extended by using a
variety of nanoparticles and conventional precursors, and so
provide a new and versatile CVD route to nanocomposite films.
JA055563V
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