# Photoactive Thin Silver Films by Atmospheric Pressure CVD

Authors: Heather M. Yates; Lucy A. Brook; David W. Sheel
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

We report the visible and UV activity of thin silver films. The films are grown using a CVD process employing aqueous-based silver precursors, flame-assisted chemical vapour deposition. This approach overcomes many of the previously encountered limitations to silver deposition by employing an atmospheric pressure process, low-cost and low-toxicity precursors. The resultant films are assessed for activity using stearic acid destruction as a model compound. We also report on the addition of titania to these silver films to increase the potential functionality. This activity is also demonstrated, where the films appear largely transparent to the eye, further widening the potential application of this work.  It is speculated that the nanoparticulate nature, of the CVD silver, is crucial in determining photoactivity.


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## Body

## 1. Introduction

In recent years, photoactive
films have attracted increasing attention, particularly based on titania.
Significant efforts have been directed at developing the photoactivity of
titania into the visible. Methods have
included use of other photoactive materials in conjunction with TiO2  including WO3
[1]
and PdO [2],
or by doping with anionic [3, 4],
or cationic [5]
species. To date, the results have been mixed.
In many cases, the doping itself leads to a reduction in photoactivity, often
attributed to disruption of the crystal structure [6]
and/or the introduction of compensation centres [7]. Some papers do claim visible activity [8],
although the form and mechanism of how the photoactivity works is often in
dispute [9].The combination of Ag and TiO2  produced mainly by sol-gel techniques has been shown to improve TiO2  UV photoactivity, under
the correct conditions. In some cases, this has been shown to relate to the
modification of the sample morphology by the Ag with, for example, a change in
crystallite size [10] or rutile/anatase [11]. The addition of Ag is
considered to promote charge separation of the electron-hole pairs from TiO2  after photon absorption by acting as an electron sink.Research literature of the behaviour
of just Ag under illuminated light relates mainly to its own transition rather
than its effect on other chemical compounds. The reduction of Ag+ ions to Ag metal is well documented, and use of UV to form the metal from ions
is common practice. Commercial uses of the reduction of Ag+ ions
include photography [12]
and photochromic glass [13]. Films of AgCl were found to be photocatalytic
for the oxidation of water, but only in the presence of excess Ag+. This activity expanding from the UV to
visible due to self-sensitisation [14].Other research relating to use
of Ag in catalysis is usually in conjunction with zeolites, where the presence
of Ag+ clusters increases the photodecomposition rate of specific
organics by acting as active sites, or electron trapping sites [15],
or decomposition of inorganics such as NO and H2O [16]. In these cases, the zeolite is needed to
isolate and stabilize the Ag+ ions
(and their clusters). The zeolite itself is playing a role in the photoactivity of the catalyst, for example, the increased
efficiency of the N2O decomposition reaction with ZSM-5 opposed to
zeolite Y [17]. There is only very limited literature
available on the photoactivity of thin Ag films with organic materials.
Research by Guo et al. [18]
states that there is a possible photocatalyic reaction under laser excitation.A wide range of techniques
have been used to deposit thin film silver including electroless deposition [19], electrostatic deposition [20],
and PVD processes (e.g., evaporation and sputtering) [21]. Previous studies on silver thin film
structure have shown that continuous sheets of silver can be produced easily by
PVD [22],
and a granular structure can be produced by MOCVD [23, 24]. Although these processes are effective at
producing thin films, the degree of silver nanostructure control is typically
limited.In this paper, we report the
use of flame-assisted chemical vapour deposition (FACVD) to produce silver
nanostructured layers and coatings. The process can use aqueous precursors,
which are in many cases very soluble, and of relatively low toxicity. Growth
rates are also rapid in comparison to previous APCVD reports, and a high degree
of nanostructure control can be achieved with this approach. Indeed, the nanostructured silver deposition
control arising from our described approach appears to give an enhancement to
photoactivity and also to the bioactivity of the various film structures, as
previously shown [25].
We will report on the photoactivity of Ag films in both the visible and the UV,
along with examples of Ag/TiO2 films. The added attraction of the
TiO2/Ag combination is the increased durability of the samples, over
that of Ag which is relatively soft and the additional high photoactivity
expected of the TiO2 under UV.


## 2. Experimental

### 2.1. Growth

All films were grown on
commercially supplied silica-coated barrier glass substrates. The barrier is a
thin (60 nm), amorphous film of SiO2 to prevent diffusion of
impurity ions within the float glass. These would all cause a reduction in the quality and photoactivity of
the films.All silver films were produced
by flame-assisted chemical vapour deposition (FACVD). The FACVD reactor used is of in-house
construction. Basically, it consists of a burner head which allows gas mixing,
a translational substrate stage and a precursor delivery system (electronic
mass-flow controllers and a nebuliser). A
schematic of the system is shown in Figure1. The substrate stage is made up of a carbon block which can be translated
beneath the flame at a rate of 3.6 cm/s. The number of passes under the burner
head is related to the sample thickness. The carbon block was held at 300∘C for
optimum growth. The nebuliser is a commercially available ultrasonic system
(ultra-neb2000 Devilbiss
200HS-042), with water
used as the transmission medium. The precursor solution sits within a cup that
in turn is in contact with the water in the nebuliser, such that ultrasonic
waves are passed through to the precursor solution, hence producing droplets of
precursor solution. Nitrogen carrier gas
then passes through the nebuliser, collecting solution droplets, which are then
transported to the flame. Fuelling the
burner is a mixture of propane and oxygen gases, flowing at 0.99 L min−1⁡ and 3.65 L min−1⁡, respectively, generating a flame output power of
1.50 kW. The precursor is carried in
1.70 L min−1⁡ of nitrogen. These
gases are also mixed with 13.90 L min−1⁡ of make-up nitrogen for flame
control. The precursor reagent used was
silver nitrate supplied by Aldrich, with a purity of 99.99% dissolved in deionised
water to the required concentration.Figure 1
Schematic of the FACVD system.Electroplated silver was also
deposited as a comparison to the FACVD films. These films were deposited using
0.15 M silver nitrate in deionised water (solution A), 0.8 M potassium hydroxide
in deionised water (solution B), and a 0.49 M glucose solution (solution C). The
glucose solution was made up of 88% deionised water, 11% ethanol, and 1% concentrated nitric acid. The solutions were mixed in the ratio 16 : 8 : 1
of A : B : C, respectively, and deionised water was added to control the speed of
the reaction. Concentrated ammonia was
used to react with any oxide precipitation.Titania films were grown using
an atmospheric pressure CVD coater. The precursors used were titanium
tetrachloride (4.9×10−4 mol min−1⁡) and ethyl acetate
(3.65×10−3 mol min−1⁡) (Aldrich), which were transported
through the reactor by a carrier gas of nitrogen. The substrate temperature was 650∘C.
Silver was deposited both under and over these films by FACVD.


### 2.2. Characterisation

X-ray diffraction (Siemens D5000) was used to confirm the sample crystallinity.
The morphology assessed by SEM (Philips XL30). Film thickness (for titania) was
estimated by relating the reflected colour to a calibrated chart for thickness
versus refractive index. Film thickness for the Ag was determined by
cross-sectional SEM and the
use of a surface profiler (Dektak 3ST) on an etched edge. X-ray photoelectron spectroscopy,
XPS, (Kratos AXIS Ultra) with an Al (monochromated)Kα radiation source was used to check the surface composition
and stoichiometry of the films.To test the photocatalytic
behaviour under UV (365 nm), the degradation of stearic acid was followed by
FTIR (Bruker, Vector 22). The software
allows integration of the area under the peaks over a range of 2800–3000cm−1. A typical stearic acid layer would have an
integrated absorbance over this range of 1.0 cm−1 corresponding to ca.
3.13×1015 molecules cm−2 [26]. The stearic acid
(100 μl of 10 mmoL in methanol) was spun coated onto the sample.
After drying in an oven, the sample was exposed to UV light with an intensity
of 3 mW/cm2. For visible light measurements, a high-intensity tungsten/halogen lamp
(300 W) with a 400 nm cut-off filter was used.


## 2.1. Growth

All films were grown on
commercially supplied silica-coated barrier glass substrates. The barrier is a
thin (60 nm), amorphous film of SiO2 to prevent diffusion of
impurity ions within the float glass. These would all cause a reduction in the quality and photoactivity of
the films.All silver films were produced
by flame-assisted chemical vapour deposition (FACVD). The FACVD reactor used is of in-house
construction. Basically, it consists of a burner head which allows gas mixing,
a translational substrate stage and a precursor delivery system (electronic
mass-flow controllers and a nebuliser). A
schematic of the system is shown in Figure1. The substrate stage is made up of a carbon block which can be translated
beneath the flame at a rate of 3.6 cm/s. The number of passes under the burner
head is related to the sample thickness. The carbon block was held at 300∘C for
optimum growth. The nebuliser is a commercially available ultrasonic system
(ultra-neb2000 Devilbiss
200HS-042), with water
used as the transmission medium. The precursor solution sits within a cup that
in turn is in contact with the water in the nebuliser, such that ultrasonic
waves are passed through to the precursor solution, hence producing droplets of
precursor solution. Nitrogen carrier gas
then passes through the nebuliser, collecting solution droplets, which are then
transported to the flame. Fuelling the
burner is a mixture of propane and oxygen gases, flowing at 0.99 L min−1⁡ and 3.65 L min−1⁡, respectively, generating a flame output power of
1.50 kW. The precursor is carried in
1.70 L min−1⁡ of nitrogen. These
gases are also mixed with 13.90 L min−1⁡ of make-up nitrogen for flame
control. The precursor reagent used was
silver nitrate supplied by Aldrich, with a purity of 99.99% dissolved in deionised
water to the required concentration.Figure 1
Schematic of the FACVD system.Electroplated silver was also
deposited as a comparison to the FACVD films. These films were deposited using
0.15 M silver nitrate in deionised water (solution A), 0.8 M potassium hydroxide
in deionised water (solution B), and a 0.49 M glucose solution (solution C). The
glucose solution was made up of 88% deionised water, 11% ethanol, and 1% concentrated nitric acid. The solutions were mixed in the ratio 16 : 8 : 1
of A : B : C, respectively, and deionised water was added to control the speed of
the reaction. Concentrated ammonia was
used to react with any oxide precipitation.Titania films were grown using
an atmospheric pressure CVD coater. The precursors used were titanium
tetrachloride (4.9×10−4 mol min−1⁡) and ethyl acetate
(3.65×10−3 mol min−1⁡) (Aldrich), which were transported
through the reactor by a carrier gas of nitrogen. The substrate temperature was 650∘C.
Silver was deposited both under and over these films by FACVD.


## 2.2. Characterisation

X-ray diffraction (Siemens D5000) was used to confirm the sample crystallinity.
The morphology assessed by SEM (Philips XL30). Film thickness (for titania) was
estimated by relating the reflected colour to a calibrated chart for thickness
versus refractive index. Film thickness for the Ag was determined by
cross-sectional SEM and the
use of a surface profiler (Dektak 3ST) on an etched edge. X-ray photoelectron spectroscopy,
XPS, (Kratos AXIS Ultra) with an Al (monochromated)Kα radiation source was used to check the surface composition
and stoichiometry of the films.To test the photocatalytic
behaviour under UV (365 nm), the degradation of stearic acid was followed by
FTIR (Bruker, Vector 22). The software
allows integration of the area under the peaks over a range of 2800–3000cm−1. A typical stearic acid layer would have an
integrated absorbance over this range of 1.0 cm−1 corresponding to ca.
3.13×1015 molecules cm−2 [26]. The stearic acid
(100 μl of 10 mmoL in methanol) was spun coated onto the sample.
After drying in an oven, the sample was exposed to UV light with an intensity
of 3 mW/cm2. For visible light measurements, a high-intensity tungsten/halogen lamp
(300 W) with a 400 nm cut-off filter was used.


## 3. Results and Discussion

In order to understand the
influence of the various layer structures, both silver (by FACVD) and titania
(by thermal CVD) were grown. The silver films were analysed, and both the
silver and titania films were tested for photoactivity in the visible and the
UV regions.


### 3.1. Visual

The Ag films were produced
with a range of thickness (60–250 nm), by increasing the number of passes of
the moving substrate under the flame. The comparison TiO2 films were
all transparent (80 nm) with a brown tinge due to interference colouration. Visually, all the Ag films were highly
reflective, with a pale pink tinge which darkens to purple for thicker
coatings. The films could withstand gentle rubbing or sonification. Those with an upper layer of TiO2 were slightly more robust.


### 3.2. X-Ray Diffraction

All Ag films (FACVD and electroplated)
were crystalline showing metallic cubic Ag only (JCPDS 04-0783). That of the
titania was anatase (JCPDS 21-1272) when grown directly on a glass substrate
(or under Ag film), while it contained some rutile (calculated at 33 wt%) when
grown on top of the Ag film. Use of Scherrers formula [27]
allows calculation of crystallite size (Table 1). The calculation is ideally for a powder not a
thin film, so will contain line width broadening from strain as well as
crystallite size. Despite these reservations, the values obtained will give an
idea of the changes occurring.Table 1
Number of passes, thickness, and crystallite size.
Deposition
time (number of passes)Thickness,
nmCrystallite
size, nm260114611230932010025038Electroplate10433


### 3.3. Chemical Composition

Confirmation that the Ag films
consisted of metallic Ag, not oxides (or sulphides) came from the XPS. The high-resolution
scan only showed the
3d signals at3d5/2=368.7⁢eV  and 3d3/2=374.7⁢eV with
a splitting of 6 eV which are characteristic of metallic Ag. From the wide scan, a small amount of Si was
detected, which almost certainly comes from the glass substrate. The O 1s signal consisted only of a single peak
at 532.7 eV relating to absorbed water on the surface. No O 1s or Ag 3d signal
was present for an oxide.Both TiO2 deposited
over Ag and the inverse (Ag over TiO2) established that there was
both TiO2 and Ag on the surface. Obviously, in the case of Ag over
TiO2 the intensity of the Ag signal was greater. The high-resolution
scan showed Ti 2p signals at 2p1/2=464.7⁢eV and 2p3/2=459.0⁢eV, with a splitting of 5.7 eV, characteristic of TiO2. This
was confirmed by the O 1s signal at 530.2 eV.


### 3.4. Morphology

From the SEM can be seen
(Figure2) that in the early stages of thin film growth the Ag deposits as
particles rather than a continuous film. The exact size and spacing depend on the growth
conditions [28]. It
is also important to note that the sample thickness is more correctly the
height of the individual islands. As the number of passes is increased the particles
gradually coalesce and form a continuous sheet. As can be seen in Figure 2(d), the
electroplate Ag forms a continuous film with a granular surface.SEM
images of (a) Ag 2 passes, (b) Ag 30 passes, (c) Ag 100 passes, (d) electroplate
Ag.
(a)(b)(c)(d)


### 3.5. Photoactivity

The photoactivity was assessed
by use of stearic acid as a model system, as stearic acid simulates a typical
type of solid organic film that deposits on glass and ceramic surfaces.
However, it is important to note that if a material is highly photoactive with
one model compound, it does not necessarily mean that it is as active with
another, or even that the same relative rates of activity can be determined.
This has been previously clearly shown with titania [29, 30].


#### 3.5.1. Photoactivity Under UV Radiation

The rate at which the stearic
acid was decomposed is shown by the integrated area under the IR signals from
the stearic acid (2957.5, 2922.8, and 2853.4cm−1), which are
directly proportional to the concentration.
For a period of up to 70 minutes, the FACVD Ag films showed signs of
photoactivity, which then levelled out, as seen below in
Figure 3. This is particularly obvious
for the thinnest sample of 2 passes (60 nm). This behaviour does not occur for
the thickest FACVD film of 100 passes (250 nm) or the electroplate film (104 nm).Figure 3
UV
photoactivity for Ag films of differing thickness (2, 4, 30, and 100 passes).
Also included are the results for the electroplate silver
() and
an 80 nm thick film of titania
().There is an increased scatter in
some of the measurements (enhanced by the normalisation). This is particularly
obvious for the thickest film (100 passes) due to increased roughness and
haziness. The hazy appearance of the
thick samples is due
to increased crystalline disorder and increased crystallite size (i.e., above a
critical particles size a film is perceived as hazy as more light is
scattered).That these decay curves for
silver are real and not an artefact of the experimental set up (or instrument
fault), a film of TiO2 was run over the same experimental period,
showing the expected zero-order curve as the stearic acid decomposes, (as seen
in Figure 3). This is of a similar order to the initial rate of reaction (0–70 minutes) of
the thinnest Ag layer (60 nm). A linear fit giving 0.0026±0.0002 cm−1 min−1⁡ (8.13×1012 molecules cm−2 min−1⁡)
for the TiO2 film and 0.0025±0.0002 cm−1 min−1⁡(7.81×1012 molecules cm−2 min−1⁡) for the
initial rate of the Ag layer (2 passes). Most of the curves shown are obviously
not linear, so have been fitted by a sigmoidal fit.The lack of activity for the
thick films may relate to differences in the morphology (continuous rather than
island formation) and the crystallite size. As previously noted the thinner
films have much smaller crystallite sizes (11–20 nm) than that for the thicker films (33–38 nm). This change in crystallite size
relates to the extent of deposition. Also, the smaller the nanoparticles, the
greater the surface area and hence increased rate of photoactivity.From Figure3, in particularly
those for 4 and 2 passes, it can be seen that a point is reached at which no
more stearic acid is decomposed. These coatings are noncontinuous, but previous research
has shown that a pollutant can be removed from incomplete coatings of TiO2 due to the mobile electron/hole during the photoactive process [31].
This is not occurring here, suggesting that the mechanism for TiO2 photoactivity is different to that of the Ag.


#### 3.5.2. Photoactivity under Visible Radiation

Before running the experiments,
the emission of the “visible” lamp with and without the 400 nm cut-off filter
was checked. This confirmed that no obvious emission below 398 nm was detected
with the filter in place.Considering the UV experiments, a sharp
reduction of the stearic acid is followed by a point of no change for the
thinner samples (Figure4), while the thicker samples do not show this
behaviour, and confirming lack of photoactivity. For reference, a film of TiO2 (which is not expected to show any visible activity) has been added. This can
be seen to show a slight reduction in values with time (10−4⁢cm−1 min−1⁡, 3.12×1012  molecules cm−2 min−1⁡),
relating to stearic acid changes due to temperature fluctuations and possibly
UV undetected by the emission spectra. This
trend is in line with that seen for the thicker samples (100 pass FACVD and
electroless plate). The thinner samples show a much more pronounced
change.Figure 4
Visible photoactivity for Ag films of differing thickness (2, 4, 30, and 100
passes). Also included are the results for the electroplate silver
() and an
80 nm thick film of titania ().As before, it is proposed that
the initial activity seen relates to the formation of nanoparticles in the
FACVD which do not occur in the electroplate silver, along with the difference
in the morphology. As noted earlier, the
thinner FACVD films exist as islands of Ag, rather than continuous film (100
pass and electroplate). Calzaferri et al. [14]
suggest that Ag clusters on the surface sensitise
the photocatalytic process. These clusters lead to the formation of empty Ag
energy levels lower than those in the bulk metal, and so enable a new
transition, extending the process from the UV into the visible.Interestingly, it was possible
to repeat this behaviour on addition of more stearic acid, as shown in
Figure5
using the thinnest Ag layer (2 passes). After
the standard stearic acid test had been carried out, more stearic acid was spun
coated onto the sample and the test repeated. As can be seen the same trend
occurs. The initial rate of the reaction
(gradient) showed no significant change at 0.0033±0.0004 cm−1 min−1⁡(1.25 × 1013
molecules cm−2 min−1⁡).
In fact, this value is compatible to the UV activity of a TiO2 film
(80 nm).Figure 5
Repeated addition of stearic acid on a single
Ag sample (2 passes). First stearic acid run, additional stearic acid added, and the
experiment repeated once ,
twice .A possible explanation for
this behaviour is that the stearic acid would react with the Ag at the islands,
but no reaction would occur between them (being on glass not Ag). When all the
stearic acid on the island was used up, no more reaction would occur. On
addition of more stearic acid, the process would begin again. This would seem
to be in agreement with the SEM (Figure6) taken after the reaction had
finished, showing a patchy coating of stearic acid rather than a film.Figure 6
SEM image of an Ag
sample (2 pass) after stearic acid decomposition has stopped.As in the case of UV photoactivity, the
presence of smaller nanoparticles will increase the surface area available and
so increase any reaction between the stearic acid and the film.As standard practice, the
samples were exposed to UV light for 4 hours before the visible experiment as
previous work had shown that this gave improved visible photoactivity of
sol-gel produced, Ag-doped TiO2 samples [32, 33].
This is considered due to the UV partially reducing the TiO2 and
favouring the electron transfer to Ag [34].
However, the above experiment with repeated layers of stearic acid showed that
this was not necessary for just Ag films (opposed to TiO2-Ag). Exposing the sample to UV light should reduce
any Ag+ ions to Ag metal, as is common practice for the formation of
Ag/TiO2 from TiO2/AgNO3 for sol-gel films [35].It has previously been
mentioned that under UV light any Ag+ would be reduced to Ag metal.
However, under visible light in the presence of O2 this process can
be reversed with O2 acting as an electron acceptor for the
photoexcited Ag. This was shown to be a reversible sequence (between UV/visible)
by Ohko et al. [36]. Films of AgCl were found to be photocatalytic
for the oxidation of water, but only in the presence of excess Ag+. This photoactivity expanding from the UV to
visible due to self-sensitisation [14].
The presence of the excess Ag+ was considered to
be responsible for the self-sensitisation with the band gap of AgCl being
decreased due to these Ag ion 5s states. For our experiments, there is no initial
high concentration of Ag+ ions, as no other chemical species than Ag
metal were detectable
by the limits of XRD or XPS.Similar experiments were done
using multilayers of TiO2  and Ag. As previously mentioned, if TiO2  was deposited on top of Ag, anatase with a small amount of rutile was produced.
Also, the layer was not
continuous,
so both Ag and TiO2 were present on the top surface.This again (Figure7) shows
the trend seen with the Ag films, along with the ability to repeat the
experiment on adding more stearic acid on completion of the first experiment.
The TiO2 film is also plotted, so the contrast between no activity
(TiO2) and activity (TiO2 on Ag) can be more clearly
seen.Figure 7
Visible
stearic acid tests for TiO2 over Ag (2 passes). First stearic acid
run , additional stearic acid and
experiment repeated , titania
film .Growth Ag (2 passes) on top of
TiO2 again led to a surface containing Ag and TiO2 (anatase only). In this case, no sign of activity was seen
(Figure 8), with the
multilayer giving the same lack of activity as seen in the comparison TiO2 sample. Both samples gave rates of 10−4⁢cm−1 min−1⁡(3.12×1012 molecules cm−2 min−1⁡),
which are
similar to that obtained by uncoated float glass. The complete lack of any activity due to the
Ag is surprising given the previous results.
Possibly there is an advantage having a mixture of rutile and anatase rather
than just anatase as rutile absorbs a higher wavelength of light than anatase,
which may in some way help the decomposition of stearic acid which occurs.
Also, the presence of rutile may favour the TiO2 over Ag sample, as
it was previously shown by Sclafani et al. [34] that rutile, but not anatase
improved the UV photoactivity of sol-gel mixed TiO2/Ag samples. However, this cannot be the full explanation,
as the films of just Ag show photoactivity. Alternatively, deposition of Ag on
top of TiO2, rather than directly on the barrier glass may alter
either the amounts or dispersion of Ag being deposited and hence may reduce its effect on
the stearic acid. If as earlier
postulated the structure of the Ag is of importance and then by growth on a
different effective substrate, this structure is changed, hence curtailing the
photoactivity. However, the Ag crystallite size itself has not been altered
whether grown on either material, so indicating the FACVD deposition process
was controlling this property.Figure 8
Visible activity of Ag on TiO2
compared to that of TiO2.The mechanism by which Ag
(under certain conditions) removes stearic acid is not clear at this stage. We
can speculate on this, as to whether this is a photocatalytic process or a
photoactive reduction/oxidation reaction.
In this regard, some key points (noted in the above section) are as
follows:(i)
under
both UV and visible light,
stearic acid can be decomposed;(ii)
such
degradation only occurs for the thin films of Ag and TiO2 over
Ag (which actually consists of TiO2 interspersed with Ag
clusters);(iii)
the
reaction tails off before completion;(iv)
the
behaviour can be repeated on the same sample;(v)
the
(active) thin films consist of noncontinuous island growth of small Ag
clusters. Significantly, the two films where no reaction occurred were for
the thick, continuous films of 100 passes FACVD, and the electroplate
film. Both also have the largest
crystallite sizes.The decomposition of the
stearic acid might initially suggest a photocatalytic process particularly as
the behaviour seems to be repeatable on the same sample. However, the reaction tails off implying that the active species may
be being depleted which would suggest a photoactive oxidation/reduction
process. It is possible that under more extended testing, full depletion may be
observable. However, such a simple silver depletion driven reaction is hard to
rationalise with the observation that reaction does not occur (or is
substantially slower) in two types of silver films. We could speculate that, in
this instance, presence of the nanoparticulate films consisting of isolated
clusters, and below a critical crystallite size, is of paramount importance for these UV and
visible activated decompositions of stearic acid. Additional studies are planned to try to
further elucidate the mechanism involved.


## 3.1. Visual

The Ag films were produced
with a range of thickness (60–250 nm), by increasing the number of passes of
the moving substrate under the flame. The comparison TiO2 films were
all transparent (80 nm) with a brown tinge due to interference colouration. Visually, all the Ag films were highly
reflective, with a pale pink tinge which darkens to purple for thicker
coatings. The films could withstand gentle rubbing or sonification. Those with an upper layer of TiO2 were slightly more robust.


## 3.2. X-Ray Diffraction

All Ag films (FACVD and electroplated)
were crystalline showing metallic cubic Ag only (JCPDS 04-0783). That of the
titania was anatase (JCPDS 21-1272) when grown directly on a glass substrate
(or under Ag film), while it contained some rutile (calculated at 33 wt%) when
grown on top of the Ag film. Use of Scherrers formula [27]
allows calculation of crystallite size (Table 1). The calculation is ideally for a powder not a
thin film, so will contain line width broadening from strain as well as
crystallite size. Despite these reservations, the values obtained will give an
idea of the changes occurring.Table 1
Number of passes, thickness, and crystallite size.
Deposition
time (number of passes)Thickness,
nmCrystallite
size, nm260114611230932010025038Electroplate10433


## 3.3. Chemical Composition

Confirmation that the Ag films
consisted of metallic Ag, not oxides (or sulphides) came from the XPS. The high-resolution
scan only showed the
3d signals at3d5/2=368.7⁢eV  and 3d3/2=374.7⁢eV with
a splitting of 6 eV which are characteristic of metallic Ag. From the wide scan, a small amount of Si was
detected, which almost certainly comes from the glass substrate. The O 1s signal consisted only of a single peak
at 532.7 eV relating to absorbed water on the surface. No O 1s or Ag 3d signal
was present for an oxide.Both TiO2 deposited
over Ag and the inverse (Ag over TiO2) established that there was
both TiO2 and Ag on the surface. Obviously, in the case of Ag over
TiO2 the intensity of the Ag signal was greater. The high-resolution
scan showed Ti 2p signals at 2p1/2=464.7⁢eV and 2p3/2=459.0⁢eV, with a splitting of 5.7 eV, characteristic of TiO2. This
was confirmed by the O 1s signal at 530.2 eV.


## 3.4. Morphology

From the SEM can be seen
(Figure2) that in the early stages of thin film growth the Ag deposits as
particles rather than a continuous film. The exact size and spacing depend on the growth
conditions [28]. It
is also important to note that the sample thickness is more correctly the
height of the individual islands. As the number of passes is increased the particles
gradually coalesce and form a continuous sheet. As can be seen in Figure 2(d), the
electroplate Ag forms a continuous film with a granular surface.SEM
images of (a) Ag 2 passes, (b) Ag 30 passes, (c) Ag 100 passes, (d) electroplate
Ag.
(a)(b)(c)(d)


## 3.5. Photoactivity

The photoactivity was assessed
by use of stearic acid as a model system, as stearic acid simulates a typical
type of solid organic film that deposits on glass and ceramic surfaces.
However, it is important to note that if a material is highly photoactive with
one model compound, it does not necessarily mean that it is as active with
another, or even that the same relative rates of activity can be determined.
This has been previously clearly shown with titania [29, 30].


### 3.5.1. Photoactivity Under UV Radiation

The rate at which the stearic
acid was decomposed is shown by the integrated area under the IR signals from
the stearic acid (2957.5, 2922.8, and 2853.4cm−1), which are
directly proportional to the concentration.
For a period of up to 70 minutes, the FACVD Ag films showed signs of
photoactivity, which then levelled out, as seen below in
Figure 3. This is particularly obvious
for the thinnest sample of 2 passes (60 nm). This behaviour does not occur for
the thickest FACVD film of 100 passes (250 nm) or the electroplate film (104 nm).Figure 3
UV
photoactivity for Ag films of differing thickness (2, 4, 30, and 100 passes).
Also included are the results for the electroplate silver
() and
an 80 nm thick film of titania
().There is an increased scatter in
some of the measurements (enhanced by the normalisation). This is particularly
obvious for the thickest film (100 passes) due to increased roughness and
haziness. The hazy appearance of the
thick samples is due
to increased crystalline disorder and increased crystallite size (i.e., above a
critical particles size a film is perceived as hazy as more light is
scattered).That these decay curves for
silver are real and not an artefact of the experimental set up (or instrument
fault), a film of TiO2 was run over the same experimental period,
showing the expected zero-order curve as the stearic acid decomposes, (as seen
in Figure 3). This is of a similar order to the initial rate of reaction (0–70 minutes) of
the thinnest Ag layer (60 nm). A linear fit giving 0.0026±0.0002 cm−1 min−1⁡ (8.13×1012 molecules cm−2 min−1⁡)
for the TiO2 film and 0.0025±0.0002 cm−1 min−1⁡(7.81×1012 molecules cm−2 min−1⁡) for the
initial rate of the Ag layer (2 passes). Most of the curves shown are obviously
not linear, so have been fitted by a sigmoidal fit.The lack of activity for the
thick films may relate to differences in the morphology (continuous rather than
island formation) and the crystallite size. As previously noted the thinner
films have much smaller crystallite sizes (11–20 nm) than that for the thicker films (33–38 nm). This change in crystallite size
relates to the extent of deposition. Also, the smaller the nanoparticles, the
greater the surface area and hence increased rate of photoactivity.From Figure3, in particularly
those for 4 and 2 passes, it can be seen that a point is reached at which no
more stearic acid is decomposed. These coatings are noncontinuous, but previous research
has shown that a pollutant can be removed from incomplete coatings of TiO2 due to the mobile electron/hole during the photoactive process [31].
This is not occurring here, suggesting that the mechanism for TiO2 photoactivity is different to that of the Ag.


### 3.5.2. Photoactivity under Visible Radiation

Before running the experiments,
the emission of the “visible” lamp with and without the 400 nm cut-off filter
was checked. This confirmed that no obvious emission below 398 nm was detected
with the filter in place.Considering the UV experiments, a sharp
reduction of the stearic acid is followed by a point of no change for the
thinner samples (Figure4), while the thicker samples do not show this
behaviour, and confirming lack of photoactivity. For reference, a film of TiO2 (which is not expected to show any visible activity) has been added. This can
be seen to show a slight reduction in values with time (10−4⁢cm−1 min−1⁡, 3.12×1012  molecules cm−2 min−1⁡),
relating to stearic acid changes due to temperature fluctuations and possibly
UV undetected by the emission spectra. This
trend is in line with that seen for the thicker samples (100 pass FACVD and
electroless plate). The thinner samples show a much more pronounced
change.Figure 4
Visible photoactivity for Ag films of differing thickness (2, 4, 30, and 100
passes). Also included are the results for the electroplate silver
() and an
80 nm thick film of titania ().As before, it is proposed that
the initial activity seen relates to the formation of nanoparticles in the
FACVD which do not occur in the electroplate silver, along with the difference
in the morphology. As noted earlier, the
thinner FACVD films exist as islands of Ag, rather than continuous film (100
pass and electroplate). Calzaferri et al. [14]
suggest that Ag clusters on the surface sensitise
the photocatalytic process. These clusters lead to the formation of empty Ag
energy levels lower than those in the bulk metal, and so enable a new
transition, extending the process from the UV into the visible.Interestingly, it was possible
to repeat this behaviour on addition of more stearic acid, as shown in
Figure5
using the thinnest Ag layer (2 passes). After
the standard stearic acid test had been carried out, more stearic acid was spun
coated onto the sample and the test repeated. As can be seen the same trend
occurs. The initial rate of the reaction
(gradient) showed no significant change at 0.0033±0.0004 cm−1 min−1⁡(1.25 × 1013
molecules cm−2 min−1⁡).
In fact, this value is compatible to the UV activity of a TiO2 film
(80 nm).Figure 5
Repeated addition of stearic acid on a single
Ag sample (2 passes). First stearic acid run, additional stearic acid added, and the
experiment repeated once ,
twice .A possible explanation for
this behaviour is that the stearic acid would react with the Ag at the islands,
but no reaction would occur between them (being on glass not Ag). When all the
stearic acid on the island was used up, no more reaction would occur. On
addition of more stearic acid, the process would begin again. This would seem
to be in agreement with the SEM (Figure6) taken after the reaction had
finished, showing a patchy coating of stearic acid rather than a film.Figure 6
SEM image of an Ag
sample (2 pass) after stearic acid decomposition has stopped.As in the case of UV photoactivity, the
presence of smaller nanoparticles will increase the surface area available and
so increase any reaction between the stearic acid and the film.As standard practice, the
samples were exposed to UV light for 4 hours before the visible experiment as
previous work had shown that this gave improved visible photoactivity of
sol-gel produced, Ag-doped TiO2 samples [32, 33].
This is considered due to the UV partially reducing the TiO2 and
favouring the electron transfer to Ag [34].
However, the above experiment with repeated layers of stearic acid showed that
this was not necessary for just Ag films (opposed to TiO2-Ag). Exposing the sample to UV light should reduce
any Ag+ ions to Ag metal, as is common practice for the formation of
Ag/TiO2 from TiO2/AgNO3 for sol-gel films [35].It has previously been
mentioned that under UV light any Ag+ would be reduced to Ag metal.
However, under visible light in the presence of O2 this process can
be reversed with O2 acting as an electron acceptor for the
photoexcited Ag. This was shown to be a reversible sequence (between UV/visible)
by Ohko et al. [36]. Films of AgCl were found to be photocatalytic
for the oxidation of water, but only in the presence of excess Ag+. This photoactivity expanding from the UV to
visible due to self-sensitisation [14].
The presence of the excess Ag+ was considered to
be responsible for the self-sensitisation with the band gap of AgCl being
decreased due to these Ag ion 5s states. For our experiments, there is no initial
high concentration of Ag+ ions, as no other chemical species than Ag
metal were detectable
by the limits of XRD or XPS.Similar experiments were done
using multilayers of TiO2  and Ag. As previously mentioned, if TiO2  was deposited on top of Ag, anatase with a small amount of rutile was produced.
Also, the layer was not
continuous,
so both Ag and TiO2 were present on the top surface.This again (Figure7) shows
the trend seen with the Ag films, along with the ability to repeat the
experiment on adding more stearic acid on completion of the first experiment.
The TiO2 film is also plotted, so the contrast between no activity
(TiO2) and activity (TiO2 on Ag) can be more clearly
seen.Figure 7
Visible
stearic acid tests for TiO2 over Ag (2 passes). First stearic acid
run , additional stearic acid and
experiment repeated , titania
film .Growth Ag (2 passes) on top of
TiO2 again led to a surface containing Ag and TiO2 (anatase only). In this case, no sign of activity was seen
(Figure 8), with the
multilayer giving the same lack of activity as seen in the comparison TiO2 sample. Both samples gave rates of 10−4⁢cm−1 min−1⁡(3.12×1012 molecules cm−2 min−1⁡),
which are
similar to that obtained by uncoated float glass. The complete lack of any activity due to the
Ag is surprising given the previous results.
Possibly there is an advantage having a mixture of rutile and anatase rather
than just anatase as rutile absorbs a higher wavelength of light than anatase,
which may in some way help the decomposition of stearic acid which occurs.
Also, the presence of rutile may favour the TiO2 over Ag sample, as
it was previously shown by Sclafani et al. [34] that rutile, but not anatase
improved the UV photoactivity of sol-gel mixed TiO2/Ag samples. However, this cannot be the full explanation,
as the films of just Ag show photoactivity. Alternatively, deposition of Ag on
top of TiO2, rather than directly on the barrier glass may alter
either the amounts or dispersion of Ag being deposited and hence may reduce its effect on
the stearic acid. If as earlier
postulated the structure of the Ag is of importance and then by growth on a
different effective substrate, this structure is changed, hence curtailing the
photoactivity. However, the Ag crystallite size itself has not been altered
whether grown on either material, so indicating the FACVD deposition process
was controlling this property.Figure 8
Visible activity of Ag on TiO2
compared to that of TiO2.The mechanism by which Ag
(under certain conditions) removes stearic acid is not clear at this stage. We
can speculate on this, as to whether this is a photocatalytic process or a
photoactive reduction/oxidation reaction.
In this regard, some key points (noted in the above section) are as
follows:(i)
under
both UV and visible light,
stearic acid can be decomposed;(ii)
such
degradation only occurs for the thin films of Ag and TiO2 over
Ag (which actually consists of TiO2 interspersed with Ag
clusters);(iii)
the
reaction tails off before completion;(iv)
the
behaviour can be repeated on the same sample;(v)
the
(active) thin films consist of noncontinuous island growth of small Ag
clusters. Significantly, the two films where no reaction occurred were for
the thick, continuous films of 100 passes FACVD, and the electroplate
film. Both also have the largest
crystallite sizes.The decomposition of the
stearic acid might initially suggest a photocatalytic process particularly as
the behaviour seems to be repeatable on the same sample. However, the reaction tails off implying that the active species may
be being depleted which would suggest a photoactive oxidation/reduction
process. It is possible that under more extended testing, full depletion may be
observable. However, such a simple silver depletion driven reaction is hard to
rationalise with the observation that reaction does not occur (or is
substantially slower) in two types of silver films. We could speculate that, in
this instance, presence of the nanoparticulate films consisting of isolated
clusters, and below a critical crystallite size, is of paramount importance for these UV and
visible activated decompositions of stearic acid. Additional studies are planned to try to
further elucidate the mechanism involved.


## 3.5.1. Photoactivity Under UV Radiation

The rate at which the stearic
acid was decomposed is shown by the integrated area under the IR signals from
the stearic acid (2957.5, 2922.8, and 2853.4cm−1), which are
directly proportional to the concentration.
For a period of up to 70 minutes, the FACVD Ag films showed signs of
photoactivity, which then levelled out, as seen below in
Figure 3. This is particularly obvious
for the thinnest sample of 2 passes (60 nm). This behaviour does not occur for
the thickest FACVD film of 100 passes (250 nm) or the electroplate film (104 nm).Figure 3
UV
photoactivity for Ag films of differing thickness (2, 4, 30, and 100 passes).
Also included are the results for the electroplate silver
() and
an 80 nm thick film of titania
().There is an increased scatter in
some of the measurements (enhanced by the normalisation). This is particularly
obvious for the thickest film (100 passes) due to increased roughness and
haziness. The hazy appearance of the
thick samples is due
to increased crystalline disorder and increased crystallite size (i.e., above a
critical particles size a film is perceived as hazy as more light is
scattered).That these decay curves for
silver are real and not an artefact of the experimental set up (or instrument
fault), a film of TiO2 was run over the same experimental period,
showing the expected zero-order curve as the stearic acid decomposes, (as seen
in Figure 3). This is of a similar order to the initial rate of reaction (0–70 minutes) of
the thinnest Ag layer (60 nm). A linear fit giving 0.0026±0.0002 cm−1 min−1⁡ (8.13×1012 molecules cm−2 min−1⁡)
for the TiO2 film and 0.0025±0.0002 cm−1 min−1⁡(7.81×1012 molecules cm−2 min−1⁡) for the
initial rate of the Ag layer (2 passes). Most of the curves shown are obviously
not linear, so have been fitted by a sigmoidal fit.The lack of activity for the
thick films may relate to differences in the morphology (continuous rather than
island formation) and the crystallite size. As previously noted the thinner
films have much smaller crystallite sizes (11–20 nm) than that for the thicker films (33–38 nm). This change in crystallite size
relates to the extent of deposition. Also, the smaller the nanoparticles, the
greater the surface area and hence increased rate of photoactivity.From Figure3, in particularly
those for 4 and 2 passes, it can be seen that a point is reached at which no
more stearic acid is decomposed. These coatings are noncontinuous, but previous research
has shown that a pollutant can be removed from incomplete coatings of TiO2 due to the mobile electron/hole during the photoactive process [31].
This is not occurring here, suggesting that the mechanism for TiO2 photoactivity is different to that of the Ag.


## 3.5.2. Photoactivity under Visible Radiation

Before running the experiments,
the emission of the “visible” lamp with and without the 400 nm cut-off filter
was checked. This confirmed that no obvious emission below 398 nm was detected
with the filter in place.Considering the UV experiments, a sharp
reduction of the stearic acid is followed by a point of no change for the
thinner samples (Figure4), while the thicker samples do not show this
behaviour, and confirming lack of photoactivity. For reference, a film of TiO2 (which is not expected to show any visible activity) has been added. This can
be seen to show a slight reduction in values with time (10−4⁢cm−1 min−1⁡, 3.12×1012  molecules cm−2 min−1⁡),
relating to stearic acid changes due to temperature fluctuations and possibly
UV undetected by the emission spectra. This
trend is in line with that seen for the thicker samples (100 pass FACVD and
electroless plate). The thinner samples show a much more pronounced
change.Figure 4
Visible photoactivity for Ag films of differing thickness (2, 4, 30, and 100
passes). Also included are the results for the electroplate silver
() and an
80 nm thick film of titania ().As before, it is proposed that
the initial activity seen relates to the formation of nanoparticles in the
FACVD which do not occur in the electroplate silver, along with the difference
in the morphology. As noted earlier, the
thinner FACVD films exist as islands of Ag, rather than continuous film (100
pass and electroplate). Calzaferri et al. [14]
suggest that Ag clusters on the surface sensitise
the photocatalytic process. These clusters lead to the formation of empty Ag
energy levels lower than those in the bulk metal, and so enable a new
transition, extending the process from the UV into the visible.Interestingly, it was possible
to repeat this behaviour on addition of more stearic acid, as shown in
Figure5
using the thinnest Ag layer (2 passes). After
the standard stearic acid test had been carried out, more stearic acid was spun
coated onto the sample and the test repeated. As can be seen the same trend
occurs. The initial rate of the reaction
(gradient) showed no significant change at 0.0033±0.0004 cm−1 min−1⁡(1.25 × 1013
molecules cm−2 min−1⁡).
In fact, this value is compatible to the UV activity of a TiO2 film
(80 nm).Figure 5
Repeated addition of stearic acid on a single
Ag sample (2 passes). First stearic acid run, additional stearic acid added, and the
experiment repeated once ,
twice .A possible explanation for
this behaviour is that the stearic acid would react with the Ag at the islands,
but no reaction would occur between them (being on glass not Ag). When all the
stearic acid on the island was used up, no more reaction would occur. On
addition of more stearic acid, the process would begin again. This would seem
to be in agreement with the SEM (Figure6) taken after the reaction had
finished, showing a patchy coating of stearic acid rather than a film.Figure 6
SEM image of an Ag
sample (2 pass) after stearic acid decomposition has stopped.As in the case of UV photoactivity, the
presence of smaller nanoparticles will increase the surface area available and
so increase any reaction between the stearic acid and the film.As standard practice, the
samples were exposed to UV light for 4 hours before the visible experiment as
previous work had shown that this gave improved visible photoactivity of
sol-gel produced, Ag-doped TiO2 samples [32, 33].
This is considered due to the UV partially reducing the TiO2 and
favouring the electron transfer to Ag [34].
However, the above experiment with repeated layers of stearic acid showed that
this was not necessary for just Ag films (opposed to TiO2-Ag). Exposing the sample to UV light should reduce
any Ag+ ions to Ag metal, as is common practice for the formation of
Ag/TiO2 from TiO2/AgNO3 for sol-gel films [35].It has previously been
mentioned that under UV light any Ag+ would be reduced to Ag metal.
However, under visible light in the presence of O2 this process can
be reversed with O2 acting as an electron acceptor for the
photoexcited Ag. This was shown to be a reversible sequence (between UV/visible)
by Ohko et al. [36]. Films of AgCl were found to be photocatalytic
for the oxidation of water, but only in the presence of excess Ag+. This photoactivity expanding from the UV to
visible due to self-sensitisation [14].
The presence of the excess Ag+ was considered to
be responsible for the self-sensitisation with the band gap of AgCl being
decreased due to these Ag ion 5s states. For our experiments, there is no initial
high concentration of Ag+ ions, as no other chemical species than Ag
metal were detectable
by the limits of XRD or XPS.Similar experiments were done
using multilayers of TiO2  and Ag. As previously mentioned, if TiO2  was deposited on top of Ag, anatase with a small amount of rutile was produced.
Also, the layer was not
continuous,
so both Ag and TiO2 were present on the top surface.This again (Figure7) shows
the trend seen with the Ag films, along with the ability to repeat the
experiment on adding more stearic acid on completion of the first experiment.
The TiO2 film is also plotted, so the contrast between no activity
(TiO2) and activity (TiO2 on Ag) can be more clearly
seen.Figure 7
Visible
stearic acid tests for TiO2 over Ag (2 passes). First stearic acid
run , additional stearic acid and
experiment repeated , titania
film .Growth Ag (2 passes) on top of
TiO2 again led to a surface containing Ag and TiO2 (anatase only). In this case, no sign of activity was seen
(Figure 8), with the
multilayer giving the same lack of activity as seen in the comparison TiO2 sample. Both samples gave rates of 10−4⁢cm−1 min−1⁡(3.12×1012 molecules cm−2 min−1⁡),
which are
similar to that obtained by uncoated float glass. The complete lack of any activity due to the
Ag is surprising given the previous results.
Possibly there is an advantage having a mixture of rutile and anatase rather
than just anatase as rutile absorbs a higher wavelength of light than anatase,
which may in some way help the decomposition of stearic acid which occurs.
Also, the presence of rutile may favour the TiO2 over Ag sample, as
it was previously shown by Sclafani et al. [34] that rutile, but not anatase
improved the UV photoactivity of sol-gel mixed TiO2/Ag samples. However, this cannot be the full explanation,
as the films of just Ag show photoactivity. Alternatively, deposition of Ag on
top of TiO2, rather than directly on the barrier glass may alter
either the amounts or dispersion of Ag being deposited and hence may reduce its effect on
the stearic acid. If as earlier
postulated the structure of the Ag is of importance and then by growth on a
different effective substrate, this structure is changed, hence curtailing the
photoactivity. However, the Ag crystallite size itself has not been altered
whether grown on either material, so indicating the FACVD deposition process
was controlling this property.Figure 8
Visible activity of Ag on TiO2
compared to that of TiO2.The mechanism by which Ag
(under certain conditions) removes stearic acid is not clear at this stage. We
can speculate on this, as to whether this is a photocatalytic process or a
photoactive reduction/oxidation reaction.
In this regard, some key points (noted in the above section) are as
follows:(i)
under
both UV and visible light,
stearic acid can be decomposed;(ii)
such
degradation only occurs for the thin films of Ag and TiO2 over
Ag (which actually consists of TiO2 interspersed with Ag
clusters);(iii)
the
reaction tails off before completion;(iv)
the
behaviour can be repeated on the same sample;(v)
the
(active) thin films consist of noncontinuous island growth of small Ag
clusters. Significantly, the two films where no reaction occurred were for
the thick, continuous films of 100 passes FACVD, and the electroplate
film. Both also have the largest
crystallite sizes.The decomposition of the
stearic acid might initially suggest a photocatalytic process particularly as
the behaviour seems to be repeatable on the same sample. However, the reaction tails off implying that the active species may
be being depleted which would suggest a photoactive oxidation/reduction
process. It is possible that under more extended testing, full depletion may be
observable. However, such a simple silver depletion driven reaction is hard to
rationalise with the observation that reaction does not occur (or is
substantially slower) in two types of silver films. We could speculate that, in
this instance, presence of the nanoparticulate films consisting of isolated
clusters, and below a critical crystallite size, is of paramount importance for these UV and
visible activated decompositions of stearic acid. Additional studies are planned to try to
further elucidate the mechanism involved.


## 4. Summary

By use of FACVD, with aqueous
precursors, it has been shown possible to controllably deposit polycrystalline
cubic Ag films. The thickness of the film and morphology can be controlled by
the number of passes of the burner head over the substrate. The comparison TiO2 is stoichiometric anatase. The silver films deposited in an island formation
which gradually closed up becoming a continuous film by a thickness of 250 nm.
This nanostructure could be varied by controlling the growth conditions. These
films were compared with a continuous film of Ag produced by the electroplate
method. All the silver films which consisted of the noncontinuous nanostructure
demonstrated an initial visible activity to stearic acid, which was shown to be
repeatable on the same sample, although they did not go to completion. No activity was
shown by the continuous films of 100 passes or the electroplate samples for UV
or visible, suggesting that the nanostructure is a significant factor in the photoactivity
seen with stearic acid in the thinner films.By use of multilayers of thin
coatings of Ag under TiO2, it is possible to combine the visible
photoactivity of the Ag and complement its UV activity with the excellent photoactivity
of TiO2. This has potential
use in the commercial sector due to the shown repeatable measurement, along
with the improved durability of the product over that of just Ag.



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*Source: 870392-2008-05-15.xml*