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Titania and silver–titania composite films on glass—potent antimicrobial
coatings
Kristopher Page,
a
Robert G. Palgrave,
a
Ivan P. Parkin,*
a
Michael Wilson,
b
Shelley L. P. Savin
c
and
Alan V. Chadwick
c
Received 15th August 2006, Accepted 19th October 2006
First published as an Advance Article on the web 3rd November 2006
DOI: 10.1039/b611740f
Titania (anatase) and Ag-doped titania (anatase) coatings were prepared on glass microscope
slides by a sol–gel dip-coating method. The resultant coatings were characterised by X-ray
diffraction, X-ray absorption near edge structure (XANES), Raman, scanning electron
microscopy (SEM), wavelength dispersive X-ray (WDX) analysis, X-ray photoelectron
spectroscopy (XPS) and UV-vis techniques and shown to consist of anatase with ca. 0.2–1 atom%
Ag
2
O. Photocatalytic activity of the coatings was determined by photomineralisation of stearic
acid, monitored by FT-IR spectroscopy. Photocatalytically-active coatings were screened for their
antibacterial efficacy against Staphylococcus aureus (NCTC 6571), Escherichia coli (NCTC 10418)
and Bacillus cereus (CH70-2). Ag-doped titania coatings were found to be significantly more
photocatalytically and antimicrobially active than a titania coating. No antimicrobial activity was
observed in the dark—indicating that silver ion diffusion was not the mechanism for antimicrobial
action. The mode of action was explained in terms of a charge separation model. The coatings
also demonstrated significantly higher activity against the Gram-positive organisms than against
the Gram-negative. The Ag
2
O–TiO
2
coating is a potentially useful coating for hard surfaces in a
hospital environment due to its robustness, stability to cleaning and reuse, and its excellent
antimicrobial response.
1. Introduction
Staphylococcus aureus is a Gram-positive bacterium which
colonises approximately 30% of individuals in developed
countries, mainly in the nose or on the skin.
1,2
In a
colonisation of this type most people experience no symptoms
or any infection, however it is able to cause a variety of
diseases ranging from the trivial (e.g. boils) to the life-
threatening (e.g. toxic shock syndrome). Most S. aureus
infections can be treated with antibiotics
1
as these are due to
infection by methicillin-sensitive S. aureus (MSSA). However,
some strains of the organism (known as methicillin-resistant S.
aureus—MRSA) are resistant to a number of antibiotics, and
infections due to such strains are very difficult to treat.
2
MRSA infections are more common in hospital environments
where the organism is usually passed on by direct contact,
usually by the hands of health care workers (nosocomial
infection).
2–4
S. aureus has achieved methicillin resistance by
evolving both an efflux mechanism, which actively and non-
specifically expels antibiotics from the cell,
5
and by the
production of an altered penicillin binding protein PBP2a
the product of the mecA gene which is insensitive to
methicillin.
6
The spread of MRSA and other infections can
be controlled effectively through a rigorous hygiene regime.
Simple hand-washing is sufficient to help control the spread of
the organism,
2,7
however this is of little use if the hospital
environment is heavily contaminated.
3
Contamination of
surfaces touched by health care staff in the hospital environ-
ment is obviously a potential reservoir for nosocomial
infection by MRSA
3,4,8,9
and the organism can survive for
up to 9 weeks when it dries onto surfaces.
4
An antimicrobial
coating that actively disinfects hard surfaces touched by
nursing staff will help to break the nosocomial infection loop.
Such a coating would be particularly useful as a means of
disinfection in high traffic communal areas and on items such
as door handles, taps and toilet flushes. An effective
antimicrobial coating would not necessarily be limited to these
areas, but could be employed in various roles across the
hospital in both surgical and communal areas.
Titanium dioxide (TiO
2
) is receiving considerable research
interest as a photocatalyst and consequently an antimicrobial
coating. TiO
2
first came to the attention of the scientific
community when Fujishima and Honda demonstrated the
photolysis of water by a TiO
2
–Pt electrochemical photocell in
1972.
10,11
However it was not until 1985 that the efficacy of TiO
2
semiconductor particles as a means of microbial disinfection
was first realised by Matsunaga et al.
12
It was found that
platinised TiO
2
, when irradiated with ultra band gap UV
radiation, acted as an antimicrobial agent, as a result of
photocatalytic processes taking place on the TiO
2
surface. Mills
and LeHunte have written a key review in this area covering
photocatalytic and antimicrobial properties of titanium dioxide
and metal-doped titanium dioxide thin films.
13
a
Department of Chemistry, University College London, 20 Gordon
Street, London, UK WC1H 0AJ. E-mail: i.p.parkin@ucl.ac.uk
b
Division of Microbial Diseases, UCL Eastman Dental Institute,
University College London, 256 Gray’s Inn Road, London, UK
WC1X 8LD
c
School of Physical Sciences, Ingram Building, University of Kent,
Canterbury, Kent, UK CT2 7NH
PAPER www.rsc.org/materials | Journal of Materials Chemistry
This journal is ßThe Royal Society of Chemistry 2007
J. Mater. Chem.
, 2007, 17, 95–104 |95
Anatase titanium dioxide has a band gap energy (E
g
)of
3.2 eV.
13
Irradiation of anatase TiO
2
with UV radiation
greater than E
g
causes promotion of an electron from the
valence band to the conduction band. This results in the
formation of an electron–hole pair. This is a free electron in
the conduction band, and a hole in the valence band.
13–16
These reactive species then participate in oxidation and
reduction processes either within the TiO
2
itself (electron and
hole recombination), or with adsorbates at the surface.
Disinfection of a surface by photocatalysed reactions on
TiO
2
is a popular possible alternative to using chemical
disinfectants such as chlorine bleach.
The effectiveness of the TiO
2
as a photocatalyst is in part
dependent upon the rate of production of hydroxyl radicals at
the surface of the semiconductor. This is in turn dependent
upon other factors. These include the energy of the light
illuminating the surface and the competition between electron–
hole recombination and the redox processes occurring on the
surface.
17
Titanium dioxide thin films have been formed on glass, steel
and other surfaces by a wide range of techniques, especially by
sol–gel and chemical vapour deposition.
18,19
Furthermore they
have been looked at as antimicrobial coatings and shown to be
efficient especially under sunlight or black light irradiation.
13
Commercial products making use of TiO
2
photocatalyst
include self cleaning glasses such as Pilkington Activ2and
Saint Gobain Bioclean, self cleaning tiles (TOTO Inc.) and in
air purifiers.
11
The formation of silver-doped titania thin films
has received less attention.
20
Silver is incorporated into the
titania film by first forming the film, often using a paste
method using Degussa P-25, followed by impregnation with an
aqueous solution that contains silver ions.
21,22
Reduction of
this film by photolysis forms nanoparticulate silver nuggets
within a host titania matrix. These films have shown to be both
more and less active than the parent titania host matrix in the
photomineralisation of organic molecules.
21,22
The destruction
of a particular pollutant has been related to the sensitivity of
its radical and the ability of the silver–titania film to stabilise
photo-produced electrons and holes. The ability of silver–
titania thin films to act as antimicrobial coatings has received
scant attention, although one report on preliminary antimi-
crobial tests showed that the coating halts E. coli colony
formation.
20
The use of silver as a microbicide is well known
and a host of commercial products exist for use in wound
dressings, ear-pieces, face masks, catheters, plasters and even
for deodorisation of socks.
23
A number of commercial
antimicrobial surface treatments also exist which rely on the
microbicidal activity of the Ag
+
ion—these include AgION2
(AgION Technologies Inc.)
24
and SilvaGard2(AcryMed
Inc.).
25
In all of these instances the silver is impregnated in
the products in its nanoparticulate form or as a silver salt such
as silver nitrate. The mode of action has been shown to
correlate directly with the diffusion of Ag
+
into solution. This
mode of action works equally well in the dark as in the light as
it is not directly related to the photocatalytic mechanism
associated with the host titania.
In this paper we report the synthesis of titania and silver-
doped titania nanoparticulate thin films from a sol–gel route.
We demonstrate that the silver-doped titania thin films are
significantly more active than titania films both as a
photocatalyst and as an antimicrobial agent when illuminated
with 365 nm light. We show that the silver is present in the
films as Ag
2
O by XPS and X-ray absorption spectroscopy
(XAS). The silver-doped titania films are rugged and have
survived multiple reuses and cleaning with no depletion in
antimicrobial effect. We provide a comparison of the
antimicrobial efficiencies of the films for Gram-positive,
Gram-negative and spore-forming bacteria. Furthermore we
observe no antimicrobial activity from these films in the dark,
indicating that the mode of action is not, unlike previous
studies, due to silver ion diffusion. We conclude that the mode
of action of these films is related to the ease of stabilisation of
the photo-generated electron–hole pair. These new films are
easy to apply at the point of manufacture and have the
potential to be used in a clinical environment for reducing
bacterial loads and hence nosocomial infections.
Experimental
The chemicals used in this investigation were all purchased
from Sigma-Aldrich Chemical Co; propan-2-ol; butan-1-ol;
pentane-2,4-dione (acetylacetone); silver nitrate; titanium (IV)
n-butoxide and acetonitrile. The thin films were prepared on
standard low iron microscope slides (BDH). These were
supplied cleaned and polished, but were nonetheless washed
with distilled water, dried and rinsed with propan-2-ol and left
to air dry before use (2 h).
Sol–gel synthesis
Ag-doped TiO
2
film. The procedure was carried out in air.
Titanium n-butoxide (17.02 g, 0.05 mol) was chelated with a
mixture of pentane-2,4-dione (2.503 g, 0.025 mol) in butan-1-ol
(32 cm
3
, 0.35 mol). A clear, straw yellow solution was
produced, with no precipitation. This was covered with a
watch glass and stirred for an hour. Distilled water (3.6 g,
0.2 mol) was dissolved in propan-2-ol (9.04 g, 0.15 mol)
and added to hydrolyse the titanium precursor. The solution
remained a clear straw yellow colour, with no precipitate.
The solution was stirred for a further hour. Silver nitrate
(0.8510 g, 0.005 mol) was dissolved in acetonitrile (1.645 g,
0.04 mol). This was added to the pale yellow titanium
solution, which was stirred for a final hour. After the final
stirring, the resultant sol was a slightly deeper yellow in
colour, but remained clear and without precipitate. The sol
was used within 30 min for dip-coating. The TiO
2
film
controls were made in a similar manner and to the same
thickness/crystallinity.
Dip-coating. For dip-coating the glass microscope slides, the
sols were transferred to a tall and narrow 50 cm
3
beaker. This
ensured that most of the slide could be immersed in the sol. A
dip-coating apparatus was used to withdraw the slide from the
sol at a steady rate of 120 cm min
21
. If more than one coat was
required, the previous coat was allowed to dry before repeating
the process. Alternative substrate materials were also coated.
These included martensitic stainless steel; aluminium; brass;
galvanised steel and Pilkington float glass.
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J. Mater. Chem.
, 2007, 17, 95–104 This journal is ßThe Royal Society of Chemistry 2007
Calcination/annealing. All films were annealed in a furnace
at 500 uC for one hour, with a rate of heating of 5 uC min
21
.
General
Characterisation of the synthesised coatings was carried out by
field emission scanning electron microscopy (SEM) (Jeol JSM-
6301F), wavelength dispersive X-ray (WDX) analysis (Philips
ESEM) and by Raman techniques (Renishaw 1000). Powder
X-ray diffraction (XRD) was carried out at glancing angles
with a 0.5 mm collimator using an AXS D8 Discover
instrument equipped with a general area detection diffraction
system (GADDS). The TiO
2
coating was examined with an
angle of incidence of 5uover an angular range of 10–66ufor a
15 min period. The Ag–TiO
2
coatings were examined with an
angle of incidence of 1.5uover an angular range of 10–62.5ufor
a 30 min period. X-Ray absorption near edge structure
(XANES) measurements were made on station 9.3 at the
CCLRC Daresbury Synchrotron Radiation Source. The
synchrotron has an electron energy of 2 GeV and the average
current during the measurements was 150 mA. Ag K-edge
extended X-ray absorption fine structure (EXAFS) spectra for
the films were collected at room temperature in fluorescence
mode using ten films added together to give effectively 20 layers
of the sample. Ag
2
O, AgO, and Ag metal powder were used as
standards, along with a Ag metal foil reference, these were
collected in standard transmission mode. The standards were
prepared by thoroughly mixing the ground material with
powdered polyvinylpyrrolidine diluent and pressing into
pellets in a 13 mm IR press. Spectra were typically collected
to k=16A
˚
21
(kis the wave vector associated with the
photoelectron) and several scans were taken to improve the
signal-to-noise ratio. For these measurements the amount of
sample in the pellet was adjusted to give an absorption of
about md= 1 (where mis the absorption coefficient and dis the
sample thickness). The data were processed in the conventional
manner using the Daresbury suite of EXAFS programmes:
EXCALIB and EXBACK.
26,27
UV-vis spectra were obtained
using a Thermo Spectronic Helios Alpha single beam
instrument. WDX (Philips ESEM) was performed on car-
bon-coated samples, and SEM imaging (JEOL JSM-6301F)
was performed on gold-coated samples. X-Ray photoelectron
spectroscopy (XPS) measurements were carried out on a VG
ESALAB 220i XL instrument using focussed (300 mm spot)
monochromatic Al-K
a
X-ray radiation at a pass energy of
20 eV. Scans were acquired with steps of 50 meV. A flood gun
was used to control charging and the binding energies were
referenced to surface elemental carbon at 284.6 eV. Depth
profile analysis was undertaken using argon sputtering.
Photocatalytic activity
The photocatalytic activity of the films was monitored by
Fourier transform infrared (FTIR) spectroscopy (Perkin
Elmer Paragon 1000). The films were firstly activated by
30 min exposure to UV radiation from a 254 nm germicidal
lamp (Vilber Lourmat VL-208G; 8W—BDH/VWR Ltd). The
IR spectrum of each stearic acid over-layer was then recorded
over the range 3000–2700 cm
21
and the areas of the peaks
between 2950–2875 and 2863–2830 cm
21
(the C–H stretching
regions of stearic acid) were integrated. Monitoring the
integrated area is directly analogous to measuring the
concentration of stearic acid on the surface, and so can be
used to monitor the degree of photomineralisation after UV
irradiation. Slides were irradiated for a set period and then the
IR measured after each irradiation. The stearic acid over-layer
was applied by dip-coating the sample slides in a 0.02 mol dm
23
solution of stearic acid in methanol. To compare the
photocatalytic ability between samples it was ensured that
the initial peak areas were as close in value as possible. At the
end of the experiments the peak areas were normalised to the
initial starting value, such that comparison could be made.
Rates of photocatalysis (in molecules cm
22
min
21
) were also
calculated when the stearic acid decay profile could be fitted to
an appropriate rate law.
28,29
Water droplet contact angle
Photoactive films often demonstrate photoinduced super-
hydrophilicity (PSH). The degree of PSH can be gauged by
observing the change in contact angle of a water droplet on the
film surface after UV illumination. The samples were pre-
irradiated for 30 min under a 254 nm germicidal lamp (Vilber
Lourmat VL-208G—BDH/VWR Ltd), and then a 4 ml droplet
of distilled water was placed on the surface. The diameter of
the drop was then measured after it had settled. The volume–
diameter data were then entered into a computer programme
to calculate the contact angle of the water droplet. If a coating
demonstrates PSH after UV illumination, the water droplet
will be seen to spread out and have a very low contact angle
with the coating surface. Droplets were added and measured
after every consecutive 30 min of illumination time for 2 h.
Antibacterial activity
The antibacterial activity of the films was assessed against
Staphylococcus aureus (NCTC 6571), Escherichia coli (NCTC
10418) and Bacillus cereus (CH70-2; mixed vegetative and
endospore). Samples were tested in duplicate against a suite of
controls (detailed below). Sample coatings and the controls
were irradiated under a 254 nm germicidal UV lamp (Vilber
Lourmat VL-208G from VWR Ltd; 8 W) for 30 min to both
activate and disinfect the films. The sample slides were then
transferred to individual moisture chambers (made from Petri
dishes with moist filter paper in the base). An overnight culture
in nutrient broth (Oxoid Ltd, Basingstoke UK) was then
vortexed and 25 ml aliquots of the culture pipetted onto each
film in duplicate. The samples were then irradiated by a black-
light UV lamp, 365 nm (Vilber Lourmat VL-208BLB; 8W
from VWR Ltd) for the desired length of time. The irradiance
of the 365 nm lamp was measured at 1.4 mW cm
22
using a
Solarmeter Model 5.0 Total UV (A + B) hand held meter
(Solartech Inc., Michigan USA). After the desired irradiation
period, the bacterial droplets were swabbed from the surface
using sterile calcium alginate swabs (Technical Service
Consultants Ltd). The swabs were transferred aseptically to
4 ml ‘Calgon’ Ringer solution (Oxoid Ltd, Basingstoke UK) in
a glass bijoux containing 5–7 small glass beads. The bijoux was
then vortexed until the entire swab had dissolved. For all
bijoux, serial 10-fold dilutions of the bacterial suspension were
This journal is ßThe Royal Society of Chemistry 2007
J. Mater. Chem.
, 2007, 17, 95–104 |97
prepared down to 1 610
26
in phosphate buffered saline
(Oxoid Ltd, Basingstoke UK) in a sterile 96 well plate. Each
dilution was then plated in duplicate onto agar. Mannitol salt
agar (Oxoid Ltd, Basingstoke UK) was used for S. aureus,
MacConkey agar (Oxoid Ltd, Basingstoke UK) was used for
E. coli and nutrient agar (Oxoid Ltd, Basingstoke UK) was
used for B. cereus. Inoculated plates were then incubated
overnight at 37 uC. After incubation a colony count was
performed for the dilution with the optimal countable number
of colonies (30 to 300 colonies). The data were then processed,
taking into account the dilution factor and the mean values of
duplicate experiments. The end result is a direct comparison of
the number of viable bacteria per ml on the samples to that on
a glass control. Experiments were repeated at least twice,
giving four data points for each sample tested. Experiments of
2, 4 and 6 h irradiation were conducted for S. aureus;
experiments of 6 h were carried out for E. coli and experiments
of 2 and 4 h were carried out for B. cereus.
Appropriate use of controls is essential in determining
whether the coating by itself, UV exposure by itself, or a
combination of the two is the cause of any observed
microbicidal effect. For each coating under test the following
system of positive and negative controls is required: (1) L+S+
(in UV light with an active substrate); (2) L+S2(in UV light
with an inactive substrate); (3) L2S+ (in the dark with an
active substrate); (4) L2S2(in the dark with an inactive
substrate). By using a system of controls as shown it is possible
to deduce from the results which conditions result in the
antibacterial effect. Photocatalytic coatings should not be
antimicrobially active without the activation by UV light, and
so only the L+S+ sample should show antibacterial activity. A
comparison of L+S+ and L2S2enables kill levels to be
calculated. (Note: depending upon the bacterium being
investigated, exposure to UV light by itself may have a
microbicidal effect. That is to say that the L+S2sample may
in some cases demonstrate a measurable kill.)
Results
Synthesis
A simple sol–gel method was used to produce both the TiO
2
and
silver-doped TiO
2
films. The general principle behind this method
is the hydrolysis of a titanium precursor and its subsequent
polymerisation into a Ti–O–Ti network. By dip-coating the
microscope slides, a thin film of titanium precursor is deposited
and the gelation of the sol is substantially accelerated.
17
Annealing of the samples in a furnace drives off the last traces
of solvent, removes carbon and further enhances the polymerisa-
tion of the precursor into a crystalline anatase network.
The synthetic technique for doping Ag nanoparticles into a
titania film was similar to that of the pure titania film. Key to a
successful synthesis is the chelation of the metal sites involved;
this prevents agglomeration of nanoparticulate Ag and also
stops the instant gelation that occurs upon addition of AgNO
3
to an acidified titanium precursor. This effect was observed in
preliminary experiments without the use of stabilising solvents.
Acetylacetone (pentane-2,4-dione) in butan-1-ol was used to
stabilise the Ti centre and acetonitrile was used as a
coordinating solvent to stabilise the Ag.
Physical characterisation
The TiO
2
and Ag-doped TiO
2
films had a multicoloured hue,
dependent upon the angle from which they are viewed. The
appearance of the coatings is due to refringence effects
resulting from a small variation in the coating thickness.
Films of different thickness were made by varying the number
of dip-coats applied—however all films had a uniform
appearance, and were smooth. All of the Ag–TiO
2
had a
bluish–purple hue (possibly due to nanoparticulate silver),
with a distinct yellow–orange tinge in certain lighting
conditions. Notably TiO
2
films without silver did not show
the distinct bluish–purple hue or the yellow–orange tinge.
Under an optical microscope the surface of the one-coat film
TiO
2
–Ag was featureless, however in the four dip-coat film,
cracking of the surface was visible. All thicknesses of the
coating were resistant to standard scratch tests with a stainless
steel spatula, could not be removed by Scotch1tape and were
generally rugged. Indeed the film could only be removed by
chipping the glass substrate. Repeated dipping of the coatings
into distilled water had no effect on the coating’s surface,
which could not be wiped off. Depositing coatings onto
alternative substrates (brass, aluminium, SnO
2
, silica and
stainless steel) produced films of identical appearance to those
made on glass microscope slides. In particular, films deposited
onto stainless steel had excellent uniformity and retained the
ruggedness and adherence of the films coated on glass. This
robust physical behaviour is significantly better than paste,
21,22
traditional sol–gel and physical vapour deposition (PVD)
prepared titania films and is most akin to those made by
chemical vapour deposition (CVD)
18,19
such as the commercial
products Pilkington Activ and Saint Gobain Bioclean (ca.
25–50 nm thick anatase TiO
2
, deposited by ‘on-line’ CVD at
650 uC).
Characterisation
Powder X-ray diffractograms of the TiO
2
films were indexed
as anatase (I4
1
/amdz,a= 3.776 A
˚,c= 9.486 A
˚). The Ag–TiO
2
diffractograms were slightly less well defined than the TiO
2
diffractogram but did show peaks attributed to anatase TiO
2
(Fig. 1). Furthermore, the Ag–TiO
2
patterns exhibited one
other significant peak at 31.5u2hwhich was absent in the TiO
2
pattern and must therefore be due to the difference in
composition—possibly due to the incorporation of a Ag
compound rather than crystalline Ag. Database patterns for
crystalline Ag do not correlate with this observed peak. The
best pattern match for this peak and the remainder of the
diffractogram is for the silver oxides AgO and Ag
2
O. Both
silver oxide species correspond well with their most intense
peaks aligning with the additional peak observed in the
experimental pattern.
Raman analysis of both TiO
2
and Ag–TiO
2
types was
attempted in the range 100 to 1000 cm
21
. A characteristic
anatase TiO
2
scattering pattern was produced (Fig. 2), with a
sharp and intense peak at 143 cm
21
, and further peaks at 197,
396, 519 and 639 cm
21
in the undoped TiO
2
pattern. The less
well defined Raman pattern for the Ag-doped samples is most
probably due to the lower level of crystallinity in the samples—
as observed by the comparatively weak anatase peaks in the
98 |
J. Mater. Chem.
, 2007, 17, 95–104 This journal is ßThe Royal Society of Chemistry 2007
XRD. No Raman patterns for silver oxides were apparent.
This is most likely due to the low concentration of Ag
2
O in the
films and the poor Raman scattering power of Ag
2
O compared
with the TiO
2
matrix.
Ag K-edge XAS spectra were collected for the three Ag-
doped TiO
2
films made from sols with Ag concentrations of
5%, 10% and 20%, Ag metal foil, Ag metal powder, Ag
2
O and
AgO powders. The Ag K-edge XANES data for the doped
samples are shown in Fig. 3(a) along with the corresponding
data for Ag metal powder, Ag
2
O and AgO. The energy scales
of all the spectra have been consistently normalised to the Ag
K-edge at 25 518 eV and the spectra shifted on the y-axis for
ease of viewing. Fig. 3(a) shows that the local environment of
the Ag atoms has a distinct effect on the shape of the XANES
spectra. This can be used to identify the local environment of
the Ag atoms in the Ag-doped TiO
2
films. In each case, the
shape of the XANES spectra for the doped films matches that
of the Ag
2
O standard, indicating that the silver is present in the
Fig. 1 Powder XRD patterns for four coat TiO
2
(lower trace) and two and four coat Ag–TiO
2
coatings (upper and middle traces respectively).
The Ag-oxide peak is marked with an asterisk (*).
Fig. 2 Raman pattern for four coat TiO
2
film.
Fig. 3 (a) The Ag K-edge XANES for Ag
2
O, AgO, Ag powder and
Ag-doped TiO
2
films. No pre edge features were observed; (b) the Ag
K-edge XANES for Ag
2
O and Ag-doped TiO
2
films.
This journal is ßThe Royal Society of Chemistry 2007
J. Mater. Chem.
, 2007, 17, 95–104 |99
films as Ag
2
O [Fig. 3(b)]. The pattern for silver metal—as also
shown in Fig. 3(a) is very different to that observed and can’t
be detected in the samples measured. No bands were observed
before the edge in any of the XANES experiments.
Furthermore as the XAS gave such a good match to Ag
2
O
[see Fig. 3(b)] it is unlikely that the silver is present within the
titania lattice as a discrete solid solution Ag
x
Ti
22x
O
2
because
this would give a different edge shape pattern. Hence the films
are best described as composites of anatase titania with small
amounts of homogeneously distributed silver (I) oxide.
SEM and WDX techniques were used to study the
composition and morphology of the coated surfaces. WDX
analysis confirmed the presence of Ag in the Ag–TiO
2
with
ratios of 1 part Ag to 100 parts Ti (or less). This was
significantly lower than the silver : titania ratio in the starting
sol (1 : 10). SEM imaging showed minor shrink cracking in the
single or double dip-coated films. The severity of shrink
cracking increased with increasing film thickness. At higher
magnifications, both coating types had similar morphologies,
consisting of granular structures. A high magnification
(6160 000) image of the two coat Ag-doped coating (Fig. 4)
displayed the granular and uneven nature of the coating. In the
top left quarter of the image, a high electron density artefact
can be observed. This indicates the presence of an agglomer-
ated island that contains Ag since this has a higher electron
density than the TiO
2
matrix—such islands were seen
randomly dispersed across the surface of the film. Also, during
the course of the SEM studies, the nanocrystalline nature of
the TiO
2
coating was observed—particles of 30 nm size on
average can be seen in a 6400 000 image (Fig. 5).
Observation of particles of this size correlates well with the
crystallite sizes calculated by the Scherrer equation from the
XRD line broadening—which corresponds best to nanocrys-
talline titania, rather than a fully crystalline phase. End on
SEM studies were also carried out to measure the thickness of
the films. The two coat materials had a thickness of
approximately 150 nm and a four coat material was
approximately twice this thickness, at ca. 300 nm.
X-Ray photoelectron spectroscopy was undertaken on two
sets of four coat Ag–TiO
2
films, one on a set exposed to UV
light and one on the films as made. Both gave the same XPS
profile. The titanium to oxygen atomic ratio was as expected
2 : 1, no other elements were detected other than carbon and
silicon at a few atom%. The percentage of the carbon
decreased dramatically on etching indicating that it was
residual carbon from within the XPS chamber. The Si
abundance was constant with etching and probably a result
of breakthrough to the underlying glass on regions where there
was a small crack in the titania coating, notably it was only
seen in one of the four samples analysed. Silver was detected
both at the surface and throughout the film and its abundance
was invariant with sputter depth. The silver was typically
detected at below 1 atom%—significantly lower than that in
the initial sol but comparable to that observed by WDX
analysis (values ranged around 0.2 atom%, however accurate
quantification was difficult at such low levels). The detection
limit of the instrument is approximately 0.1 atom% and for
quantification it is 0.2 atom%. XPS spectra were collected and
referenced to elemental standards. The Ti 2p
3/2
and O 1s
binding energy shifts of 458.6 eV and 530.1 eV match exactly
literature values for TiO
2
.
30
In the sample exposed to UV light
just prior to measurement there was a small shoulder to both
the Ti and O peaks that correspond to Ti
2
O
3
. Interestingly the
silver 3d
5/2
XPS showed a single environment centred at
367.8 eV which gave a best match for Ag
2
O (literature reports
at 367.7–367.9 eV) rather than for silver metal 368.3 eV
(Fig. 6).
30
Hence the XPS is consistent with the silver being
oxidised as Ag(I) rather than a metallic form in the thin films.
Furthermore sputtering studies showed no change in the silver
environment with sputter depth. This indicates that the silver is
present as Ag
2
O and not a Ag
2
O coated Ag particle; as
otherwise an asymmetry to the peak shape would have
occurred.
UV-vis spectroscopy of the TiO
2
and Ag–TiO
2
thin films on
glass was carried out in the range 300–800 nm. A band edge for
the O
22
to Ti
4+
transition in anatase TiO
217
was observed in all
of the types of coating at approximately 380 nm. This coupled
with XRD and Raman evidence showed that the anatase form
of TiO
2
was present in all films. An approximate value of the
optical band gap for the coatings was obtained by extrapola-
tion on a plot of (ahn)
1/2
versus hn, where ais the absorbance of
Fig. 4 SEM image of two coat Ag–TiO
2
coating 6160 000, scale bar
100 nm.
Fig. 5 SEM image of TiO
2
coating 6400 000, scale bar 10 nm.
100 |
J. Mater. Chem.
, 2007, 17, 95–104 This journal is ßThe Royal Society of Chemistry 2007
the film (a=2log T/T
0
;T, sample optical transmission; T
0
,
substrate optical transmission) and hnthe photon energy. The
band gap for the TiO
2
coating was in the region of 3.0 eV—
which is to be expected for the anatase form of TiO
2
(3.2 eV).
13
Band gap plots for the Ag-doped coatings were not as
easy to interpret as that of TiO
2
, giving a band gap range of
2.8–3.4 eV. Ag metal nanoparticles could not be detected by
the observation of a plasmon band
31,32
in the UV visible
spectra of the Ag–TiO
2
films. However, nanoparticulate silver
was detected in the initial starting sol by this method,
exhibiting a broad plasmon band at 430 nm. There was,
therefore, considering the UV, XAS and XPS spectra, little
evidence for the incorporation of these Ag metal nanoparticles
into the coatings intact without transformation into an oxide.
Functional properties I: photocatalysis and water droplet contact
angle
All of the films showed photocatalytic activity with 254 nm
germicidal lamp illumination over a period of eight hours. The
reason for choosing the 254 nm (4.88 eV) lamp was to make
sure that the radiation was of greater energy than the TiO
2
band gap (3.2 eV). The degree of photocatalysis observed
varied between the different coatings, as shown in Fig. 7. It can
be clearly seen that the Ag-doped coatings were significantly
more photocatalytically active than the undoped TiO
2
coating
of the same thickness. Amongst the different thickness
Ag-doped coatings there was also a difference in the
photocatalytic activity. The two-coat Ag–TiO
2
film had the
highest initial rate of photocatalysis. The zero order rate
constants for the degradation of stearic acid were calculated at
4.05 610
12
molecules cm
22
min
21
for TiO
2
and 5.85 6
10
12
molecules cm
22
min
21
for a Ag
2
O–TiO
2
coating of the
same thickness. The photoactivity of the TiO
2
films generated
in this study to photomineralise stearic acid was slightly lower
than our previous work using CVD and sol–gel prepared
films.
17
In previous work depositions had been conducted on
barrier glass which has a diffusion layer to stop sodium ion
diffusion from the glass substrate into the film. It has been
noted previously that sodium diffusion during calcinations can
reduce the photocatalytic ability of titania films.
28,33
However
our XPS and WDX studies did not detect any sodium in the
titania films so if present it must be less than the 0.1 atom%
detection limit of these techniques.
Initial water contact angle measurements showed that all of
the samples were hydrophilic as they made ca.15uwater
contact angles and they became superhydrophilic upon
exposure to UV radiation. The Ag–TiO
2
samples had contact
angles of around 1uafter only the initial 30 min of irradiation
with 254 nm. These angles decreased further upon subsequent
exposure to the germicidal lamp (254 nm)—but as they were so
low they were difficult to quantify. However, it showed that
the 2-coat Ag–TiO
2
film had a very high degree of
photoinduced superhydrophilicity, as did the three and four
coat versions of the same coating. Photoinduced super-
hydrophilicity was not observed in the coatings deposited
onto metal substrate materials, with initial contact angles
being significantly higher (ca.20u) than for equivalent coatings
on glass. This may be due to metal ions diffusing into the
coating during the annealing step.
Functional properties II: microbicidal activity
The antimicrobial activity of the coatings was assessed against
three different micro-organisms; Staphylococcus aureus (NCTC
6571), Escherichia coli (NCTC 10418) and Bacillus cereus
(CH70-2). These organisms represent a spectrum of different
classes of bacterium. S. aureus is perhaps the most important
target for this investigation, because of its direct link with MRSA
and hospital acquired infections. S. aureus is also a fairly typical
example of a Gram-positive organism, so it serves as a useful
indicator of the behaviour of a sample coating towards this class
of micro-organism. In the interests of completeness and
experimental rigour, the coatings were also tested against E.
coli, a Gram-negative organism and with B. cereus,aGram-
positive spore-forming organism. It should be noted that the
same coatings were reused for all antimicrobial testing and that
all experiments were carried out in duplicate and repeated twice.
The samples were cleaned between uses by wiping with
isopropanol wipes (as commonly used to clean hard surfaces in
hospitals). The Ag-doped coatings performed very well under
conditions of reuse, maintaining a constant level of effectiveness
despite being handled, cleaned and reused.
Staphylococcus aureus (NCTC 6571). Experiments with
S. aureus were carried out on timescales of two hours, four
Fig. 6 XPS Ag 3d profile for a four coat Ag–TiO
2
coating.
Fig. 7 Relative photocatalytic abilities of all coatings.
This journal is ßThe Royal Society of Chemistry 2007
J. Mater. Chem.
, 2007, 17, 95–104 |101
hours and six hours. Both the Ag-doped and un-doped TiO
2
coatings displayed antibacterial activity towards S. aureus,
although to varying degrees (Fig. 8). The two-coat Ag–TiO
2
coating proved to be extremely effective against S. aureus.
After six hours of illumination under 365 nm UV radiation the
two coat Ag–TiO
2
coating proved to be 99.997% effective
against an inoculum of approximately 2.15 610
9
cfu ml
21
(colony-forming units ml
21
)S. aureus. As a point of reference,
the analogous TiO
2
coating displayed an effectiveness of
49.925% against the same inoculum. Supplementary studies
carried out at four and two hours of illumination enabled
elucidation of relative antimicrobial activity between coating
types, and also of the relationship between UV light dose and
antimicrobial activity.
Escherichia coli (NCTC 10418). Six hour experiments were
carried out with the two coat Ag–TiO
2
coating against E. coli.
The results were not as striking as with S. aureus, but
nonetheless revealed that the coatings exerted an antimicrobial
effect. The coating averaged an effectiveness of 69% against an
inoculum of ca. 2.61 610
9
cfu ml
21
E. coli. The coating was
noticeably less effective against E. coli than S. aureus, even
though the size of the inoculum was similar.
Bacillus cereus (CH70-2). The two-coat Ag–TiO
2
coating
was also tested against B. cereus, another Gram-positive
organism, but one that forms spores under adverse environ-
mental conditions. Four and two hour experiments were
carried out against this organism using the two coat Ag–TiO
2
coating only. The coating achieved greater than 99.9% kills of
this organism at both 2 h and 4 h exposure periods with 365 nm
UV. It should be noted however, that this was from an initial
concentration of ca. 1.0 610
8
cfu ml
21
B. cereus. Further, the
UV light control L+S2showed no measurable kill at 2 h and a
64% kill at 4 h of exposure. This demonstrates that the coating
is extremely effective after just 2 h against an inoculum in the
region of one hundred million cfu ml
21
. This level of
contamination is still significantly greater than what would
be found on a contaminated surface. For example, S. aureus
contamination of a surface was shown typically to be between
4 and 7 cfu cm
22
.
34
Discussion
There are a number of avenues that can be followed in an
attempt to provide an explanation for the enhanced activity of
the Ag-doped TiO
2
coating over that of an un-doped TiO
2
film. In reality, there is likely no one single reason for the
increased activity, rather the observation results from a
combination of effects. The simplest explanation is one of
surface microstructure. The Ag-doped films displayed islands
with a high silver density. This in itself is a good explanation
for the difference in activities, but it does not take into account
other evidence from the characterisation of the coatings. XRD,
XPS and XANES analysis elucidated the presence of the silver
oxide Ag
2
O. It is possible that these species act as a source of
electrons and as charge separators because of their high
electron density relative to the TiO
2
matrix. These factors
would enhance the overall photoactivity of the coating by
firstly donating extra electrons to the conduction band which
in turn are able to produce more reactive species at the catalyst
surface, and secondly by blocking electron–hole recombination
which stops the production of radicals at the surface. Indeed,
this explanation is supported by the photocatalysis results.
There have also been reports in the literature of some silver
oxides exhibiting semiconductor behaviour
35,36
and Ag
2
Ois
quoted in the literature as having a band gap of 2.25 eV
(550 nm).
37
This may go some way to explaining the apparent
change in the optical band gap of the Ag–TiO
2
films over the
TiO
2
coating.
It is difficult to compare photocatalysis results with the
literature since there is not as yet an agreed universal reference
against which photocatalysis can be measured. However, the
use of Pilkington Activ2glass (which is TiO
2
coated) as a
reference photocatalyst has been proposed, since this would
make a reliable standard.
38
Preliminary photocatalytic results
in our laboratory indicate the Ag–TiO
2
films are considerably
more active. It is equally difficult to compare the micro-
biological results of this investigation with other work in the
literature because of the great diversity in techniques used, and
in the precise details of the experiments performed. The vast
majority of studies of TiO
2
antimicrobials are carried out in
solution using a suspension of Degussa P25 TiO
2
.
14,15,38
This is
fundamentally different from the thin film coatings prepared in
this study because the surface area of active catalyst in
suspension would be significantly greater than that available
on a thin film surface (perhaps up to 10 000 times greater).
Furthermore, titania particles in suspension can be ingested by
cells via phagocytosis—this has been shown to cause rapid
cellular damage in addition to that caused by photocataly-
sis.
39,40
Consequently, literature results from this method differ
greatly from those obtained in this study. Most studies also
examined only E. coli. However, the efficacy against E. coli
when using a suspended powder is variable from study to
study. One study used an inoculum of 1 610
6
cfu ml
21
E. coli
in a P25 suspension and observed 85% effectiveness after 20 min
exposure to UV (peak wavelength 356 nm), and 100%
effectiveness after an hour.
39
This compares with 69%
Fig. 8 Bacterial kills for the two coat Ag–TiO
2
sol–gel prepared
coating against Staphylococcus aureus after 2, 4 and 6 h illumination
times with 365 nm radiation. The viable counts are expressed as
colony-forming units ml
21
. L+S+ refers to the exposure of an active
coating (identity in brackets) to UV light. L+S2refers to the exposure
of an uncoated slide to UV light. L2S+ refers to an active coating
(identity in brackets) kept in the dark and L2S2refers to an uncoated
slide kept in the dark.
102 |
J. Mater. Chem.
, 2007, 17, 95–104 This journal is ßThe Royal Society of Chemistry 2007
effectiveness against 1 610
9
cfu ml
21
E. coli after 6 h UV
illumination (365 nm) for the two-coat Ag–TiO
2
coating
prepared in this study.
The antimicrobial effect of titania coatings is derived from
the production of hydroxyl radicals,
15,16
hence a rationalisa-
tion for the relative effectiveness of the coating against Gram-
positive and Gram-negative organisms can be offered.
Previous research examining the toxicity mechanism of TiO
2
against micro-organisms showed that the lethal action
involved breach of the cytoplasmic membrane and the
resultant leakage of intracellular components.
39,41
For this to
occur, the hydroxyl radicals produced at the coating surface
must be able to directly attack the cytoplasmic membrane. The
differing morphologies of Gram-positive and Gram-negative
cell envelopes means that the passage of hydroxyl radicals
from coating surface to cytoplasmic membrane is hindered to
differing extents. For S. aureus, the only barrier is the
peptidoglycan layer and the periplasmic space. Despite having
a thick layer of peptidoglycan, S. aureus is likely afforded little
protection from the hydroxyl radicals. This is because the
peptidoglycan is composed of a fairly open network polymer
of N-acetylmuramic acid and N-acetylglucosamine polysac-
charide chains, with peptide bridges. In contrast, the passage
of hydroxyl radicals towards the cytoplasmic membrane of
E. coli is significantly hindered by the morphology of the cell
envelope. In Gram-negative organisms, such as E. coli, the
cytoplasmic membrane is protected by a thin layer of
peptidoglycan, followed by an outer membrane. The outer
membrane presents a significant barrier to hydroxyl radical
passage since it is comprised of a complex layer of lipids,
lipopolysaccharides and proteins. The outer membrane layer
presents an attractive target for approaching hydroxyl radicals
because of this composition. Although the outer membrane is
semi-permeable, many of the hydroxyl radicals will react with
the lipid constituents of the membrane rather than pass
through it. Once the membrane is breached, however, there are
no further significant obstacles blocking the approach of the
radicals to disrupt the cytoplasmic membrane and cell death
can be observed.
39
This interpretation is supported by a recent
study of the photokilling of E. coli by TiO
2
thin films.
40
The
bactericidal action was found to be a two step process in which
the outer membrane is compromised first, followed by the
cytoplasmic membrane. Hence the Gram-negative envelope
affords better protection against the hydroxyl radical as a
cytotoxic agent. This rationale would therefore account for the
higher antimicrobial activity of the Ag–TiO
2
towards Gram-
positive organisms than Gram-negative.
In the films prepared here, no antibacterial activity was
observed from the Ag–TiO
2
films in the absence of light—this
implies that the silver has no direct role in promoting increased
bacterial kills. The presence of silver as an oxide within the film
enhanced the antimicrobial and photocatalytic properties.
Solutions of silver sols have applications as antibacterial
agents where the active component of these solutions is the
Ag
+
ions which disrupt bacterial metabolism.
42
The silver sols
display a large surface area and are known to be partially
oxidised by atmospheric oxygen to give Ag
2
O. While this is
only sparingly soluble in water it is sufficient to provide
antibacterial effects. These antibacterial effects are manifested
independently of whether a light source is used or not. In the
films made in this study no bacterial kill was observed in the
dark. This is strong evidence that the films are not functioning
as microbicides due to the presence of silver ions, as we would
have observed some kill in the absence of light. Silver metal is
normally quite resistant to oxidation in air and requires
stronger oxidising agents such as ozone to convert to the oxide.
The fact that the silver is present as Ag
2
O in these films is a
consequence of the high temperature anneal and the fact that
the silver is embedded in a titanium dioxide matrix. Although
the silver is present as the oxide, UV illumination of titania can
in principle convert this to the native metal in the presence of
titanium dioxide. However XPS studies of the Ag
2
O–TiO
2
film
both before and after UV irradiation did not show any change
in the silver environment—the binding energy shifts match well
for Ag
2
O and no lower energy peak was seen as would be
characteristic of silver formation. Hence this combined with
the lack of any antibacterial activity in the dark seems, even
after 12 cycles of UV irradiation, to indicate that any possible
formation of silver metal in this system occurs below the
detection limits of the experiments used. Recent work has
shown that at elevated temperature in the presence of oxygen
the most stable thermodynamic form is Ag
2
O.
43
This correlates
nicely with what was observed in this study. However, the
presence of the silver oxide Ag
2
O in conjunction with titania
did show a marked enhancement over a pure titania film as a
photocatalyst. This is most likely due to stabilisation of
photogenerated electron–hole pairs at the titania surface by
localisation of the photogenerated electron onto the silver
oxide.
Conclusion
Photocatalytically-active and antimicrobially-active coatings
were synthesised by a simple sol–gel dip-coating technique.
The resultant coatings were characterised by glancing angle
X-ray diffraction, XPS, XANES, Raman spectroscopy, SEM,
WDX and UV-vis spectroscopy and shown to consist of
anatase titania with embedded Ag
2
O particles. Photocatalytic
activity of the coatings was determined by photomineralisation
of stearic acid and monitored by FT-IR spectroscopy.
Coatings demonstrating high photocatalytic activity against
stearic acid were then screened for antibacterial efficacy
against Staphylococcus aureus (NCTC 6571), Escherichia coli
(NCTC 10418) and Bacillus cereus (CH70-2). Ag-doped
coatings were found to be significantly more photocatalytically
and antimicrobially active than a regular TiO
2
coating. This
was explained in terms of a charge separation model. Notably
the coatings showed no activity against bacteria in the dark—
indicating that their efficacy is not due to silver ions acting as a
microbicide. The coatings also demonstrated significantly
higher activity against the Gram-positive organisms than
against the Gram-negative. This was explained in terms of
the comparative morphologies of the cell envelopes and the
permeability of these envelopes to the likely toxic agent, the
hydroxyl radical. The two coat Ag–TiO
2
coating would appear
to be a potentially useful coating for hard surfaces in a hospital
environment due to its robustness, stability to cleaning and
reuse, and its excellent antimicrobial response to all organisms
This journal is ßThe Royal Society of Chemistry 2007
J. Mater. Chem.
, 2007, 17, 95–104 |103
tested thus far. Such a coating would need to be applied at the
point of manufacture of a particular item—and could not be
retrofitted to existing surfaces because of the heat treatment
required to generate the active coatings. However on new
products it could create a very potent antimicrobial coating.
Acknowledgements
The Horshall fund is thanked for financial support. Professor
Parkin is a Royal Society Wolfson Trust merit holder. K.P.
would like to thank Ms Vale´rie Decraene for her help and
advice during the antimicrobial testing. Mr Kevin Reeves is
thanked for his assistance with SEM imaging and WDX
analysis. CCLRC Daresbury is thanked for provision of
XANES time.
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