# A Study of Factors that Change the Wettability of Titania Films

Authors: Andrew Mills; Matthew Crow
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

The effect of several pretreatment methods on the wettability of polycrystalline titania-coated glass (Pilkington Activ) and plain glass are investigated. UV/ozone, immersion in aqua regia, and heating (T>500∘C) render both substrates superhydrophilic (i.e., water contact angle (CA)<5∘). The dark recovery of the contact angles of these superhydrophilic substrates, monitored after treatment in either an evacuated or an ambient atmosphere, led to marked increases in CA. Ultrasound treatment of superhydrophilic Activ and glass samples produced only small increases in CA for both substrates, but rubbing the samples with a cloth produced much larger increases.  The results of this study are discussed in the context of the current debate over the mechanism of the photo-induced superhydrophilic effect.


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## Body

## 1. Introduction

Titanium dioxide films have been widely observed to become superhydrophilic,
that is, water droplet contact angle <5°, when exposed to ultraviolet (UV)
light, a phenomenon termed photoinduced superhydrophilicity (PSH) [1–7]. Its effects have been pivotal in the development of self-cleaning
surfaces for glazing and other applications, but the precise mechanism by which
this change is effected remains the subject of some debate, although it is
clear that PSH is initiated by the photogeneration of electron/hole pairs and
their migration to the surface. An essential feature of the overall PSH phenomenon
is not only the initial, photo-induced hydrophobic to hydrophilic change, but also
the reverse, dark process, whereby a superhydrophilic titania film recovers its
original hydrophobic form. Any mechanism of PSH must embrace both the
light-induced superhydrophilic process and the dark hydrophobic recovery step.In 1985 Kume and Nozu of Nihon Itagarasu, Japan reported that sheet
glass coated with TiO2 had the ability to stay clean by “…rapidly
and automatically decomposing and removing organic stains adhered to the glass
surface…” [8]. Most importantly, part of
their patent application involved the observation that the contact angle made
by a water droplet on the glass, that is, CA, was reduced as a function of UV irradiation
time. This appears to be the first clear claim of a PSH effect relating to
titania films on glass and explanation for the effect, namely, it is due to the
removal of hydrophobic organic stains on the surface, via the following,
well-established, photocatalytic oxidative (PCO) process: (1)organic+O2⟶hν≥3.2⁢eVTiO2minerals(e.g.,CO2⁢and⁢H2O). In contrast, in 1997, Wang et al. [6], reporting on the UV-induced PSH of TiO2/SiO2 films, proposed that the effect was not due to PCO but rather the dissociative
adsorption of water at surface defects created by UV light [9]. The defects, it was suggested, were formed by the trapping of
holes at bridging oxygen lattice points close to the surface, the Ti4+ sites being reduced to Ti3+ by the electrons and oxygen atoms being ejected
to form vacancies [10]. The oxygen vacancies were thought to cause an increase in the
adsorbed hydroxyl group density and lead to the formation of hydrophilic
regions. It was proposed that storage in the dark, in the presence of oxygen,
removed the defects replacing the chemisorbed hydroxyl groups with oxygen and
so returning the surface to its original hydrophobic state. A very simple
summary of this surface reorganisation mechanism for PSH is as follows: (2)≡Ti−O−Ti≡+H2O⁢⇆darkO2,hν≥EBG≡Ti−OH⁢HO−Ti≡. It has been suggested that the above mechanism is specific to only a
few materials of which titania is most notable, for example, it is not exhibited
by silica substrates, that is, glass. This photo-induced surface reorganisation
(PISR) model of PSH has received support from a number of different studies
including examination of the creation of Ti3+ defects on
single-crystal rutile TiO2 using XPS [11]; FTIR measurements that reveal anatase TiO2 is able to adsorb water reversibly
upon UV irradiation [10]; and AFM studies which show apparent “roughening” of the surface as
a result of UV exposure (attributed to defect formation) [12]. The importance of bridging oxygen, an integral part of the PISR
model, has also been illustrated using rutile single crystals, with the (001)
face—which does not feature bridging oxygens—showing the least propensity to photo-induced
hydrophilic change [10, 13].Recently, support for the more simplistic PCO model of PSH, based on
reaction (1), namely, the photocatalytic removal of hydrophobic organic surface
contaminants, has increased with a number of different groups reporting
evidence for this model using test hydrophobic surface contaminants such as
trimethyl acetate [14] and hexane [15], and the detection of previously unobserved layers of hydrocarbons
on titania surfaces [16]. These results have presented a significant challenge to the PISR
model of PSH.Key to any model of PSH is an explanation of the dark recovery step,
and significant support for this aspect of the PISR model of PSH effectively a
dark, dehydration step (see reaction (2)) has been provided by studies [10, 17, 18] which show that titania
films regain their original hydrophobicity at an increased rate when stored at
increased temperatures [18, 19] or under an evacuated (H2O
free) atmosphere [17]. The PISR model rationale behind this is that heat and vacuum cause
the metastable water/hydroxyl groups, responsible for the superhydrophilic
surface of titania, to dissociate more quickly. It has also been demonstrated
that hydrophilic TiO2-glass films exhibit a measurable increase in
contact angle when subjected to ultrasound [20]. The use of ultrasound to render a superhydrophilic film slightly
more hydrophobic was interpreted initially by advocates of the PISR model as being
due to the reoxidation of the surface by sonically produced OH radicals [20], but more recently as simply the breaking down of the photo-induced
hydrophilic metastable state by an external stimulus [18]. A similar interpretation has been used to explain the effect of wet
rubbing [21], which reconverts superhydrophilic titania to its original
hydrophobic state via a mechanochemical effect.In this paper, the effects of UV/O3, heating, and strong
acid pretreatment on initially hydrophobic samples of Activ and plain glass
were studied. In addition, the effects of vacuum, ultrasound, and rubbing on
the wettability of clean, superhydrophilic samples of titania-coated,
self-cleaning glass (Activ), and plain glass were examined and the results
discussed in terms of the two different mechanisms of the PSH effect.


## 2. Experimental

All reagents, unless otherwise stated, were supplied by Aldrich and used
as received. The titania-coated substrates used in this work were 2 cm× 2 cm Activ
samples, supplied by Pilkington Glass, comprising 4 mm thick float glass coated
with an approximately 15 nm layer of TiO2, deposited by an inline
CVD process [22]. The reverse side of Activ provided the samples of uncoated
float-glass used in all “plain glass” blank experiments.  Additional experiments revealed no difference
in behaviour between that of the reverse side of  Activ and uncoated float glass samples.  Typically, all glass samples, that is, Activ
or plain, were cleaned with chloroform and left in a sealed box in the dark for
in excess of one month before use. Unless otherwise stated, both types of glass
were rendered superhydrophilic by pretreating with UVC/ozone, effected by
irradiation with 2×6 W UVC (254 nm) bulbs (Vilber-Lourmat, Torcy, France), irradiance ~1 mW cm-2,
in a water saturated, that is, 100% relative humidity  (RH), oxygen atmosphere containing ~1700 ppm
ozone, produced by a corona-discharge ozone generator (OZ500, Dryden Aqua, Edinburgh, UK).Other pretreatment methods used included a heat-treatment process,
using a muffle furnace, in which the samples were kept at the desired
temperature for 1 hour before removing to cool to room temperature in a covered
Petri dish. Aqua regia was also used as a pretreatment method and involved immersing
the samples for 1 hour in the aqua regia before being removed, rinsed with
doubly-distilled, deionised water and dried in a stream of cylinder air.Water droplet contact angles, CAs, were measured using an FTA100
system comprising a CCTV camera interfaced with a computer, which provided a
continuous stream of images of any water droplet after its initial deposition
onto the surface of the substrate under test. Droplets were deposited using a
500μL Gastight (Hamilton)
syringe, via a 30-gauge stainless steel needle with 0° bevel (Kahnetics), which
deposited reproducible 5μL water droplets.
The software package associated with the instrument allowed calculation
of the contact angle made by the water droplet deposited on the substrate by
curve fitting the droplet image outline. Three droplets were deposited per
sample and the average of the contact angles determined with a typical variance
of <5%.The study of the kinetics of contact angle recovery of samples held under
vacuum was carried out using a vacuum desiccator connected to a vacuum line. Noncovered
samples were simply left on the benchtop in a light-free laboratory. For the
investigation of ultrasonic (US)
recovery, samples of superhydrophilic Activ or glass were immersed in
doubly-distilled, deionised water and placed in a US
bath (VWR model: USC100T). All
glassware for this section was thoroughly cleaned with chloroform and rinsed
with water before use. Sample handling was carried out using plastic forceps
and the samples were dried with compressed air from a cylinder.


## 3. Results and Discussion

### 3.1. Pretreatment Cleaning Methods

The combination of short wavelength ultraviolet light and ozone to
create electronically excited ozone,O3*, a very effective oxidising
agent, is well documented in its use for destroying organic materials and
scouring the latter from surfaces such as semiconductor wafers [23, 24]. Recent work by this
group has found that the combination of UVC and ozone as a pretreatment
produces a superhydrophilic surface on Activand plain glass. These
findings imply that the intrinsic state of a titania film or pristine glass is
superhydrophilic, as indicated by other work on both plain glass [25] and titania [26]—such as found on Activ—and that this state requires simply the
removal of all surface contaminants, particularly hydrophobic organics for its
production.The finding that UV/O3 pretreatment renders Activ
superhydrophilic fits in with both the PSIR and PCO models of PSH. However, the
observation that plain glass is also rendered superhydrophilic indicates that
the initial hydrophobic nature of the plain glass, and most probably Activ, is
due to contamination of the surface by hydrophobic organics, at a level too low
to be detected by simple FTIR. Thus, there appears no need to invoke a surface
reorganisation mechanism, that is, PISR to explain PSH in titania,  since the above results imply  any oxidative process that removes hydrophobic surface
organics, such as UVC/O3, will render the surface of titania and glass
superhydrophilic. The typical water droplet contact angles of plain and Activ
glass samples before and after UV/O3 treatment are given in Table 1.Table 1
Table of water contact angles for Activ and glass samples before
and after pretreatment.
Pretreatment methodActivGlassinitial CA/°final CA/°initial CA/°final CA/°UVC/O3(a)71<535<5Δ 500°C(b)70<537<5Aqua regia(c)70<538<5(a) Samples irradiated by 2×6 W UVC bulbs
in  ~1700 ppm O3 for 1 hour.(b) Samples heated to 500°C and held at
temp for 1 hour before cooling under Petri dish.(c) Immersion in 25 mL aqua regia for 1
hour before rinsing and drying.A series of experiments were carried out to investigate a variety of
other oxidising pretreatment methods, including heating and strong acid
immersion, to ascertain if all were able to render Activ and plain glass
superhydrophilic (i.e., CA < 5°). Thus, heating Activ and plain glass at
different temperatures and testing their hydrophilic natures, via CA
measurements, produced the results illustrated in Figure 1. These results show
that for both substrates the CA decreases with increasing temperature, with
glass falling from an initial contact angle, CAi, ~35° to  <5° by 300°, and Activ from CAi  ~70° to <5° by 550°.Figure 1
Plot of Contact angle versus pre-treatment temperature forActiv(∘) and for glass (•).That the titania substrates become hydrophilic with heat treatment appears
at odds with the PSIR model of PSH, that is, reaction (2),  which suggests that
the dark dehydration process should be promoted by high temperatures [27], leading to either no change, or an increase in its hydrophobic
nature and not, as observed in  Figure 1, a decrease.  Instead, the results in Figure 1 indicate that
the initial hydrophobic character of Activ and plain glass is due to organic
surface contaminants that are readily oxidised by high-heat treatment to reveal
the intrinsic superhydrophilic nature of glass and titania films.  From the data in Figure 1, it appears that the
plain glass samples become hydrophilic at lower temperatures than Activ,
probably due to a lower level of contamination on glass, which is much smoother
than the titania films, rendering the latter more susceptible to organic
adsorption.In a final set of experiments, Activ and plain glass samples that were
initially hydrophobic were pretreated with the very oxidising reagent, aqua
regia, and all found to be rendered superhydrophilic (see Table1).From the above results, it appears that all three methods of
pretreatment, that is, UVC/O3, heat (>550°C), and aqua regia are
able to effect the conversion of both Activ and plain glass samples from an
initial hydrophobic state to a superhydrophilic state. All three methods use
very strong oxidising agents/conditions to render plain and Activ glass samples
superhydrophilic, most probably due to the oxidation of any surface hydrophobic
organic species. These results provide significant support for the PCO model of
PSH and reveal that the pristine, that is, organic-free, surfaces of these
materials are superhydrophilic.


### 3.2. Studies of the “Dark” Hydrophilic to Hydrophobic Process

The “dark” recovery of contact angle, that is, the regeneration of
hydrophobicity, exhibited by superhydrophilic titania films has been widely
reported [10, 17, 22, 28], and opinion
over its cause is polarised between a recontamination of the surface by
airborne organics (the PCO model of PSH) and the reverse of reaction (2), that
is, a dehydroxylation/dehydration process on the surface of the titania (the PISR
model of PSH). Work carried out by others [17] shows that superhydrophilic samples of titania become more
hydrophobic when stored in an evacuated atmosphere. These results are
interpreted as providing strong evidence of the PISR model, in which vacuum
storage accelerates water desorption and therefore CA recovery in superhydrophilic
samples of titania. In replicating this work, we also found that the CA of
Activ increases markedly when the samples are stored under vacuum, but also,
and more revealing, that plain glass exhibits the same feature, although to a
lesser extent. These results, illustrated in Figure 2, indicate that the
phenomenon of the CA increasing for superhydrophilic titania films held under vacuum—used to support the “surface reorganisation”
model, that is, reaction (2)—is not specific to titania
films but is also exhibited by plain glass and so not likely to be associated
with a PISR mechanism, which is attributed to titania but not to glass.
Instead, the more likely explanation is that it is due to organic contamination
of a clean surface, that is, glass or titania in this work, produced by
exposing the sample to air when making the CA measurement after storage in a
vacuum.Figure 2
Water droplet contact (CA) angle versus exposure time in an evacuated
atmosphere forActiv(∘) and glass (•); CA
measurements recorded under ambient conditions.The effect of airborne contamination of a pristine, superhydrophilic
glass, and Activ surface was demonstrated by leaving two freshly prepared
samples out on the bench and monitoring the change in CA as a function of time.
The results of this work are illustrated in Figure3 from which  it is
clear that, on the open bench, a pristine, superhydrophilic sample of glass, or
Activ can be rendered noticeably hydrophobic within few hours of exposure. Note
that, as expected from its lower surface roughness, over the same time plain
glass is rendered less hydrophobic than Activ due to this surface
contamination. Simple IR analysis of these glass samples revealed no indication
of contamination of their surfaces by hydrophobic organics, implying that the
level is very low and below that detectable by conventional FTIR spectroscopy.Figure 3
Variation in CA versus time for initially pristine superhydrophilic sample
ofActivTM(▫) and glass (▪) under ambient laboratory
conditions.Previous work carried out by others [20] has shown that a superhydrophilic titania film is rendered more
hydrophobic upon exposure to ultrasound (10 minutes). In another set of experiments,
both glass and titania films, rendered superhydrophilic by treatment with UVC/O3,
were placed in high-purity water in clean beakers and treated with ultrasound.
The measured CAs of these samples before and after exposure to ultrasound are
given in Table 2 and show that for both samples upon sonication the CA rises slightly, by 3–5°, whereas without
sonication the CA remains largely unchanged. These results imply that the effect of ultrasound on
superhydrophilic films, be they glass or titania, is very small and most
probably due to a roughening of the substrate, and/or some organic contamination.  This effect is, once again, not specific to titania and would have
been expected if the
PISR model was appropriate.Table 2
Table of water contact angle for Activ and glass samples before
and after either sonication or immersion for 10 minutes in deionised (DI) water.
TreatmentActivGlassinitial CA/°final CA/°initial CA/°final CA/°Sonication for 10 minutes
in DI water<58.93.88.2DI water immersion for
10 minutes<56.44.35.3Other work shows that any wiping or rubbing of a pristine
superhydrophilic glass or titania film using a tissue or cloth renders the film
hydrophobic, and this is attributed, once again, to organic contamination of
the surfaces of these substrates. It appears unlikely that rubbing is able to
effect the reverse of reaction (2) via a mechanochemical effect as claimed by others [21] since plain glass is affected in the same way as titania.


## 3.1. Pretreatment Cleaning Methods

The combination of short wavelength ultraviolet light and ozone to
create electronically excited ozone,O3*, a very effective oxidising
agent, is well documented in its use for destroying organic materials and
scouring the latter from surfaces such as semiconductor wafers [23, 24]. Recent work by this
group has found that the combination of UVC and ozone as a pretreatment
produces a superhydrophilic surface on Activand plain glass. These
findings imply that the intrinsic state of a titania film or pristine glass is
superhydrophilic, as indicated by other work on both plain glass [25] and titania [26]—such as found on Activ—and that this state requires simply the
removal of all surface contaminants, particularly hydrophobic organics for its
production.The finding that UV/O3 pretreatment renders Activ
superhydrophilic fits in with both the PSIR and PCO models of PSH. However, the
observation that plain glass is also rendered superhydrophilic indicates that
the initial hydrophobic nature of the plain glass, and most probably Activ, is
due to contamination of the surface by hydrophobic organics, at a level too low
to be detected by simple FTIR. Thus, there appears no need to invoke a surface
reorganisation mechanism, that is, PISR to explain PSH in titania,  since the above results imply  any oxidative process that removes hydrophobic surface
organics, such as UVC/O3, will render the surface of titania and glass
superhydrophilic. The typical water droplet contact angles of plain and Activ
glass samples before and after UV/O3 treatment are given in Table 1.Table 1
Table of water contact angles for Activ and glass samples before
and after pretreatment.
Pretreatment methodActivGlassinitial CA/°final CA/°initial CA/°final CA/°UVC/O3(a)71<535<5Δ 500°C(b)70<537<5Aqua regia(c)70<538<5(a) Samples irradiated by 2×6 W UVC bulbs
in  ~1700 ppm O3 for 1 hour.(b) Samples heated to 500°C and held at
temp for 1 hour before cooling under Petri dish.(c) Immersion in 25 mL aqua regia for 1
hour before rinsing and drying.A series of experiments were carried out to investigate a variety of
other oxidising pretreatment methods, including heating and strong acid
immersion, to ascertain if all were able to render Activ and plain glass
superhydrophilic (i.e., CA < 5°). Thus, heating Activ and plain glass at
different temperatures and testing their hydrophilic natures, via CA
measurements, produced the results illustrated in Figure 1. These results show
that for both substrates the CA decreases with increasing temperature, with
glass falling from an initial contact angle, CAi, ~35° to  <5° by 300°, and Activ from CAi  ~70° to <5° by 550°.Figure 1
Plot of Contact angle versus pre-treatment temperature forActiv(∘) and for glass (•).That the titania substrates become hydrophilic with heat treatment appears
at odds with the PSIR model of PSH, that is, reaction (2),  which suggests that
the dark dehydration process should be promoted by high temperatures [27], leading to either no change, or an increase in its hydrophobic
nature and not, as observed in  Figure 1, a decrease.  Instead, the results in Figure 1 indicate that
the initial hydrophobic character of Activ and plain glass is due to organic
surface contaminants that are readily oxidised by high-heat treatment to reveal
the intrinsic superhydrophilic nature of glass and titania films.  From the data in Figure 1, it appears that the
plain glass samples become hydrophilic at lower temperatures than Activ,
probably due to a lower level of contamination on glass, which is much smoother
than the titania films, rendering the latter more susceptible to organic
adsorption.In a final set of experiments, Activ and plain glass samples that were
initially hydrophobic were pretreated with the very oxidising reagent, aqua
regia, and all found to be rendered superhydrophilic (see Table1).From the above results, it appears that all three methods of
pretreatment, that is, UVC/O3, heat (>550°C), and aqua regia are
able to effect the conversion of both Activ and plain glass samples from an
initial hydrophobic state to a superhydrophilic state. All three methods use
very strong oxidising agents/conditions to render plain and Activ glass samples
superhydrophilic, most probably due to the oxidation of any surface hydrophobic
organic species. These results provide significant support for the PCO model of
PSH and reveal that the pristine, that is, organic-free, surfaces of these
materials are superhydrophilic.


## 3.2. Studies of the “Dark” Hydrophilic to Hydrophobic Process

The “dark” recovery of contact angle, that is, the regeneration of
hydrophobicity, exhibited by superhydrophilic titania films has been widely
reported [10, 17, 22, 28], and opinion
over its cause is polarised between a recontamination of the surface by
airborne organics (the PCO model of PSH) and the reverse of reaction (2), that
is, a dehydroxylation/dehydration process on the surface of the titania (the PISR
model of PSH). Work carried out by others [17] shows that superhydrophilic samples of titania become more
hydrophobic when stored in an evacuated atmosphere. These results are
interpreted as providing strong evidence of the PISR model, in which vacuum
storage accelerates water desorption and therefore CA recovery in superhydrophilic
samples of titania. In replicating this work, we also found that the CA of
Activ increases markedly when the samples are stored under vacuum, but also,
and more revealing, that plain glass exhibits the same feature, although to a
lesser extent. These results, illustrated in Figure 2, indicate that the
phenomenon of the CA increasing for superhydrophilic titania films held under vacuum—used to support the “surface reorganisation”
model, that is, reaction (2)—is not specific to titania
films but is also exhibited by plain glass and so not likely to be associated
with a PISR mechanism, which is attributed to titania but not to glass.
Instead, the more likely explanation is that it is due to organic contamination
of a clean surface, that is, glass or titania in this work, produced by
exposing the sample to air when making the CA measurement after storage in a
vacuum.Figure 2
Water droplet contact (CA) angle versus exposure time in an evacuated
atmosphere forActiv(∘) and glass (•); CA
measurements recorded under ambient conditions.The effect of airborne contamination of a pristine, superhydrophilic
glass, and Activ surface was demonstrated by leaving two freshly prepared
samples out on the bench and monitoring the change in CA as a function of time.
The results of this work are illustrated in Figure3 from which  it is
clear that, on the open bench, a pristine, superhydrophilic sample of glass, or
Activ can be rendered noticeably hydrophobic within few hours of exposure. Note
that, as expected from its lower surface roughness, over the same time plain
glass is rendered less hydrophobic than Activ due to this surface
contamination. Simple IR analysis of these glass samples revealed no indication
of contamination of their surfaces by hydrophobic organics, implying that the
level is very low and below that detectable by conventional FTIR spectroscopy.Figure 3
Variation in CA versus time for initially pristine superhydrophilic sample
ofActivTM(▫) and glass (▪) under ambient laboratory
conditions.Previous work carried out by others [20] has shown that a superhydrophilic titania film is rendered more
hydrophobic upon exposure to ultrasound (10 minutes). In another set of experiments,
both glass and titania films, rendered superhydrophilic by treatment with UVC/O3,
were placed in high-purity water in clean beakers and treated with ultrasound.
The measured CAs of these samples before and after exposure to ultrasound are
given in Table 2 and show that for both samples upon sonication the CA rises slightly, by 3–5°, whereas without
sonication the CA remains largely unchanged. These results imply that the effect of ultrasound on
superhydrophilic films, be they glass or titania, is very small and most
probably due to a roughening of the substrate, and/or some organic contamination.  This effect is, once again, not specific to titania and would have
been expected if the
PISR model was appropriate.Table 2
Table of water contact angle for Activ and glass samples before
and after either sonication or immersion for 10 minutes in deionised (DI) water.
TreatmentActivGlassinitial CA/°final CA/°initial CA/°final CA/°Sonication for 10 minutes
in DI water<58.93.88.2DI water immersion for
10 minutes<56.44.35.3Other work shows that any wiping or rubbing of a pristine
superhydrophilic glass or titania film using a tissue or cloth renders the film
hydrophobic, and this is attributed, once again, to organic contamination of
the surfaces of these substrates. It appears unlikely that rubbing is able to
effect the reverse of reaction (2) via a mechanochemical effect as claimed by others [21] since plain glass is affected in the same way as titania.


## 4. Conclusions

UV light is well known to render titania samples superhydrophilic,
but this can also be effected for glass and titania films by the combination of
UVC and ozone or pretreatment with a highly oxidising solution (e.g., aqua regia)
or by heating atT>500°C, suggesting that a common process is responsible
for the cleaning of both substrates, namely, the removal of adventitious hydrophobic
organic surface species via oxidation. These findings indicate that the
pristine surfaces of plain glass and titania are intrinsically superhydrophilic
and imply that the
photocatalytic oxidation of organics on titania (PCO) is the most likely cause
of the PSH effect.Investigations of the recovery of the initial hydrophobicity
exhibited by untreated titania or plain glass showed that, once rendered
superhydrophilic, the dark hydrophilic to hydrophobic process is exhibited by
both titania and plain glass films and is most likely due to contamination by
airborne, hydrophobic organics. Thus, storage of titania or plain glass under
vacuum, or on the open bench, leads to a noticeable increase in CA. The effect
of ultrasonic treatment on contact angle recovery is very small for both plain
and titania-coated glass, and probably due to surface roughening. Rubbing or
wiping the hydrophilic plain glass or titania films produces a marked rise in
hydrophobicity and is attributed to contamination of the surfaces by
hydrophobic organics.The work presented here provides further support for the proposal
that the PSH effect is due to the UV-driven removal of hydrophobic surface
organics by PCO, and the dark, slow readsorption of airborne organics.



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*Source: 470670-2008-04-13.xml*