# A Rapid Method of Assessing the Photocatalytic  Activity of ThinTiO2 Films Using an Ink Based on  the Redox Dye 2,6-Dichloroindophenol

Authors: Andrew Mills; Mark McGrady; Jishun Wang; James Hepburn
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

An indicator ink based on the redox
dye 2,6-dichloroindophenol (DCIP) is described, which allows
the rapid assessment of the activity of thin,
commercial photocatalytic films, such as Activ.  The ink works via a photoreductive mechanism, DCIP being reduced to dihydro-DCIP within ca. 7.5 minutes exposure to UVA irradiation of moderate intensity (ca. 4.8 mWcm−2).  The kinetics of photoreduction are found to be independent
of the level of dye present in the ink formulation, but are highly sensitive to the level of glycerol.  This latter observation may be associated with a solvatochromic effect, whereby the microenvironment in which the dye finds itself and, as a consequence, its reactivity is altered significantly by small changes in the glycerol content.  The kinetics of photoreduction also appear linearly dependent on the UVA light intensity with an observed quantum efficiency of ca. 1.8×10−3.


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## Body

## 1. Introduction

Semiconductor photocatalysis  (SPC) has
progressed significantly since the early research into the possible use of
semiconductor photochemistry to split water 30 years ago [1], such that it is
now one of the most actively researched areas in photochemistry and is
associated with several, major commercial products [2]. Its potential use in environmental
remediation, based on the following reaction: (1)Organicmaterial+O2⟶UVAlightTiO2Mineralacids+H2O+CO2, has been the main reason for much of
the interest and the semiconductor which has been the most heavily exploited
for this purpose is titania (TiO2). Despite having a relatively large bandgap (ca. 3.2 eV), thus
necessitating the use of ultraviolet light (UV) to initiate the photocatalytic
process, titania is chemically and biologically inert, mechanically robust,
inexpensive, and highly active. Of the
three known forms of titania (anatase, rutile, and brookite), it is generally
accepted that anatase TiO2 is the most active form for SPC
applications.Upon activation of the titania with
UV light, an electron-hole pair is produced within the bulk of the
material. These species usually
recombine rapidly; however, for reaction (1) to occur, the electron-hole pair must
migrate to the surface of the semiconductor, whereupon the photogenerated electron
reduces atmospheric O2 to superoxide, O2−, which
can be reduced further to water through a number of highly active intermediates,
one of which is believed to be the strongly oxidising hydroxyl radical, OH•. Conversely, the photogenerated hole is
capable of oxidising surface TiOH groups to radicals, that is, TiOH•, that are
capable of oxidising many organic materials/pollutants into mineral acids and
CO2.One of the reasons why titania has proved
successful in the removal of both aqueous and atmospheric contaminants is that
it is easily adhered to a suitable supporting substrate, which can subsequently
be exposed to either an aqueous or airborne pollutant. Suitable methods for preparing titania films
on the surface of a substrate include using a sol-gel method [3, 4], physical
vapour deposition (i.e., sputtering) [5, 6], and chemical vapour deposition
(CVD) [7, 8]. All of these techniques
have been exploited in the research laboratories, but the method which has met
with the most commercial success is CVD. Indeed, the world first self-cleaning glass, Activ [9], formulated by Pilkington
Glass (Lathom, UK),  is manufactured using an atmospheric pressure CVD (APCVD) technique
[10]. Many of the World's other major
glass manufacturers have followed the lead of Pilkington Glass and now
manufacture their own self-cleaning product [11, 12]. More recently, there has also been the
emergence of self-cleaning tiles [13], paints [14], and concrete [15] products,
particularly from Japan.With so many photocatalytic
materials appearing on the market, there is an increasing interest in the
development of one or more standard methods for assessing the activity of both
laboratory and commercial samples. To date,
there are many tests which have been used in the research laboratories to
elucidate film activity, including the destruction of nitrogen oxides [16–18], the
mineralization/degradation of dyes [19, 20] (particularly methylene blue [21, 22]), and the destruction of stearic acid [23, 24]. However, with all the methods outlined above,
whilst suitable for use in the research laboratories, the prospect of using them
in the field, for example, to demonstrate the efficacy of the photocatalytic
film to potential customers, or test samples in situ, is limited, since each
involves some degree of sample preparation, use of expensive, often bulky,
analytical equipment, and technical skill to run the method and assess the
results. From an industrial perspective
at least, what is needed is a rapid, semiquantitative (if not quantitative) method
of demonstrating and assessing rapidly the activity of any photocatalytic
coating.One possible route is to use a
photocatalyst indicator ink which, when deposited onto a photocatalytic
coating, changes colour upon UV activation of the coating. Such an ink has been proposed recently based
on the blue redox dye, 2,6-dichloroindophenol (DCIP) [25], the structure of
which is illustrated in Figure 1. The
overall process involving the ink is as follows: (2)DCIP(blue)+glycerol⟶UVAlightTiO2leuco‐DCIP(colourless)+glyceraldehyde. The DCIP ink has the striking feature
that it is blue before UV irradiation, but colourless, due to the formation of
leuco-DCIP (the structure of which is also shown in Figure 1), afterwards. The photocatalytic process is very rapid, that
is, within 7 minutes using typical solar UVA levels (ca. 4.5 mW cm−2)
on a commercial self-cleaning glass, such as Activ. In
addition, the photoreduction of the dye has also been shown to be
unaffected by both the level of O2 and the humidity of the
environment under which the ink is being used.
This article reports the characterisation of a DCIP photocatalyst indicator
ink when used on a typical, commercial self-cleaning glass (Activ).Figure 1
The structures of the sodium salt of DCIP (left) and its reduced product,
dihydro-DCIP.


## 2. Experimental

Unless stated otherwise, all
reagents were Analar grade and used, as received, from Sigma-Aldrich. The conditions referred to as “ambient atmospheric conditions” are on the open bench at room temperature (ca. 20°C)
and a relative humidity of ca. 60%.A typical DCIP ink was prepared as
follows: 3 g of a 1.5 wt%
hydroxyethylcellulose (Fluka, medium viscosity) solution in water were placed in
a sample bottle and to it were added 0.3 g of glycerol. The resulting solution was gently stirred
whilst 5 mg of 2,6-dichloroindophenol (supplied by Alfa-Aesar, sodium salt
hydrate, 98%) were added slowly. The
solution was placed in an ultrasound bath and sonicated for 5 minutes in order
to ensure complete dissolution of the dye, before being subjected to further
gentle stirring for 30 minutes. Ink
produced in this manner appeared stable and unchanged for at least 1month, and
was used as required. In terms of parts
per hundred resin (pphr), the level of polymer/SED/dye in the ink formulation
is equivalent to a ratio of 100/667/11.  Such a DCIP ink is referred to henceforth as
the standard formulation.The photocatalyst substrate used in
this work was Activ™, kindly supplied by Pilkington Glass. Full details on its preparation can be found
elsewhere [10]. The bulk Activ™ samples
received were cut into 25 mm × 25 mm × 4 mm thick samples so that it was possible
to place them in a home-made cell holder for making spectral measurements using
a UV-vis spectrophotometer. Coating of a
typical glass sample was achieved by smearing approximately 1 mL of the ink under
test over the sample surface and spinning the sample, using an Electronic Micro
Systems Model 4000-1 spin coater, at 500 rpm for 15 s, which generates a film
ca. 900 nm thick on the photocatalyst surface. The ink was then further dried by placing the spun-coated Activ™
substrate in an oven at 70°C for 10 minutes, although similar kinetic results
were also observed when the ink-coated substrate was simply allowed to dry in air in the
absence of light for 1 hour.The UV-driven reduction of the dye
on the photocatalyst surface was monitored via UV-visible spectroscopy, using a
Cary 50 Bio UV-visible spectrophotometer. The instrument was configured to scan in the wavelength range 300–800 nm, with scans taken at time intervals
between UV irradiations typically ranging from 10 s to 10 minutes depending
upon the observed rate of photoreduction (i.e., photobleaching). UV irradiations were conducted using a
hand-held, 2 × 4 W Black Light Blue (BLB) UVA source (peak wavelength 365 ± 20 nm).  Unless stated otherwise, the UVA irradiance
used for all irradiations was ca. 4.8 mW cm−2, which is comparable
to the standard UVA irradiance on a sunny day (ca. 4.5 mW cm−2) [26].


## 3. Results and Discussion

Both the DCIP ink and the resulting
dried ink film on the desired substrate are blue in colour, and this blue
colour persists when the ink is cast on plain glass, or indeed any substrate
with nil photocatalyst, and irradiated with UVA light for an extended (ca. 1
hour) time period. With such films, no
significant change in the absorbance was noted over this time period at the
wavelength of maximum absorbance,λmax (ca. 629 nm and HPBW ca. 100 nm). In aqueous
solution, a dilute (i.e., 10−4 M) solution of DCIP has a λmax at 600 nm and a half peak band width (HPBW) of ca. 130 nm (measured in house). Therefore, in the ink it is observed that the
λmax is
slightly red shifted compared to what is observed in solution. However, a similar HPBW is observed with the
dried ink film, which is indicative of multimer (dimer, trimer, etc.) formation. The formation of said multimers in the dried
film is not surprising given the effective dye concentration of 0.064 M.In contrast, when cast on Activ, the blue ink film
is observed to be rapidly photobleached upon irradiation with UVA light.
Figure2(a) shows the observed absorption
spectra obtained for a DCIP ink cast as a film on the surface of Activ and subsequently irradiated
with UVA light. Thus, upon UV
irradiation of a typical DCIP film on Activ, the absorbance of the film decreases with
time (as indicated by the arrow) and the ink is completely bleached, that is,
the dye is fully reduced to dihydro-DCIP, after ca. 7.5 minutes. The change in the absorbance (ΔAbs=Abst−Abs∞, where Abst and Abs∞ are the absorbance of the film at UV irradiance times t and after complete
photobleaching, resp.) at λmax is shown
more explicitly in Figure 2(b). The
initial rate of reduction of the ink, ri, is derived from the
absorbance data recorded over the time necessary for the initial absorbance of
the ink to decrease by ca. 20% of its initial value. Based upon the data presented in Figures 2(a)
and 2(b), the initial rate of reduction of a typical DCIP ink under ambient
atmospheric conditions using 4.8 mW cm−2 UVA light is ca. 4.5 × 10−4 AbsU s−1, where AbsU is used to denote absorbance units, thus
avoiding confusion that the value quoted is a rate constant.(a) The spectral shape obtained for a standard DCIP ink cast on the surface of Activ, and the subsequent
changes (from top to bottom) in this spectral shape as the sample is irradiated
with UVA light at an intensity of ca. 4.8 mW cm−2. Spectra were recorded at 30-second intervals.
(b) The change in the absorbance (ΔAbs) at λmax (629 nm) with UVA irradiation of a DCIP ink, based on the data presented in Figure 2(a). (c) The reduction, and subsequent recovery, of a DCIP ink on the surface
of Activ at 629 nm. The reduction conditions were
similar to those detailed previously in Figure 2(a), whereas the ink recovery
was monitored in the dark with spectra recorded at 1 hour intervals.
(a)(b)(c)Since DCIP is a redox dye (E°(DCIP/dihydroDCIP)
= + 0.228 V at pH 7 versus SHE [27]), it is possible that, once DCIP has been reduced
on the surface of Activ
to dihydro-DCIP, the latter may be reoxidised by atmospheric O2, regenerating
the initial level of DCIP, and thus restoring the original blue colour of the
ink film. However, monitoring the
absorbance at λmax of a
typical DCIP film on Activ
before and after UV bleaching revealed that the photobleached DCIP only
partially (ca. 38%) recovers its original colour over a period of many hours
(see Figure 2(c)).The initial rate of recovery in
absorbance at 629 nm of the ink, from Figure2(c), is typically 1.8 × 10−6 AbsU s−1, a rate which is more than 100 times slower than its
initial rate of reduction, thus allaying any concerns that the “dark” reoxidation of dihydro-DCIP by ambient O2 would compete with the
reduction of the dye during irradiation, that is, effectively, the
photobleaching of the DCIP ink on Activ appears an irreversible process. This observation supports the results
of previous work on the ink, where the kinetics of photoreduction by Activ were assessed under
varying levels of oxygen and noted to be the same under both anaerobic and O2-saturated
conditions [25].


### 3.1. The Effect of Dye Loading on the Initial Rate of Photoreduction of a Typical DCIP Film on Activ

The variation ofri as a
function of dye loading (5 mg dye per 45 mg polymer in a typical ink
formulation) was studied over the range 1–10 mg (equivalent to 2.2–22 pphr) and
the results of this work are illustrated in Figure 3. From these results, it appears that ri is approximately independent of dye loading, with an observed average ri of 5.2 ×
10−4 AbsU s−1. This result is in stark contrast to the behaviour of the Rz photocatalyst
indicator ink reported previously [28] over a similar dye loading range, where
the initial rate of reduction was observed to decrease by a factor of 4 as the
dye level was increased from 1 to 8 mg. This latter effect was attributed to an increasing level of less
reactive Rz dimers with increasing dye loading, supported by spectral changes
showing a shoulder peak emerging at 570 nm as the dye loading was increased up
to 40 mg. In contrast, the shape of the
absorption spectrum of the DCIP ink did not change as a function of dye loading. The invariance of ri as a function
of [substrate] is not unusual in semiconductor photochemistry and implies that
either all the reactive sites associated with dye reduction are occupied by
molecules of DCIP; or that the rate-determining step in reaction (2) does not
involve the reduction of DCIP.Figure 3
The variation in the initial rate of reduction(ri) of a DCIP ink on
Activ as the level
of dye in the ink formulation is altered between 1 and 10 mg. The dotted line indicates the calculated
average ri of 5.21 × 10−4 AbsU s−1.


### 3.2. Effect of Glycerol Level on the Initial Rate of Photoreduction

In the standard ink formulation,
glycerol is present to act as a sacrificial electron donor (SED); essentially, the
glycerol molecules act as “hole traps,” preventing recombination of the
photogenerated electron-hole pair by undergoing oxidation, thus allowing any
photogenerated electrons to reduce the dye molecules in the ink at the
photocatalyst surface. If an ink formulation
is prepared with no glycerol present, cast on the surface of Activ, and irradiated
with UVA light over a prolonged irradiation period (ca. 1 hour), there is no
observed photobleaching/reduction of the dye on the photocatalyst surface,
indicating that the removal of the SED from the DCIP ink results simply in electron-hole
recombination becoming the predominant process.Experiments show thatri is independent of glycerol loading over the range 0.1–0.9 g (typically, 0.3 g glycerol is present in
the formulation), as demonstrated in Figure 4. However, ri does vary markedly from 0–0.1 g (0–222 pphr) glycerol loading, as illustrated by
the results in Figure 5. The initial
rate of reduction, ri, is observed to increase steadily as the
glycerol level is increased from 0 to ca. 0.03 g (0–66 pphr), after which point
a plateau value is achieved. In addition
to the above kinetic data, Figure 5 also charts the observed variation in the λmax of the
resulting DCIP ink as the glycerol level is varied over the same range. It would appear from this plot that the
variations in λmax and ri are linked in some way, with λmax being blue-shifted
as the glycerol level is increased. The
likely cause of this shift in the peak wavelength is a solvatochromic effect, brought
about by a change in the solvation shell of the dye molecules in the ink
formulation and subsequent ink film as the glycerol  level is increased. Such a change in the solvation shell is
likely to alter the reactivity of the DCIP and may be responsible for the
variation in ri with glycerol loading illustrated
in Figure 5.Figure 4
The variation inri as the level of glycerol in the ink formulation
is altered between 0.1 and 0.9 g. As in
Figure 3, the dotted line in the plot indicates the average ri of
5.15 × 10−4 AbsU s−1.Figure 5
The variation inri(□) and the λmax(∘) as the level of
glycerol in the ink formulation is altered between 0 and 0.1 g.However, there may be other effects
working here since the work by Van Keuren and Schrof [29] showed  that in a PVA/glycerol
solution, where the glycerol content was varied between 10 and 40 wt%, the
diffusion coefficient for a dye, rhodamine B, increased by a factor of 104. Beyond 40 wt% glycerol, the dye exhibited a
diffusion coefficient similar to that observed for the dye in neat glycerol. In the DCIP ink formulations presented here, with
a 0.1 g glycerol loading, the glycerol wt% is 66 wt%, and thus the variation in
ri as a function of glycerol loading illustrated in Figure 5 could
be simply due to the variation in the diffusion coefficient of the dye from the
bulk film to the surface of Activ,
assuming that this is the rate determining step for the photoreduction/bleaching
process. In this instance, the maximum rate
observed at glycerol loading ≥0.1 g is due to the diffusion coefficient for
DCIP in the film (HEC/glycerol) reaching a limiting value, as was observed by
Van Keuren for rhodamine B in PVA/glycerol.


### 3.3. The Effect of UVA Light Irradiance on the Initial Rate of Photoreduction

In a final set of experiments, the
initial rate of reduction of a typical DCIP film was determined as a function
of UVA light irradiance, varied from 0.44 to 7.41 mW cm−2, and the
results are illustrated in Figure 6. Not
surprisingly, ri increases with increasing light irradiance, since
the number of photons reaching the photocatalyst surface, and hence the number
of photogenerated electrons produced, will increase. The strong linear dependence of ri on the UV light irradiance is attributed to the relatively low light
intensities used here to reduce the dye, the low absorbance of the underlying Activ photocatalyst layer,
and the fact that, at such low UV irradiance levels, the reduction reaction
will be controlled by the reactions at the ink/photocatalyst interface of the
photogenerated electron-hole pairs, as opposed to their recombination (which is
expected at high irradiance levels). A similar linear dependence of ri on the UV irradiance was observed
for the Rz-based photocatalyst indicator ink [28].Figure 6
The variation inri with UVA irradiance.SinceΔAbst for the reduction of DCIP is ca. 0.085 AbsU, as
illustrated earlier in Figure 2(b), for the standard ink formulation, and since
it is known that 1 mW cm−2 of 365 nm light is equivalent to 1.1 × 1017 photons per cm2 per minute, the quantum efficiency for the process
can be estimated to be 0.0018 molecules per photon. It is also known from spectral studies
conducted elsewhere that Activ
only absorbs ca. 7.2% of the incident UVA irradiation from black light blue
lamps [30], thus the quantum yield for the reduction of DCIP on the substrate
can be estimated to be 2.5%. Both of
these estimated values are greater than those observed for the Rz ink (2.5×10−4  molecules per photon and 0.35%, resp.) [28], although in practice the initial rate of photoreduction of DCIP is slower than that of Rz in the ink
formulation under ambient atmospheric conditions [25].


## 3.1. The Effect of Dye Loading on the Initial Rate of Photoreduction of a Typical DCIP Film on Activ

The variation ofri as a
function of dye loading (5 mg dye per 45 mg polymer in a typical ink
formulation) was studied over the range 1–10 mg (equivalent to 2.2–22 pphr) and
the results of this work are illustrated in Figure 3. From these results, it appears that ri is approximately independent of dye loading, with an observed average ri of 5.2 ×
10−4 AbsU s−1. This result is in stark contrast to the behaviour of the Rz photocatalyst
indicator ink reported previously [28] over a similar dye loading range, where
the initial rate of reduction was observed to decrease by a factor of 4 as the
dye level was increased from 1 to 8 mg. This latter effect was attributed to an increasing level of less
reactive Rz dimers with increasing dye loading, supported by spectral changes
showing a shoulder peak emerging at 570 nm as the dye loading was increased up
to 40 mg. In contrast, the shape of the
absorption spectrum of the DCIP ink did not change as a function of dye loading. The invariance of ri as a function
of [substrate] is not unusual in semiconductor photochemistry and implies that
either all the reactive sites associated with dye reduction are occupied by
molecules of DCIP; or that the rate-determining step in reaction (2) does not
involve the reduction of DCIP.Figure 3
The variation in the initial rate of reduction(ri) of a DCIP ink on
Activ as the level
of dye in the ink formulation is altered between 1 and 10 mg. The dotted line indicates the calculated
average ri of 5.21 × 10−4 AbsU s−1.


## 3.2. Effect of Glycerol Level on the Initial Rate of Photoreduction

In the standard ink formulation,
glycerol is present to act as a sacrificial electron donor (SED); essentially, the
glycerol molecules act as “hole traps,” preventing recombination of the
photogenerated electron-hole pair by undergoing oxidation, thus allowing any
photogenerated electrons to reduce the dye molecules in the ink at the
photocatalyst surface. If an ink formulation
is prepared with no glycerol present, cast on the surface of Activ, and irradiated
with UVA light over a prolonged irradiation period (ca. 1 hour), there is no
observed photobleaching/reduction of the dye on the photocatalyst surface,
indicating that the removal of the SED from the DCIP ink results simply in electron-hole
recombination becoming the predominant process.Experiments show thatri is independent of glycerol loading over the range 0.1–0.9 g (typically, 0.3 g glycerol is present in
the formulation), as demonstrated in Figure 4. However, ri does vary markedly from 0–0.1 g (0–222 pphr) glycerol loading, as illustrated by
the results in Figure 5. The initial
rate of reduction, ri, is observed to increase steadily as the
glycerol level is increased from 0 to ca. 0.03 g (0–66 pphr), after which point
a plateau value is achieved. In addition
to the above kinetic data, Figure 5 also charts the observed variation in the λmax of the
resulting DCIP ink as the glycerol level is varied over the same range. It would appear from this plot that the
variations in λmax and ri are linked in some way, with λmax being blue-shifted
as the glycerol level is increased. The
likely cause of this shift in the peak wavelength is a solvatochromic effect, brought
about by a change in the solvation shell of the dye molecules in the ink
formulation and subsequent ink film as the glycerol  level is increased. Such a change in the solvation shell is
likely to alter the reactivity of the DCIP and may be responsible for the
variation in ri with glycerol loading illustrated
in Figure 5.Figure 4
The variation inri as the level of glycerol in the ink formulation
is altered between 0.1 and 0.9 g. As in
Figure 3, the dotted line in the plot indicates the average ri of
5.15 × 10−4 AbsU s−1.Figure 5
The variation inri(□) and the λmax(∘) as the level of
glycerol in the ink formulation is altered between 0 and 0.1 g.However, there may be other effects
working here since the work by Van Keuren and Schrof [29] showed  that in a PVA/glycerol
solution, where the glycerol content was varied between 10 and 40 wt%, the
diffusion coefficient for a dye, rhodamine B, increased by a factor of 104. Beyond 40 wt% glycerol, the dye exhibited a
diffusion coefficient similar to that observed for the dye in neat glycerol. In the DCIP ink formulations presented here, with
a 0.1 g glycerol loading, the glycerol wt% is 66 wt%, and thus the variation in
ri as a function of glycerol loading illustrated in Figure 5 could
be simply due to the variation in the diffusion coefficient of the dye from the
bulk film to the surface of Activ,
assuming that this is the rate determining step for the photoreduction/bleaching
process. In this instance, the maximum rate
observed at glycerol loading ≥0.1 g is due to the diffusion coefficient for
DCIP in the film (HEC/glycerol) reaching a limiting value, as was observed by
Van Keuren for rhodamine B in PVA/glycerol.


## 3.3. The Effect of UVA Light Irradiance on the Initial Rate of Photoreduction

In a final set of experiments, the
initial rate of reduction of a typical DCIP film was determined as a function
of UVA light irradiance, varied from 0.44 to 7.41 mW cm−2, and the
results are illustrated in Figure 6. Not
surprisingly, ri increases with increasing light irradiance, since
the number of photons reaching the photocatalyst surface, and hence the number
of photogenerated electrons produced, will increase. The strong linear dependence of ri on the UV light irradiance is attributed to the relatively low light
intensities used here to reduce the dye, the low absorbance of the underlying Activ photocatalyst layer,
and the fact that, at such low UV irradiance levels, the reduction reaction
will be controlled by the reactions at the ink/photocatalyst interface of the
photogenerated electron-hole pairs, as opposed to their recombination (which is
expected at high irradiance levels). A similar linear dependence of ri on the UV irradiance was observed
for the Rz-based photocatalyst indicator ink [28].Figure 6
The variation inri with UVA irradiance.SinceΔAbst for the reduction of DCIP is ca. 0.085 AbsU, as
illustrated earlier in Figure 2(b), for the standard ink formulation, and since
it is known that 1 mW cm−2 of 365 nm light is equivalent to 1.1 × 1017 photons per cm2 per minute, the quantum efficiency for the process
can be estimated to be 0.0018 molecules per photon. It is also known from spectral studies
conducted elsewhere that Activ
only absorbs ca. 7.2% of the incident UVA irradiation from black light blue
lamps [30], thus the quantum yield for the reduction of DCIP on the substrate
can be estimated to be 2.5%. Both of
these estimated values are greater than those observed for the Rz ink (2.5×10−4  molecules per photon and 0.35%, resp.) [28], although in practice the initial rate of photoreduction of DCIP is slower than that of Rz in the ink
formulation under ambient atmospheric conditions [25].


## 4. Conclusions

A characterisation study of an
indicator ink based on the redox dye 2,6-dichloroindophenol for assessing the
photocatalytic activity of thin photocatalytic films of titania has been
conducted. The initial rate of photoreduction
of the dye in the ink formulation is independent of the dye loading level but
highly sensitive to the level of glycerol present, reaching a plateau above a
loading of 0.1 g glycerol. Accompanying
this trend in the reduction rate is an inverse shift in theλmax of the
dye, believed to be due to changes to the solvent shell surrounding the dye
with changing glycerol content. As
anticipated, the rate of photoreduction increased with increasing UV
irradiation intensity, similar to what has been reported for an Rz ink. The quantum yield for the photoreduction of
DCIP by an Activ
film was estimated to be 2.5%, which is greater than that observed for the
Rz-based ink communicated previously.



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*Source: 504945-2008-04-24.xml*