Res. Chem. Intermed., Vol. 34, No. 4, pp. 381– 392 (2008)
© Koninklijke Brill NV, Leiden, 2008.
Also available online - www.brill.nl/rci
Photodegradation of methylene blue in water, a standard
method to determine the activity of photocatalytic
coatings?
JESSICA TSCHIRCH 1 , RALF DILLERT 1 , DETLEF BAHNEMANN 1,∗ ,
BERND PROFT 2 , ANDREAS BIEDERMANN 3 and BERNHARD GOER 4
1 Institut
für Technische Chemie, Leibniz Universität Hannover, Germany, Callinstrasse 3,
D-30167 Hannover, Germany
2 Sachtleben Chemie, Dr.-Rudolf-Sachtleben-Str.4, D-47198 Duisburg, Germany
3 Titam Oberflächenschutz, An der Höhe 11, D-88356 Ostrach, Germany
4 Pilkington Deutschland AG, Haydnstr. 19, D-45884 Gelsenkirchen, Germany
Received 9 April 2007; accepted 31 August 2007
Abstract—The degradation of methylene blue (MB) in aqueous solutions has been re-examined as a
method to characterize the photocatalytic activity of transparent TiO2 coatings. Increasing irradiation
intensities leads to a change in the observed kinetic behavior from zero-order to pseudo-first-order
regarding the concentration of MB. This is due to a diffusion inhibition of MB. In order to obtain data
within a zero-order kinetic regime at an initial MB concentration of 10 μmol/l and, thus, to avoid the
diffusion control, irradiation intensities below E = 5 W/m2 for substrates comprising higher photonic
efficiencies than ζ = 0.09% have to be applied. Recommendations for a standard protocol are given.
Keywords: Photodegradation; methylene blue; photocatalysis; TiO2 coatings; kinetics.
INTRODUCTION
Research and development in the area of photocatalysis today is mainly concerned
with the topic of active macroscopic surfaces. Initially, the particle surfaces of
the photocatalysts themselves suspended in water have been investigated and in
the past decades titanium dioxide (TiO2 ) has emerged as the most widely used
photocatalyst. The experiments showed that TiO2 under UV-illumination is able to
mineralize nearly every organic compound dissolved in water and, consequently,
water purification was proposed as a potential application [1 –4]. Since the
∗ To
whom correspondence should be addressed. Tel.: (49-511) 762-5560; Fax: (49-511) 762-2774;
e-mail: [email protected]
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applicable part of the excitation spectrum lies in the short visible and the UV range,
in particular the sun came into the focus as a low-cost radiation source for these
applications. However, powder suspensions are difficult to handle, and mainly
experimental and pilot-scale setups for water purification have been built and tested
so far [5]. The experimental data for the photocatalytic activity were encouraging
and soon the photocatalysts had to prove their performance after fixation in surface
coatings or on photoactive structures. Besides the purification of water, now the
purification of air and the so-called “self-cleaning effect” of the surface came into
consideration as potential applications for photocatalysis [2, 4, 6]. Driven by the
first commercial products with photoactive coatings [2, 6], e.g., glass sheets, tiles
and roof pans, the question of quantification of the photoactivity of coatings arose.
As a measure for the photocatalytic efficiency of a heterogeneous photocatalyst the
photonic efficiency, i.e., the initial reaction rate divided by the molar flux of incident
absorbable photons, has been suggested [7].
The photocatalyzed bleaching of dye molecules dissolved in a test solution has
been used as an activity test system already in many cases [8 –11]. Since the
measurement of the dye concentration can be easily achieved employing even an
online technique by using, e.g., a photometer, relative data referring to a “standard
surface” can be obtained in good quality by holding as many parameters of the test
equipment constant as possible. Being aware of at least part of the complexity of
the system it seems to be a good approach to employ aqueous solutions of selected
dye molecules for establishing a test procedure and for generating absolute data
to characterize the photoactivity of surfaces for water cleaning applications. One
of the most prominent dyes used as probe molecule is methylene blue (MB) (see
Scheme 1) [10 –13].
The photocatalytic degradation of MB on or close to a photoactive coating
upon irradiation has previously been investigated under aerobic and anaerobic
conditions [12]. One usually observes the decolorization of this dye molecule
during illumination of the photoactive system by UV(A)-radiation. It has been
proposed that the oxidation of surface adsorbed water and the reduction of O2 by
the illuminated semiconductor lead to the formation of hydroxyl and hydroperoxy
radicals, respectively [1]. These radicals are mobile along the surface of the
semiconductor and potentially able to oxidize organic compounds such as MB at
or near to this surface. To make things more complicated, MB can also be easily
reduced, yielding “leuco methylene blue” [12]. However, the latter decolorization
is reversible through the addition of gaseous O2 to the solution. In a photocatalytic
Scheme 1. Structure of methylene blue.
Photodegradation of methylene blue in water
383
experiment the reduction of MB rather than of O2 by the photocatalyst is very likely
a competing reaction, even under aerobic conditions, in particular in acidic solution.
In the absence of any acid, however, O2 acts as the scavenger for the conduction
band electrons [12].
On national (DIN) and international (ISO) level the determination of the photonic efficiency of the light-driven decolorization of MB solutions in contact with
a photocatalyst is considered as a standard method for testing and comparison of
photocatalytic coatings. In this context it is necessary to acertain that the photocatalytic reaction of MB is a simple and robust method which meets the requirements
of non-specialised laboratories in industry. Therefore, results of the photocatalytic
bleaching reaction of MB are presented in this publication and some limitations of
this method are discussed.
EXPERIMENTAL
Two different setups were used for the photoactivity tests.
Setup 1: Figure 1 shows the setup with discrete photometer measurement of the
absorbance of MB (εMB (660 nm) = 71.547 × 106 cm2 /mol [14]) at λ = 660 nm by
sampling. A glass cylinder was fixed to the surface of the substrate. 30 ml of MB
solution with appropriate concentration was added and the cylinder was covered by
an UV-A transparent glass pane. The irradiation with an ensemble of 16 Osram
Eversun L 40 W bulbs, showing a λmax at 360 nm, and a translucent poly(methyl
methacrylate) (PMMA) plate was adjusted by choosing different distances to
the sample. The optical absorption spectra were measured employing a Varian
Carry 100 bio UV-photometer. Reference measurements were carried out under
identical conditions but without illumination. Prior to each of these time-resolved
measurements a MB solution containing the concentration of the test solution was
filled into the cylinder and kept for 24 h in the dark to ensure that an adsorption
equilibrium has been reached.
Figure 1. Experimental setup for sampled photometer measurements.
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J. Tschirch et al.
Figure 2. Experimental setup with online photometer; for reference measurements the photoactive
sample is covered with a blind.
The samples were prepared by coating a glass sheet (90 × 60 mm2 ) with a slurry
of 180 mg nanoscale TiO2 in 3 ml isopropanol. The fixation was achieved by heating
for 1 h at 100◦ C. The coatings are stable in aqueous solution.
Setup 2: The experimental setup for online measurements was built as depicted
in Fig. 2. The absorbance was measured by a Titam built-in-photometer at λ =
660 nm. Temperatures of photometer and samples were controlled. The radiation
employed for these measurements at λ = 660 nm was chopped to eliminate
interferences by ambient light. The samples were immersed in 40 ml of a MB
solution of appropriate concentration and the beaker was covered with a UV-A
transparent glass sheet. Two Philips bulbs (Cleo 15 W), emitting at λmax = 360 nm,
were used as radiation source; the spectrum is similar to that of the Eversun bulbs.
The irradiation was measured with a TW30SX (SolGel Technologies) sensor and
adjusted by chopping light with an irradiation intensity of E = 10 W/m2 employing
different period lengths for bright and dark mode. Reference measurements were
carried out simultaneously with a second channel by using a blind cover in the
solution above an identical second sample.
The TiO2 -coated samples were made by a CVD coating process of glass sheets
with Ti(Oi Pr)4 as precursor.
Photolysis experiments of MB in the absence of TiO2 have been performed using a
Suntest (Atlas Material Testing Technology) and a Fluotest (UV-Consulting Peschl).
RESULTS AND DISCUSSION
Figure 3 shows the decolorization of MB in a radiation field comprising an
UV-irradiance of E = 108 W/m2 and a very intense visible radiation in the
absence of any photocatalyst. After 5 h of illumination the initial MB concentration
Photodegradation of methylene blue in water
385
Figure 3. Absorption changes observed in an aqueous MB solution (c0 = 16 μmol/l) illuminated in
a radiation field with an UV-A intensity of 108 W/m2 after different times of illumination.
Figure 4. Absorption changes observed in an aqueous methylene blue solution (c0 = 16 μmol/l)
illuminated in a Fluotest radiation field with an UV-A intensity of 50 W/m2 after different times of
illumination.
of 16 μmol/l decreased to 1.3 μmol/l. This is due to a photolytic reaction of
MB induced by the absorption of UV photons below λ = 350 nm (sensitization
of O2 ) or in the visible region around 660 nm [12]. Both pathways lead to the
decolorization of a MB solution. In order to avoid any type of photolysis reaction
of MB the wavelength for the excitation of the photocatalyst should lie in the range
of λ = 350–480 nm, i.e., a range where MB shows only slight absorption (cf., the
absorption spectra in Fig. 3).
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Figure 5. Principle decolorization of an aqueous methylene blue solution at a photocatalytic surface
under UV irradiation (M) and at a covered photocatalytic surface under UV irradiation (R1).
Under a narrow band width Fluotest UV-lamp with an emission maximum at
λ = 366 nm MB is fairly stable in aqueous solution (Fig. 4). The observed decrease
in the intensity of the absorption bands of the MB solution is most-likely the result
of the adsorption of MB on the walls of the photoreactor.
Reference measurements, e.g., employing a blind covered photoactive surface
should show a possible “phantom-photoactivity” due to, e.g., scattered UV-radiation
or adsorption of MB to the sample or the reactor surfaces. As shown in Fig. 2 the
cover is immersed in the solution and located closely above the sample to prevent
direct illumination of the sample.
Figure 5 shows the typical trace of the time-resolved decolorization of a MB
solution during the conditioning procedure (before t1 ), when MB adsorbs to the
sample and the reactor surfaces as well as during the irradiation period (after t1 ).
Curve M exhibits the results obtained without the cover while curve R1 shows the
concentration change observed during the same experiment with the cover (details
of the experimental setup are shown in Fig. 2). The latter thus represents the “dark”
decolorization of the solution.
MB tends to adsorb to surfaces very easily [15]. This process can be enhanced or
delayed by controlling the charge of the surface, since MB is a cationic dye at pH 7.
A negatively charged surface should adsorb the cation to a greater extent than a
neutral or a positively charged surface. The surface charge might change during the
photocatalytic process due to, e.g., the adsorption of negatively charged oxidation
products, resulting in an increase of the adsorption of MB. On the other hand, the
adsorption of protons due to the formation of mineral acids has not been found to
hinder the adsorption of MB [15].
The photocatalytic degradation pathway of MB has been elucidated previously
[15, 16]. The mineralization of 1 mol MB according to equation (1) consumes
Photodegradation of methylene blue in water
387
25.5 mol O2 .
C16 H18 N3 SCl + 25.5TiO2 → 16CO2 + 6H2 O + 3HNO3 + H2 SO4 + HCl
(1)
The net reaction (1) implies several parameters that could influence the reaction
rate. The reaction rate of the decolorization of MB might decrease with decreasing
pH and O2 concentration, as well as increasing CO2 and anion (NO3 − , SO4 2− , Cl− )
concentration. In case of the generation of water insoluble organic intermediates, an
inhibiting effect during the reaction can be expected also as a result of the coverage
of active sites on the surface of the catalyst. CO2 , in the form of carbonate, as
well as chloride and sulfate ions are indeed effective inhibitors of the photocatalytic
decolorization of MB [13]. These parameters will potentially have an effect on
the reaction rate during extended illumination. Under starting conditions, however,
only small concentrations of well-soluble products and, moreover, a sufficient
concentration of dissolved O2 (c = 250 μmol/l) are present in solution.
A potential inhibition effect caused by product molecule adsorption on the
photoactive surface can be excluded as the result of an additional experiment using
setup 2. A concentrated MB solution was added three times to the same starting test
solution after its photocatalytic MB concentration degradation at E = 10 W/m2
was decreased from about c0 = 5 μmol/l to c = 1 μmol/l. Figure 6 reveals
unchanged reaction rates, even after a threefold addition. Apparently, at least for
the photocatalyst tested here the degradation products at low concentrations do not
inhibit the photoactive sites responsible for the photocatalytic degradation of MB.
The irradiation intensity dependence of the degradation rate has been tested
by degrading MB solutions with an initial concentration of 10 μmol/l on glass
sheets with a photocatalyst powder coating (Fig. 1; setup 1, Experimental section).
Figure 7 shows a linear decrease of the MB concentration for irradiation intensities
up to E = 6 W/m2 . Beyond that, the concentration vs. time dependency is not
linear any more.
The values for the photon irradiance EQ , the reaction rate RM and the photonic
efficiency ζMB are calculated according to equation (5), (2) and (6), respectively
(whereas the RM is taken for PMB in equation (5)).
cM V
,
tA
cR1 V
,
RR1 =
tA
PMB = RM − RR1 ,
λE
,
EQ =
hcNA
PMB
× 100,
ζMB =
EQ
RM =
(2)
(3)
(4)
(5)
(6)
where A is geometrical test area of sample, V : test volume, E: irradiation intensity,
h = 6.63 × 10−34 J s, c = 3 × 108 m/s and NA = 6.023 × 1023 mol−1 . The values
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J. Tschirch et al.
Figure 6. Test for possible product inhibition by addition of MB to the same test solution; c0 =
10 μmol/l, E = 10 W/m2 .
Figure 7. Degradation of MB under different irradiation intensities, experimental setup 1, c0 =
10 μmol/l.
Photodegradation of methylene blue in water
389
Table 1.
Reaction rate and photonic efficiency observed at different irradiation intensities (c0 = 10 μmol/l)
E (W/m2 ), λmax = 360 nm
RM (μmol/(m2 /h))
EQ × 10−3 (μmol/(m2 /h))
ζMB (%)
4
6
8
10
38
43.2
0.088
40
64.8
0.062
60*
86.4
0.069*
68*
108
0.063*
* R and ζ for E = 8 and 10 W/m2 are calculated using the first four and first two data points,
M
respectively.
derived from the experimental results shown in Fig. 7 are given in Table 1. Table 1
reveals increasing reaction rates RM and decreasing photonic efficiencies ζ upon
increasing the irradiation intensity with slight deviations due to experimental error.
For these samples photonic efficiencies ζ between ζ = 0.09 and 0.06% at reaction
rates between RM = 39 and 68 μmol/(m2 /h) have been determined under irradiation
with intensities between 4 and 10 W/m2 .
As generally known from heterogeneous reactions kinetics, diffusion processes
during the course of the irradiation experiment might gain a significant influence on
the reaction rate. However, such diffusion limitations have usually been neglected
in previous investigations. Hence, diffusion processes have to be taken into account
for the design of a photoactivity test system.
To check a possible influence of the diffusion on the photonic efficiency setup 2
(Fig. 2) with a glass sheet coated with TiO2 by a CVD process was used for
experiments in which the total amount of the absorbed photons was reduced by
chopping the light and keeping the irradiation intensity constant at about E =
10 W/m2 (initial MB concentration = 10 μmol/l). Moreover, the reaction rate
RM was corrected by a reference measurement RR1 performed in a parallel run
with a blind covered sample evincing that the impact of the ‘dark’ decolorization
is low. The results are depicted in Fig. 8 and Table 2. Figure 8 shows the decay of
MB observed during a single run with an increasing amount of incident irradiation.
The accompanying specific activity PMB and the photonic efficiency ζMB are given
also in Table 2. As can be seen from Table 2, the quantum efficiency is strongly
influenced by the ratio between dark and illuminated time intervals. This influence
can be explained by assuming a transport limitation by diffusion.
The diffusion coefficient for molecules like MB lies in the range of D = 5 ×
10−10 m2 /s [17] and the diffusion path length is estimated to be 100 μm [18]. With
an initial MB concentration of 10 μmol/l or 6.03 × 1021 molecules/m3 , the diffusion
rate as determined by Fick’s law equals −dN/dt = 1.08 × 1020 molecules/(m2 /h)
or 180 μmol/(m2 /h).
The comparison of the employed irradiation intensity of E = 10 W/m2 (λ =
360 nm) or the corresponding photon absorption rate EQ = 108 × 103 μmol/(m2 /h)
with the calculated diffusion rate of 180 μmol/(m2 /h) for a MB concentration of
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J. Tschirch et al.
Figure 8. Photocatalytic test with reduced irradiation by chopping the light.
Table 2.
Chopping of the irradiation (c0 = 10 μmol/l; domain, see Fig. 1)
Domain UV on
(s)
A
B
C
D
0
7.5
7.5
7.5
Every time
UV off
(s)
Ratio Average
EQ × 10−3
on/ irradiation
(μmol/
(m2 /h))
off
(E, W/m2 ,
λmax = 360 nm)
Specific
Photonic
photoactivity
efficiency
(PMB = RM − RR1 , (ζMB , %)
μmol/(m2 /h))
Every time
52.5
22.5
7.5
0
0
0.125
0.25
0.5
1
0
9.2
19.2
35.6
48.9
0
0.9
2.1
5.5
9.6
0
9.7
22.7
59.5
104
–
0.095
0.085
0.060
0.047
10 μmol/l leads to the presumption of a potential diffusion control for the photocatalytic degradation of MB at highly efficient photocatalytic surfaces. Consequently,
any photonic efficiency ζ 0.15% should be regarded with great caution, i.e., it
will be necessary to carry out duplicate measurements at high initial MB concentration, and/or lower photon fluxes. Only if the results of the latter tests are identical
to those obtained under standard conditions can the values be taken as reliable!
CONCLUSIONS
Starting point of our work described above was the question whether the lightinduced decolorization of an aqueous methylene blue (MB) solution is a simple and
Photodegradation of methylene blue in water
391
robust method to determine the photocatalytic activity of TiO2 coatings. The results
presented above clearly show that this question can be affirmed with a standard
protocol to fix some important points:
1. It has been shown that MB is easily adsorbed not only at the TiO2 surface but also
at other surfaces in contact with the aqueous MB solution. A standard protocol
has to define materials with a low capability to adsorb MB.
2. Aqueous solutions are decolorized by homogeneous photolytic reactions initiated by the excitation of the probe molecule with light with wavelengths below
350 nm and above 480 nm. In order to avoid any type of photolysis reaction
of MB the wavelength for the excitation of the photocatalyst has to lie in the
range λ = 350–480 nm. Figure 9 summarizes the spectral conditions for a reasonable experimental setup. Using this setup, most of the possible undesired
photolysis reactions can be avoided or at least reduced. The rate of formation
of electron/hole pairs in the excited TiO2 upon illumination is a function of the
rate of photon absorption. Figure 9 shows the transmission spectrum of a TiO2
coating (anatase, thickness of 450 nm) on quartz glass. Absorption of light starts
below a wavelength of λ = 400 nm and in agreement with the energy required to
activate the anatase surface. The rate of decolorization of a MB solution under
these conditions should be a function of the irradiation intensity and the activity
of the photocatalytic surface.
3. For a given photoactive surface and an initial MB concentration, irradiances
below a specific maximum irradiance will yield zero-order kinetic behavior of
the system. Higher irradiation intensities cannot enhance the photocatalytic
degradation of MB linearly and lead to an increased influence (i.e., rate control)
of the diffusion-controlled process. A decrease of the MB concentration during
the reaction leads also to an increased influence of the diffusion controlled
transport of MB to the photocatalyst surface. Therefore, a standard protocol
Figure 9. Principal spectral conditions of the ensemble employed for the activity tests.
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has to fix the initial MB concentration and the light intensities. At an initial MB
concentration of 10 μmol/l irradiation intensities below 5 W/m2 for substrates
comprising quantum efficiencies greater than ζ = 0.09% have to be applied.
4. In order to characterize the activity of photocatalytic coatings by using the degradation of dissolved probe molecules, reaction rates and photonic efficiencies
should be calculated by using data obtained from the linear region at the beginning of the degradation reaction.
A standard protocol defining these points carefully should meet the requirements
of non-specialised laboratories in industry.
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