# Characterization and Evaluation of the Efficiency of TiO2/Zinc Phthalocyanine Nanocomposites as Photocatalysts for Wastewater Treatment Using Solar Irradiation

Authors: Antonio E. H. Machado; Marcela D. França; Valdemir Velani; Gabriel A. Magnino; Hosana M. M. Velani; Flávio S. Freitas; Paulo S. Müller; Christian Sattler; Martin Schmücker
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

This work reports the characterization of composites prepared by the association between zinc phthalocyanine (ZnPc) and titanium dioxide. These composites are better photocatalysts for wastewater decontamination mediated by solar radiation than pureTiO2, performance that remains even when reused. The UV-Vis diffuse reflectance absorption spectra show for these composites two intense absorption bands. The first covers the ultraviolet and part of the visible spectrum region until 460 nm (2.7 eV), whereas the second, nonstructured, goes from 475 nm until the near infrared with an absorption peak at 683 nm attributed to the Q band of ZnPc. The production of additional e−/h+ pairs by these aggregates when photoexcited, their capability to act as charge carrier, the thickness and regularity of their distribution on the TiO2 surface seem to be important parameters for the performance observed for these composites.


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## Body

## 1. Introduction

Water recycling by destruction of the pollutant load using advanced oxidative
processes (AOPs) has been suggested as
an alternative for environmental remediation
[1–10]. Between the AOP, heterogeneous photocatalysis can be considered as one of the new
“advanced oxidation technologies” (AOTs) for air and water purification [5–7].For many reasons, TiO2 has been
considered as the best choice among the semiconductor oxides with photocatalytic
activity for wastewater treatment, aiming the elimination of organic and inorganic species [2, 3, 5–10]. However, its band-gap energy, between 3.0 and 3.2 eV
[2, 3], limits its application when solar photocatalysis is desirable [3, 8, 11, 12].
To get round this problem, many research groups have proposed alternatives to amplify the photocatalytic activity of TiO2, improving the pickup of ultraviolet radiation and also making feasible the use of components of the visible
radiation [8–20].Electron
transfer at the interface between a photoactive species and the semiconductor
surface is a fundamental aspect for organic semiconductor devices [21, 22]. Certain photoactive compounds have proven to be able, when
electronically excited, to inject electrons in the conduction band of
semiconductors [21–33], increasing the performance of dye-sensitized solar cells. In particular, ultrafast charge separation led
by electron injection from electronically excited photoactive molecules to a
conduction band of a wide-gap metal oxide, and a good electronic coupling
between dye molecules and surface of the substrate are key steps for improving
the performance of these materials [23–25].In this work, we
report the characterization by
different methodologies, such as diffuse reflectance absorption spectra,
specific surface area, thickness of ZnPc coating, and their distribution on
the TiO2 particles, evaluated by transmission electron microscopy (TEM) coupled to energy disperse X-ray (EDX) of composites prepared
by the association between zinc phthalocyanine (ZnPc) and titanium dioxide. Additionally,
the quantum yield of hydroxyl radical
production, the capacity of the studied composites to
perform the degradation of organic matter present in wastewaters in reactions
mediated by solar irradiation, and the possibility of reuse of such
photocatalysts were also estimated.


## 2. Experimental

### 2.1. Chemicals

Titanium
dioxide, P25, was nicely provided by Degussa Hülls. Zinc phthalocyanine (ZnPc),
98%, acetylacetone, 99%, and a sodium salt of lignossulphonic acid ((LSA), possessing a mean molecular
mass of 52,000 D), were
purchased from Sigma - Aldrich (São Paulo, Brasil). Methanol, UV/HPLC, dimethylformamide and dimethylsulfoxide, P.A., and
ethanol, 95%, were purchased from Vetec Química Fina (Rio de Janeiro, Brasil). Ammonium phosphate
buffer (pH 6.0) was prepared using a standard procedure. Potassium ferrioxalate
solutions (10−2 mol L−1) were used as chemical
actinometer [34]. All chemicals were used as received.


### 2.2. Preparation of ZnPc/TiO2 Composites

The composites were prepared coating TiO2 particles with zinc phthalocyanine [8, 13, 14], dissolved in a solvent mixture containing
50% dimethylsulphoxide, 20% ethanol, and 30% dimethylformamide, at 60°C.
The necessary amount of TiO2 (P25, Degussa), covering the
compositions of 1.0%, 1.6%, 2.5%, 5.0%, 7.5%, and 10%, was gradually added to
the dye solution, under stirring and heating, resulting in a suspension with
homogeneous appearance. The suspension was maintained, stirred, and heated until
almost complete solvent evaporation. The material, presenting a creamy consistency,
was washed several times with distilled water, under vigorous stirring, to remove
residues and remaining organic solvent. Next, the composite is dryed at a
temperature between 70 and 80°C, for 24 hours. After milling, the
final product is a finely divided bluish powder with improved photocatalytic activity
[8–14], insoluble in
water, capable to give stable suspensions in this solvent.


### 2.3. Characterization

The diffuse reflectance absorption spectra of
the composites were recorded using a Shimadzu UV-2501PC spectrophotometer equipped with an integrating
sphere. Barium sulphate was used as reference in these experiments.The specific surface area of the composites was estimated
from BET (Brunauer, Emmett, and Teller) isotherms, based on the adsorption of gaseous nitrogen, using a
MICROMERITICS ASAP 2000 system.The thickness of ZnPc coating and their distribution on
the TiO2 surface was evaluated by transmission electron microscopy (TEM) coupled to energy disperse X-ray (EDX). TEM investigations were carried out
using a Philips Tecnai F30 microscope equipped with a field emission gun, a scanning transmission
electron microscope (STEM) and an EDX analytical system. The specimens for TEM analysis were
prepared by crushing the powder aggregates in an agate mortar, dispersing in
distilled water and depositing onto a carbon film supported by a 300 mesh copper grid.All measurements were done with unused catalyst.


### 2.4. Quantum Yield of Hydroxyl Radical Production (ΦHO•)

The quantum yield of hydroxyl radicals production was estimated using a
procedure proposed by Gao et al. [35], based on Nash’s method [36]. It
consists in the quantification of the formaldehyde concentration formed during
the oxidation of methanol by the hydroxyl radicals [37], generated during the
photocatalytic process. An aqueous suspension (5 mL), 0.2 mol L−1 in methanol, containing TiO2 or one of the composites (50 mg L−1),
placed in a closed borosilicate glass reactor with a cooling jacket, in which water
was continuous circulated to minimize heating by IR irradiation, was photolysed
during 30 minutes using a 400 W high-pressure mercury vapor lamp (HPL-N) positioned at 10 cm from the reactor. The
suspension was mixed during 20 minutes (magnetic stirrer), prior to
illumination and during it.The
radiation was filtered by the borosilicate glass (2 mm thick, cutting below 300 nm), to guarantee the excitation of the catalysts under equivalent conditions. Due
to technical limitations, only the light intensity in the UV-A region was
monitored during the irradiation. This was done using a solar light PMA-2100
radiometer. The illuminated area was 3.0 cm2.Formaldehyde
formed due to the oxidation of methanol was determined by the Nash method [36],
which is based on the Hantzsch reaction: an 1.5 mL aliquot of the photolysed
sample, previously filtered using a PTFE Millipore filter (0.45 μm of porosity), was added to 1.5 mL of 0.18 mol L−1ammonium phosphate buffer (pH = 6.0) and 15 μL of acetylacetone. A yellow color gradually
develops owing to the synthesis of diacetyldihydrolutidine [36]. Under optimum
conditions, the molecular extinction in terms of formaldehyde has a smooth
maximum of 8,000 M−1cm−1at 412 nm, regardless of
dilution. The needed spectrophotometric measurements were done
in a range covering this wavelength.The photonic flux [34] of the mercury lamp was evaluated in the
range between 295 and 815 nm using a radiometric/photometric
setup built with components furnished by Ocean Optics, Inc. (an SD2000 UV/VIS
diode array spectrometer coupled to a personal computer, a fiber optic
irradiance probe with a detection head of 0.3 cm diameter, possessing a
cosine corrector). These measurements were done using an integration time of 50
milliseconds. A 1 mW He-Ne laser (632.8 nm), two semiconductor lasers (650 and
670 nm), and calibrated LEDs (380, 450, 470, and 640 nm of peak output) were used as standards for the quantification of the
number of emitted photons. The radiation that can be effectively absorbed by these catalysts was
considered as being in the 295 to 710 nm range, with a photonic flux of about
of 3.3×10−6 einsteins s−1.


### 2.5. Photodegradation Essays

The capacity
of the studied composites to degradate organic matter present in wastewaters,
in reactions mediated by solar irradiation, and the possibility of reusing of
such photocatalysts was evaluated monitoring the decrease of the organic matter content during the treatment of three 50 L batches of a model
effluent (an aqueous solutions containing 160 mg L−1 of LSA) [8]. The reactions were done at pH 3, with the addition of 1 mL of H2O2 30% v/v
per liter of effluent (about 9 mM), and monitored by chemical oxygen demand (COD) analysis
of aliquots of effluent samples collected at different accumulated doses of
UV-A radiation. (This option was due to operational limitations. The spectral pattern
of the visible light does not change significantly during the execution of the
experiments.) To evaluate the
observed (global) reaction kinetics, the temporal variations were substituted
by the UV-A accumulated dose, which warrants the reproduction of these
experiments under different latitude and weather conditions.A same sample (100 mg per liter of effluent) of the photocatalyst, containing initially 2.5% of ZnPc, was used to treat the three effluent batches. The treatment
of each batch was performed using a compound parabolic
concentrator (CPC) reactor, after the recovery of the photocatalyst used in the previous experiment.The CPC reactor was constructed in our
laboratory [11, 12]. It was designed to process up to 150 L of effluent. This reactor consists in a
module with an aperture of about 1.62 m2, elevation angle adjusted to
the latitude of Uberlândia, Brasil (19°S), containing 10
borosilicate glass tubes (external diameter 32 mm, wall thickness of 1.4 mm, and length of 1500 mm),
mounted in parallel, each on double parabolic-shaped aluminum reflector
surfaces (Figure 1). The concentration ratio is about 1.0 and the reflectivity of the
reflector surface within the band gap of TiO2 (300–390 nm) is 89.5%.
The flow rate is 34 L min−1. The effluent was circulated in the reactor which is open to
guarantee a sufficient oxygen concentration in the bulk. The fluid flow
rate in the tubing was chosen to assure a turbulent transport regimen of the
effluent, warranting a better homogenization of the suspension. This and the
fact that the irradiated effluent returns to the reservoir by a tubing positioned
50 cm above the surface of the remaining effluent guarantees an adequate aeration of the
effluent during the photocatalytic process. The fittings and piping were made of
polypropylene.Figure 1
Details of the CPC reactor experimental setup.This reactor is similar to
the SOLARDETOX prototypes used at the Plataforma Solar de Almería and the German Aerospace Center, Cologne [38, 39], but the mirrors were calculated
exclusively for this reactor and the ratio between irradiated surface and dark
tubing is higher. Since the BRITE EURAM SOLARDETOX project CPC-type reactors
are known as very powerful solar reactors for water treatment up to a pilot
plant scale [38, 39].As reference, an additional effluent batch was treated under similar
conditions using pure TiO2 P25 as catalyst.For all reactions, hydrogen peroxide (30 mg L−1) was used as
additional source of reactive species [8].The incident UV-A radiation was monitored using a solar light PMA-2100 radiometer.
All reactions were stopped when the accumulated dose of UV-A radiation reached
900 kJ m−2.The COD measurements were based on an Environmental
Protection Agencies recommended method [40, 41], in which an aliquot
of the effluent reacts under heating and closed reflux, at 423 K during 2 hours,
reducing dichromate ions to chromic ions in a strongly acid medium. From
absorbance measurement, done at 620 nm using a Hach DR-4000U spectrophotometer
and a resident program, the COD of the samples was determined.


## 2.1. Chemicals

Titanium
dioxide, P25, was nicely provided by Degussa Hülls. Zinc phthalocyanine (ZnPc),
98%, acetylacetone, 99%, and a sodium salt of lignossulphonic acid ((LSA), possessing a mean molecular
mass of 52,000 D), were
purchased from Sigma - Aldrich (São Paulo, Brasil). Methanol, UV/HPLC, dimethylformamide and dimethylsulfoxide, P.A., and
ethanol, 95%, were purchased from Vetec Química Fina (Rio de Janeiro, Brasil). Ammonium phosphate
buffer (pH 6.0) was prepared using a standard procedure. Potassium ferrioxalate
solutions (10−2 mol L−1) were used as chemical
actinometer [34]. All chemicals were used as received.


## 2.2. Preparation of ZnPc/TiO2 Composites

The composites were prepared coating TiO2 particles with zinc phthalocyanine [8, 13, 14], dissolved in a solvent mixture containing
50% dimethylsulphoxide, 20% ethanol, and 30% dimethylformamide, at 60°C.
The necessary amount of TiO2 (P25, Degussa), covering the
compositions of 1.0%, 1.6%, 2.5%, 5.0%, 7.5%, and 10%, was gradually added to
the dye solution, under stirring and heating, resulting in a suspension with
homogeneous appearance. The suspension was maintained, stirred, and heated until
almost complete solvent evaporation. The material, presenting a creamy consistency,
was washed several times with distilled water, under vigorous stirring, to remove
residues and remaining organic solvent. Next, the composite is dryed at a
temperature between 70 and 80°C, for 24 hours. After milling, the
final product is a finely divided bluish powder with improved photocatalytic activity
[8–14], insoluble in
water, capable to give stable suspensions in this solvent.


## 2.3. Characterization

The diffuse reflectance absorption spectra of
the composites were recorded using a Shimadzu UV-2501PC spectrophotometer equipped with an integrating
sphere. Barium sulphate was used as reference in these experiments.The specific surface area of the composites was estimated
from BET (Brunauer, Emmett, and Teller) isotherms, based on the adsorption of gaseous nitrogen, using a
MICROMERITICS ASAP 2000 system.The thickness of ZnPc coating and their distribution on
the TiO2 surface was evaluated by transmission electron microscopy (TEM) coupled to energy disperse X-ray (EDX). TEM investigations were carried out
using a Philips Tecnai F30 microscope equipped with a field emission gun, a scanning transmission
electron microscope (STEM) and an EDX analytical system. The specimens for TEM analysis were
prepared by crushing the powder aggregates in an agate mortar, dispersing in
distilled water and depositing onto a carbon film supported by a 300 mesh copper grid.All measurements were done with unused catalyst.


## 2.4. Quantum Yield of Hydroxyl Radical Production (ΦHO•)

The quantum yield of hydroxyl radicals production was estimated using a
procedure proposed by Gao et al. [35], based on Nash’s method [36]. It
consists in the quantification of the formaldehyde concentration formed during
the oxidation of methanol by the hydroxyl radicals [37], generated during the
photocatalytic process. An aqueous suspension (5 mL), 0.2 mol L−1 in methanol, containing TiO2 or one of the composites (50 mg L−1),
placed in a closed borosilicate glass reactor with a cooling jacket, in which water
was continuous circulated to minimize heating by IR irradiation, was photolysed
during 30 minutes using a 400 W high-pressure mercury vapor lamp (HPL-N) positioned at 10 cm from the reactor. The
suspension was mixed during 20 minutes (magnetic stirrer), prior to
illumination and during it.The
radiation was filtered by the borosilicate glass (2 mm thick, cutting below 300 nm), to guarantee the excitation of the catalysts under equivalent conditions. Due
to technical limitations, only the light intensity in the UV-A region was
monitored during the irradiation. This was done using a solar light PMA-2100
radiometer. The illuminated area was 3.0 cm2.Formaldehyde
formed due to the oxidation of methanol was determined by the Nash method [36],
which is based on the Hantzsch reaction: an 1.5 mL aliquot of the photolysed
sample, previously filtered using a PTFE Millipore filter (0.45 μm of porosity), was added to 1.5 mL of 0.18 mol L−1ammonium phosphate buffer (pH = 6.0) and 15 μL of acetylacetone. A yellow color gradually
develops owing to the synthesis of diacetyldihydrolutidine [36]. Under optimum
conditions, the molecular extinction in terms of formaldehyde has a smooth
maximum of 8,000 M−1cm−1at 412 nm, regardless of
dilution. The needed spectrophotometric measurements were done
in a range covering this wavelength.The photonic flux [34] of the mercury lamp was evaluated in the
range between 295 and 815 nm using a radiometric/photometric
setup built with components furnished by Ocean Optics, Inc. (an SD2000 UV/VIS
diode array spectrometer coupled to a personal computer, a fiber optic
irradiance probe with a detection head of 0.3 cm diameter, possessing a
cosine corrector). These measurements were done using an integration time of 50
milliseconds. A 1 mW He-Ne laser (632.8 nm), two semiconductor lasers (650 and
670 nm), and calibrated LEDs (380, 450, 470, and 640 nm of peak output) were used as standards for the quantification of the
number of emitted photons. The radiation that can be effectively absorbed by these catalysts was
considered as being in the 295 to 710 nm range, with a photonic flux of about
of 3.3×10−6 einsteins s−1.


## 2.5. Photodegradation Essays

The capacity
of the studied composites to degradate organic matter present in wastewaters,
in reactions mediated by solar irradiation, and the possibility of reusing of
such photocatalysts was evaluated monitoring the decrease of the organic matter content during the treatment of three 50 L batches of a model
effluent (an aqueous solutions containing 160 mg L−1 of LSA) [8]. The reactions were done at pH 3, with the addition of 1 mL of H2O2 30% v/v
per liter of effluent (about 9 mM), and monitored by chemical oxygen demand (COD) analysis
of aliquots of effluent samples collected at different accumulated doses of
UV-A radiation. (This option was due to operational limitations. The spectral pattern
of the visible light does not change significantly during the execution of the
experiments.) To evaluate the
observed (global) reaction kinetics, the temporal variations were substituted
by the UV-A accumulated dose, which warrants the reproduction of these
experiments under different latitude and weather conditions.A same sample (100 mg per liter of effluent) of the photocatalyst, containing initially 2.5% of ZnPc, was used to treat the three effluent batches. The treatment
of each batch was performed using a compound parabolic
concentrator (CPC) reactor, after the recovery of the photocatalyst used in the previous experiment.The CPC reactor was constructed in our
laboratory [11, 12]. It was designed to process up to 150 L of effluent. This reactor consists in a
module with an aperture of about 1.62 m2, elevation angle adjusted to
the latitude of Uberlândia, Brasil (19°S), containing 10
borosilicate glass tubes (external diameter 32 mm, wall thickness of 1.4 mm, and length of 1500 mm),
mounted in parallel, each on double parabolic-shaped aluminum reflector
surfaces (Figure 1). The concentration ratio is about 1.0 and the reflectivity of the
reflector surface within the band gap of TiO2 (300–390 nm) is 89.5%.
The flow rate is 34 L min−1. The effluent was circulated in the reactor which is open to
guarantee a sufficient oxygen concentration in the bulk. The fluid flow
rate in the tubing was chosen to assure a turbulent transport regimen of the
effluent, warranting a better homogenization of the suspension. This and the
fact that the irradiated effluent returns to the reservoir by a tubing positioned
50 cm above the surface of the remaining effluent guarantees an adequate aeration of the
effluent during the photocatalytic process. The fittings and piping were made of
polypropylene.Figure 1
Details of the CPC reactor experimental setup.This reactor is similar to
the SOLARDETOX prototypes used at the Plataforma Solar de Almería and the German Aerospace Center, Cologne [38, 39], but the mirrors were calculated
exclusively for this reactor and the ratio between irradiated surface and dark
tubing is higher. Since the BRITE EURAM SOLARDETOX project CPC-type reactors
are known as very powerful solar reactors for water treatment up to a pilot
plant scale [38, 39].As reference, an additional effluent batch was treated under similar
conditions using pure TiO2 P25 as catalyst.For all reactions, hydrogen peroxide (30 mg L−1) was used as
additional source of reactive species [8].The incident UV-A radiation was monitored using a solar light PMA-2100 radiometer.
All reactions were stopped when the accumulated dose of UV-A radiation reached
900 kJ m−2.The COD measurements were based on an Environmental
Protection Agencies recommended method [40, 41], in which an aliquot
of the effluent reacts under heating and closed reflux, at 423 K during 2 hours,
reducing dichromate ions to chromic ions in a strongly acid medium. From
absorbance measurement, done at 620 nm using a Hach DR-4000U spectrophotometer
and a resident program, the COD of the samples was determined.


## 3. Results and Discussion

### 3.1. Diffuse Reflectance Spectra

Figure2 presents the diffuse reflectance spectra of ZnPc, TiO2, and
some of the studied composites.Diffuse reflectance spectra of TiO2/ZnPc composites, prepared with
different percent in mass of ZnPc: (a) TiO2 P25. Composite containing:
1.0% of ZnPc (b); 2.5% of ZnPc (c); 5.0% of ZnPc (d); (e) composite containing 2.5%
of ZnPc (TiO2 P25 was used as reference), showing the Soret (370 nm)
and Q (683 nm) bands, typical of ZnPc; (f) diffuse reflectance spectrum of pure
ZnPc. Barium sulphate was used as reference for (a) to (d).Comparison of these figures shows that the UV-Vis absorption spectrum of
these composites is not simply the result of an additive effect between the
absorption spectra of TiO2 and zinc phthalocyanine (ZnPc) (compare Figures 2(a), 2(e), and 2(f)).
The later figure corresponds to the diffuse reflectance
spectra of pure ZnPc. The intense absorption peak at 552 nm, is due to the Q
band, relative to π→π* transitions [42, 43]. The observed blue
shift for this peak can be attributed to noninteracting molecules and the absence
of interaction between them and solvents [42, 44]. The Soret band is evident in the ultraviolet, with an absorption maximum
at 301 nm. A low intensity and nonstructured absorption band with the
absorption peak centered at 416 nm is related to an n→π* transition linking the eu azanitrogen lone pair orbital with the eg LUMO [43, 45]. A set of three very
small low-energy bands, above the Q one, can also be observed. The spectral structure of ZnPc and TiO2 is
lost in the composites. For the spectrum of the composite with 2.5% m/m of ZnPc
obtained using pure TiO2 as a reference, for example, the bands in the
ultraviolet and visible are very different from that observed for pure ZnPc. In
the visible, presents a large and intense three peak band centered by a red-shifted
Q band, with maximum at 683 nm [12]. The
Soret (B) band presents a different shape and is red shifted when compared to
its equivalent in Figure 2(f).Thus, these absorption spectra are very different from the typical absorption
profiles of TiO2 (Figure 2(a)) and pure ZnPc in the solid state (Figure 2(f))
and in very dilute liquid solutions [44]. The absorption spectrum of these
composites is characterized by an intense absorption band below 460 nm, and a large
and intense absorption band, above 475 nm, most probably due to an extense superposition
of the electronic states of TiO2 and ZnPc, indicating electronic
interaction between both species, resulting in several common electronic states
for both species. Szaciłowski et al. [46] reported in a recent work that the diffuse reflectance spectrum
of TiO2/prussian blue does not
correspond directly to the sum of the spectra of both components, indicating
electronic interaction between the particulate semiconductor and the polymeric
matrix.The spectrum shown in Figure2(e) is very similar to the absorption spectrum
for a flash-evaporated ZnPc thin film deposited on a glass substrate, as
reported by Senthilarasu et al. [47], in which the two energy bands characteristic
of phthalocyanines is evident, one in the region
between 500 and 900 nm, with an absorption peak at 690 nm, related to the Q
band, and the other, very intense, at 330 nm, attributed to Soret (B) band [48]. Mi et al. have reported a
similar absorption spectrum for thin films of Magnesium Phthalocyanine [49]. The
unstructured visible region band and the red shift of the Q band of these
composites can be attributed to the strong intermolecular interactions due to
ZnPc aggregation (ZnPcagg), resulting in exciton coupling effects of
the allowed transitions. This causes an important charge carrier mobility, as
suggested by Hoffmann [50], or to the lift of degeneracy increasing the band
splitting due to molecular distortion [49, 51, 52]. Based on the spectral characteristics of these composites, mainly in
the visible, we can conclude that the excitons formed by the aggregation of ZnPc
molecules behave as Frenkel’s J aggregates [53, 54].TheEg value
for ZnPc/TiO2 composites, estimated by diffuse
reflectance, is lower (2.7 eV) than the one observed for pure TiO2 (between
3.0 and 3.2 eV) [2, 3], and similar to the value for iron(II) phthalocyanine
excitons (2.6 eV) in FePc/TiO2 blends [55] and other metal phthalocyanine associated to semiconductor
oxides [16]. For ZnPc thin films, Senthilarasu et al. [47] have assigned an Eg of 1.97 eV, with a directly allowed optical
transition, near to the value estimated by us for the peak absorption Q-band
(2.25 eV) of pure ZnPc in the solid state (Figure 2(f)).TheEg for these composites might be related
to the electronic coupling between TiO2 and ZnPc, and its positive
implications. Similar to FePc/TiO2 blends [55] and ZnPc thin films [22, 47], the photoexcitation
of ZnPc aggregates results in the formation of e−/ZnPc+ pairs, parallel reactions from these species and the electron
injection from these ZnPc excitons to bulk TiO2, which explains
at least in part the improved photocatalytic activity observed for some of the ZnPc/TiO2 composites [11, 12]. The electronic coupling strength between donor and acceptor is one of
the critical factors for electron transfer to occur [22, 23, 47, 48]. Experiments based
on femtosecond pump-probe laser spectroscopy have provided valuable information
for understanding the electron transfer from excited dye molecules to
nanocrystalline TiO2 films or TiO2 nanoparticles [25, 56–58]. Sharma
et al. [55] observed a strong photoluminescence quenching
of FePc in FePc/TiO2 composite film, while the photosensitivity of a
device formed by FePc/TiO2 composite sandwiched between Al and ITO
is significantly enhanced when compared to pure FePc film. The authors
concluded that both effects arise from the charge transfer from FePc to TiO2 and charge separation after photoexcitation, resulting in FePc(h+) and
TiO2(e−). Additionally, they reported that the charge
transport and the current leakage through FePc films and the photogeneration
are due to the efficient dissociation of exciton at the donor-acceptor
interface of the bulk, and that the higher holes mobility in the organic
material layer, combined with lower conductance leakage, leads to the more
efficient collection of photogenerated carriers.The excited-state dynamics of metal phthalocyanines in
solution and thin filmshas been
extensively studied by transient absorption spectroscopy and time-resolved
photoluminescence measurements [47, 49, 59]. Despite the occurrence of several S1 state relaxation
processes in thin solid films (fluorescence, internal conversion, intersystem
crossing, ultrafast exciton-exciton annihilation, exciton-phonon coupling), the nonradiative processes tend to
be predominant [49, 60, 61]. The fluorescence quantum yields, for
example, tend to be extremely low in thin films owing to enhanced intersystem
crossing [61]. Sakakibara et al. in studies on photoluminescence properties of
phthalocyanine solid films, observed that fluorescence quantum yields at room
temperature were reduced to 10−5–10−4, much smaller than those of
the corresponding monomers (>0.5) [61]. In addition to these relaxation processes,
if these films are deposited onto bulk TiO2, the electron transfer
from metal phthalocyanine excitons has been reported [22, 55, 59].


### 3.2. Transmission Electron Microscopy

For composites containing 1%, 2.5%, and 5% m/m of
ZnPc, coatings of ZnPc with irregular thickness covering the TiO2 grains were observed (Figures 3(a)–3(c)).
For the first one, the coating thickness varies between 0 and 10 nm, whereas
for the composite containing 5% m/m of ZnPc, thicknesses between 0 and 25 nm
were observed.Transmission electron micrographs of granules of the different TiO2/ZnPc composites: (a) 1% ZnPc, (b) 2.5% ZnPc, and (c) 5% ZnPc.
(a)(b)(c)On the other hand, the composite containing 2.5% m/m of ZnPc presented the most regular coating thickness, ranging between 5 and
10 nm. Some uncoated surface areas also occur
but to a much lower degree than verified for the other compositions.ZnPc amounts exceeding about 2.5% lead to an
increasingly unhomogeneous distribution.
Microstructural investigations and energy dispersive X-ray (EDX) analyses of
the 5% m/m coated composite (Figure4) reveal two different regions: amorphous
areas consisting of aggregates rich in carbon, zinc and sodium and crystalline regions
formed by almost pure agglomerates of TiO2.
Signals corresponding to copper, from TEM support grids, can be also observed. Sodium
observed in the amorphous areas was probably incorporated during the
preparation of the samples.Figure 4
Transmission
electron micrography and X-ray dispersive energy of the composite containing 5%
of ZnPc.


### 3.3. Quantum Yield of Production of Hydroxyl Radicals

The following set of reactions [3, 17] can be
considered an approximate representation of the main reactions involved in the
photocatalytic process induced by TiO2 and composites on methanol
degradation:
(1)TiO2→hvTiO2/eCB−+TiO2/h+,(2)ZnPcagg→hv(1ZnPcagg)*,(3)(1ZnPcagg)*→isc(3ZnPcagg)*,(4)(ZnPcagg)*+O2→O2•−+(ZnPcagg)+,(5)(ZnPcagg)*→eCB−/ZnPcagg+,(6)eCB−/ZnPcagg++TiO2→ZnPcagg++TiO2/eCB−,(7)H2O+hVB+→HO•+H+,(8)H2O+O2•−→HO2•+HO−,(9)eCB−+O2→O2•−,(10)2O2•−+2H2O→H2O2+O2+2HO−,(11)eCB−+H2O2→HO•+HO−,(12)HO•+CH3OH→CH•2OH+H2O,(13)HO•+CH•2OH→HCHO+H2O,(14)CH2•OH+O2→O2•CH2OH→HCHO+HO2•,(15)HO−+hVB+→HO•,(16)TiO2/(ZnPcagg)+hVB+→TiO2/(ZnPcagg)+,(17)TiO2/(ZnPcagg)++HO−→TiO2/(ZnPcagg)+HO•,(18)(ZnPcagg)+HO•⇒oxidationproducts,(19)TiO2/(ZnPcagg)++CH3OH⇒oxidationproducts,(20)2HO•→H2O2,(21)H2O2+O2•−→HO•+HO−+O2,(22)H2O2+hVB+→HO2•+H+,(23)TiO2/(ZnPcagg)++H2O2→HO2•+H++TiO2/ZnPcagg,(24)2HO2•→H2O2+O2,(25)2HO2•+O2•−→HO•+HO−+2O2.The methodology used
to estimate the quantum yield of hydroxyl radicals production [35–37] can
be considered useful and safe since the reactions related to the generation of
hydroxyl radicals are sufficiently fast. For example, reaction (12) presents expressive
values for the rate constant, in the range between 8.3 and 9.7×108 L mol−1 s−1 [62, 63], whereas reaction (13), between the dehydro-methanol
radical and HO•, the
bimolecular rate constant is
in the limit for diffusion-controlled reactions
(109–1010 L mol−1 s−1). On the other hand, the trapping of
hydroxyl radicals by hydrogen peroxide, a possible side reaction, occurs at a
considerably slower rate (2.7×107 L mol−1 s−1) [62]. The primary steps that culminate in the formation of HO•
are also in the limit for
diffusion-controlled reactions.Figure5 shows the expected dependence between the quantum yield of hydroxyl
radicals generation (ΦHO•) and the amount of ZnPc adsorbed on TiO2 for experiments at
the laboratory scale (Section 2.4). Initially, an increase in the ΦHO• is observed, with a maximum value (ΦHO•=0.60) for a ZnPc concentration equals to 2.5% m/m. For pure TiO2,
ΦHO• is 0.06.A similar trend should
be expected during solar photocatalysis. However, since hydrogen peroxide is involved
as an additional source of hydroxyl radicals
[8–12], it is
expected that this must cause a
significant increase in ΦHO•. Sun and Bolton [37], investigating the generation
of hydroxyl radicals by aqueous suspensions of TiO2 (Anatase, Aldrich), reported
ΦHO• changed from 0.04 to 0.22 with the addition of hydrogen peroxide in the reactional medium. They also observed that when the H2O2 concentration in the
suspension is larger than 18 mM, a plateau for the ΦHO• values is reached.
The presence of H2O2 in the suspension might influence
the redox reactions occurring at the TiO2 particles surface, acting
as electron acceptor, competing with the O2, reactions (9) and (11), in
the redox reactions, and also producing additional HO• radicals, reactions (11), (21), (22), and (23). According to the data published by Sun and Bolton [37], the H2O2 concentration (about 9 mM)
used in our experiments under solar irradiation is capable of increasing ΦHO• at least 4.5 times.Figure 5
Dependence between the hydroxyl radicals quantum yield and the
amount of zinc phthalocyanine adsorbed on TiO2.In principle,
the light
intensity (1.1×10−6 einsteins s−1 cm−2) in the 295 to 710 nm range, capable to trigger the photocatalytic
processes mediated by the composites in the experiments at the laboratory scale,
also could accelerate the e−/h+ recombination, impairing
the production of HO• radicals, implying in ΦHO• values lower than desired
[35, 37]. However, it is probable that, at this wavelength range, nor all photons are capable to generate e−/h+ pairs in TiO2, or favor the electron injection
from ZnPc excitons to bulk TiO2. An estimate based on the spectral distribution of
the mercury lamp shows that the light intensitybetween 295 and 390 nm, capable to
photoexcite TiO2, generating e−/h+ pairs, is about 2.0×10−7 einsteins s−1 cm−2. The light intensity between 295 and 460 nm, 3.9×10−7 einsteins s−1 cm−2, should be capable to photoexcite TiO2/ZnPc composites, generating e−/h+ pairs in both species, and consequently favoring the
electron injection from ZnPc excitons to TiO2 conduction band (Scheme 1),
since in this wavelength range the superposition of the electronic states of
TiO2 and ZnPc implies in several common electronic states for both
species. The rest of
the emitted photons,between 460 and 710 nm (about 5.1×10−7 einsteins s−1 cm−2), should be related to other processes
directly mediated by the ZnPc excitons formed by the electronic excitation in
this wavelength range. This explains the reason by which the estimated ΦHO• value in our measurements using pure TiO2 (Degussa P25) is
equal to 0.06, 50% higher than the value estimated by Sun and Bolton [37] for aqueous suspensions of pure TiO2 (anatase), but equivalent to the value reported for TiO2 Degussa P25 (ΦHO•≈0.06) [64]. Several authors have shown that ΦHO• is inversely proportional to the
square root of the incident light intensity [35, 37, 65, 66], behavior attributed to an increase in the production rate of e−/h+ pairs, which favors the electron/hole recombination, thus decreasing the relative number of photoinduced
carriers taking part in the redox reactions at the catalyst surface [35, 37]. On the other
hand, as occurs in the solar photocatalytic process [8–12], the addition of H2O2 should contribute to reducing the recombination
of photoinduced electrons and holes [37], which must increase the ΦHO•.Scheme 1
Main processes involved in a photocatalytic process mediated by a TiO2/ZnPc composites.Based on these facts, considering the TiO2/ZnPc composites, the
participation of excitons formed by electronic excitation of the ZnPc aggregates is not
only restricted to the electron injection into the conduction band of TiO2,
which must occur only at wavelengths lower than 460 nm. Other processes should
be triggered by these charge carriers, especially after electronic excitation at
wavelengths higher than 460 nm, otherwise, we should observe a strong quenching
of the photocatalytic activity and not a synergic behavior involving TiO2 and ZnPcagg (observed in results to be published). This synergism explains
the higher ΦHO• values estimated for
three of the studied composites (Figure 5).Although the
values estimated by usΦHO• are not definite, they suggest an important trend, since they converge to
the results obtained in studies involving the photocatalytic degradation of
organic matter, mediated by solar radiation, that show some TiO2/ZnPc combinations
present significant increments in their photocatalytic activity. For example,
the ratio between ΦHO• for the composite containing 2.5% m/m of ZnPc and the estimated for
pure TiO2 shows an improvement of at least 1000% in the production of hydroxyl radicals.The comparison of the
photocatalytic degradation of the organic matter (LSA) present in a model
effluent [8–10], promoted by these composites and pure TiO2 in a solar photocatalytic process, confirms the better performance of the first
ones, especially the composite containing 2.5% m/m of ZnPc, to perform wastewater treatment using photocatalytic processes mediated by solar irradiation
[11, 12].Despite the fact that part of the adsorbed ZnPc is degraded during the photocatalytic
process, surprisingly, the photocatalytic efficiency of the recovered composite
does not decrease significantly when reused. Preliminary results (not shown)
suggest that a load of composite can be used at least five times before presenting
photocatalytic efficiency similar to pure TiO2. This can be due to, in
great part, the gradual ZnPc degradation, which occurs during the cycles of use
of the catalyst. In a minor extent, limited by its very low water
solubility, ZnPc also tends to migrate to the aqueous medium.Figure6 shows an increase in the
degradation rate, with significant changes in the degradation profile due to
the use of a recovered composite. This suggests that some reactions, less
probable to occur in virtue of ZnPc aggregates formation, become important for
the global process. For example, the production of singlet oxygen from 3ZnPc*
can be a plausible event if the level of
ZnPc aggregation is drastically lowered, which is consistent
with the fact that photoactive compounds like ZnPc, when aggregated, are
incapable to sensitize type I and II reactions [67, 68]. This behavior is attributed to the fact that aggregates in the
electronically excited state tend to be deactivated preferentially by
nonradiative processes [68]. On the other hand, the
better hydration of the catalyst surface, due to many cycles of use, should
favor reactions involving the holes (Scheme 1).Figure 6
Degradation of the organic load present in 50 L of an aqueous
model-effluent containing LSA (160 mg/L). Reactions done proceeded at pH 3,
using 100 mg of catalyst per liter of effluent, and addition of 1.00 mL of
hydrogen peroxide per liter of
effluent. Effluent treated using: (1) pure TiO2; (2) TiO2/ZnPc
2.5% composite; (3) a recycled TiO2/ZnPc 2.5% composite; (4) TiO2/ZnPc
2.5% in its second recycle. Inset: Pseudofirst-order
rate profiles for the effluent degradation under the above mentioned
conditions.For ZnPc concentrations above 2.5%,ΦHO• tends to decrease, most probably due to the occurrence of undesirable processes
related to the increasing size of the ZnPc aggregates, such as exciton-exciton
annihilation due to the excessive coating of the TiO2 surface. Essays
involving a ZnPc/SiO2 composite coated with 2.5% m/m of ZnPc reinforce,
as expected, that the observed process is not exclusively due to ZnPc or TiO2,
but to a synergism between these species, indeed when the composite is excited
by visible radiation (unpublished results).It is well known that the degradation of organic
matter induced by photocatalysis follows Langmuir-Hinschelwood kinetics [3, 8]. Assuming
the concentration of reactive species quickly reaches a stationary state at the
beginning of the photocatalytic process [8], the global rate law seems to follow
pseudofirst-order behavior with respect to the organic matter (Figure 6-Inset).The adsorption of the
organic matter at the surface of the catalyst in this situation occurs at a low
rate [8], since that LSA is constituted by fragments of large size and molecular
mass. The formation and desorption of reactive species,
derived from small molecules (water, H2O2, O2,
undegraded small fragments of LSA, etc.) should compensate the slow adsorption rate
of the large fragments, reacting with them in the catalyst/solvent interface. Considering
this aspect, we can consider that LSA degradation is a particular case in which
at least one of the steps does not follow the Langmuir-Hinshelwood mechanism.In the experiments using solar irradiation, to permit comparison with the
results obtained under different latitude and weather conditions, it is
necessary to use UV-A accumulated dose instead of the temporal variation data. Therefore, the
estimated parameters refer to
a constant related to the COD decrease rate relative to the variation of the accumulated
dose of UV-A radiation. These constants were calculated considering an
expression similar to the first-order reaction rate law in which the termd(COD)/dt was substituted by d(COD)/d(dose),
since d(dose)αdt. As
COD is dimensionally equivalent to the concentration of organic matter, the
calculated constant (now named degradation
constant, D) has the dimensions of the reciprocal of the radiation dose. The
monitoring of only the UV-A dose was adopted exclusively due to operational limitations.
However, this should have only a minor effect on the results since the spectral
pattern of the visible light between a set of experiments does not vary
significantly.Thedegradation constants, D, calculated
for the degradation of LSA mediated by TiO2 and TiO2/ZnPc
2.5% are presented in Table 1. Using pure
TiO2 or an unused sample of the composite as catalyst, the reaction
seems to occur in two steps being the second faster than the first one. The
observed increase in the degradation
constant of the second step is most probably due to the increasing
hydration of the surface of the catalyst and the participation of new reactive
species. These results also confirm previous results which show that the
apparent rate constant for reactions mediated by these composites are
systematically higher than the ones using pure TiO2 [8, 11, 12].Table 1
Degradation constants (D) for LSA degradation under different conditions.
CatalystD1 (m2J−1)D2 (m2J−1)D1(X)/D1(A)D2(X)/D2(A)Pure TiO2(A)5.6×10−72.0×10−61.01.0TiO2/ZnPc 2.5% (B)1.3×10−66.7×10−62.334Composite after
reuse(C)4.2×10−6—7.5—X=A,B,orC.Thedegradation constant for
the first step of the reaction mediated by the composite containing 2.5% m/m of
ZnPc, (D1), is about 130%
higher than the one for pure TiO2, whereas the corresponding value
for the second step is 3,300% higher. Although premature to present conclusions
on the involved processes, a comparison between these values and the estimated
increase in ΦHO• reinforces that the photocatalytic processes triggered by these
composites is much more complex than the process catalyzed by pure TiO2 [3], as suggests the set of reactions presented in Section 3.3.The two steps trend is also verified in the degradation mediated by reused
composite. However, in this caseD2 seems do be near zero. The
estimated value for the degradation
constant of the first step (D1) of these processes is valid until an accumulated UV-A dose of about 600 kJ m−2 (Figure 6-Inset) and is 650% higher than the observed value for the first step of the process
mediated by pure TiO2, and 230% higher when compared with the first
one for the composite used for the first time.As
a consequence of the reactions triggered by the composite containing 2.5% m/m
of ZnPc after the first time use,the final COD of the effluent after an
accumulated UV-A dose of 900 kJ m−2 (this value corresponds to about
3 hours of solar irradiation during a sunny day, or 5 to 6 hours during a day
with moderate to high nebulosity, in Uberlândia, MG, Brasil [12]) corresponds
to 4% of the initial value. For a reused composite, this value corresponds to
9%, whereas using pure TiO2 under similar conditions, the observed reduction
in COD was of 60%.Although
it is possible to perform the photocatalytic treatment until complete
mineralization of the organic matter, often it is economically more suitable to
use the photocatalytic process as an effluent pretreatment, completing the
treatment with othermore efficient processes. In a previous investigation [10], a
treated lignosulphonate wastewater was tested for its biochemical oxygen demand
(BOD). The results observed suggest that when a high bioavailability is reached
at the point where the performance of the photocatalytic treatment decreases, its
discontinuation and the beginning of a treatment based on the biological degradation
technology is more favorable.


### 3.4. Specific Surface Area

Table2 shows the specific surface
area (SSA) of pure TiO2 and of ZnPc-coated TiO2, at
different concentrations.Table 2
Specific surface area values for TiO2 and TiO2/ZnPc
composites at different m/m ratios.
CompositionSSA (m2g−1)P2552TiO2/FtZn 1%38TiO2/FtZn 2.5%35TiO2/FtZn 5%36The SSA of the composites is about 30% smaller
than the one of pure TiO2, suggesting
ZnPc adsorption reduces the TiO2 porosity. The changes in the SSA of the composites do not imply in distortions of
the TiO2 crystal structure. For pure TiO2 and composites,
the peak positions and lattice parameters of the anatase and rutile phases (not
shown) remain practically unchanged, supporting the structural integrity of the
TiO2 phases. The increase in the amount of dye results in a decrease
of the relative intensity of the peaks, evidencing the adsorption of the dye on
the semiconductor surface, and a partial absorption of the incident radiation
by the dye.X-ray powder diffraction patterns of
TiO2 and the different composites only show peaks due to the anatase
and rutile phases (figure not shown), suggesting ZnPc is adsorbed on the
semiconductor surface. This agrees with the results based
on scanning tunneling microscopy using different metal phthalocyanines, which concluded
that this class of compounds lies flat on the semiconductor surface [69].


### 3.5. Final Remarks

The spectral distribution of a 400 W high-pressure
(HP) mercury lamp, in
the 295 to 815 nm range, is shown in Figure7 .Figure 7
Spectral
distribution of the radiation emitted by a 400 W HPL-N mercury lamp in the 295 to
815 nm spectral range.The analysis of the spectral distribution shows that
the photonic flux in the 295 to 390 nm range, useful for the electronic
excitation of pure TiO2, ZnPc, and their aggregates (Figure 2),
is of 6.0×10−7 einsteins/s. The photonic
flux of 3.3×10−6 einsteins/s in the 295 to 710 nm range is 5.5
times higher. Considering an ideal situation in which 100% of the incident
photons furnished by HP, mercury lamps are able to trigger the photocatalytic
process, the use of these composites is very advantageous, since these
materials pick up 450% more photons than pure TiO2. This makes these composites very
attractive for use in processes induced by solar radiation, the main advantage inherent
to these composites, considering that more than 90% of the the incident
radiation on biosphere has wavelength above 390 nm. Hence an increase in the
population of excited ZnPc aggregates should imply in an electron/hole ratio higher than unity as observed for pure TiO2 [22, 50, 70], besides the other processes triggered by ZnPc excitons.Additional experiments using a metal vapor lamp to
excite the composites exclusively with visible radiation were done and
confirmed the effective participation of the visible radiation components in
the photocatalytic process (not shown).The increase in theΦHO• value indicates the important
role of ZnPc aggregates combined with TiO2 on the photocatalytic
processes triggered by these composites. On the other hand, the decrease of the photocatalytic efficiency observed
for composites containing ZnPc amounts higher than 2.5% m/m can be attributed
to an excessive ZnPc aggregation, which increases the chances of
exciton-exciton annihilation affecting the charge transport.One of the causes for the higher efficiency presented
by some of these composites, mainly the one containing 2.5% m/m of ZnPc, can be
attributed to the regular distribution and coating thickness of ZnPc on the TiO2 surface. Control of the coating thickness of the photosensitizing dye can
prevent/minimize the exciton-exciton annihilation, favoring electron injection in
the semiconductor oxide [22, 50, 59, 70] as well as other processes directly
mediated by ZnPc excitons.
Besides, good electronic coupling between this one and TiO2 is
fundamental for the electron transfer process from the exciton [22]. Terasaki et al. [60] using
femtosecond spectroscopy technique showed the excited-state dynamics of vanadyl
phthalocyanines in various molecular arrangements to be strongly dependent on
the molecular arrangements between the formed phases. The composites containing 2.5% m/m of ZnPc, who presented the best
photocatalytic performance, possesses the most homogeneous coatings with an
average thickness between 5 and 10 nm.At this point, the available informations are not yet sufficient
to propose a detailed mechanism involved in the enhancement of the photocatalytic
activity of these composites. However, previous studies give important
evidences. Based on the experimental facts reported here and the contribution
of other researchers, it is reasonable to assume that the reactions triggered
by these composites are based on the formation of electron/hole pairs from TiO2 and also from ZnPcagg. For these composites, the key steps which
lead to the formation of active species at the solid/solution interface are (a)
formation of e−/h+ pairs by the direct excitation of TiO2 (λexc≤390 nm); (b) formation of e−/ ZnPcagg+ by the photoexcitation of ZnPcagg; (c) electron transfer from ZnPc excitons to the TiO2 conduction band; (d) charge transport by ZnPcagg+ and its implications, and other reactions
triggered by e−/ ZnPcagg+ pairs.Ways to prevent/minimize the vulnerability of ZnPc during the photocatalytic process are under study, even though the results presented in this work show that, despite ZnPc
degradation, these composites are able to perform efficiently wastewater
treatment using solar radiation.


## 3.1. Diffuse Reflectance Spectra

Figure2 presents the diffuse reflectance spectra of ZnPc, TiO2, and
some of the studied composites.Diffuse reflectance spectra of TiO2/ZnPc composites, prepared with
different percent in mass of ZnPc: (a) TiO2 P25. Composite containing:
1.0% of ZnPc (b); 2.5% of ZnPc (c); 5.0% of ZnPc (d); (e) composite containing 2.5%
of ZnPc (TiO2 P25 was used as reference), showing the Soret (370 nm)
and Q (683 nm) bands, typical of ZnPc; (f) diffuse reflectance spectrum of pure
ZnPc. Barium sulphate was used as reference for (a) to (d).Comparison of these figures shows that the UV-Vis absorption spectrum of
these composites is not simply the result of an additive effect between the
absorption spectra of TiO2 and zinc phthalocyanine (ZnPc) (compare Figures 2(a), 2(e), and 2(f)).
The later figure corresponds to the diffuse reflectance
spectra of pure ZnPc. The intense absorption peak at 552 nm, is due to the Q
band, relative to π→π* transitions [42, 43]. The observed blue
shift for this peak can be attributed to noninteracting molecules and the absence
of interaction between them and solvents [42, 44]. The Soret band is evident in the ultraviolet, with an absorption maximum
at 301 nm. A low intensity and nonstructured absorption band with the
absorption peak centered at 416 nm is related to an n→π* transition linking the eu azanitrogen lone pair orbital with the eg LUMO [43, 45]. A set of three very
small low-energy bands, above the Q one, can also be observed. The spectral structure of ZnPc and TiO2 is
lost in the composites. For the spectrum of the composite with 2.5% m/m of ZnPc
obtained using pure TiO2 as a reference, for example, the bands in the
ultraviolet and visible are very different from that observed for pure ZnPc. In
the visible, presents a large and intense three peak band centered by a red-shifted
Q band, with maximum at 683 nm [12]. The
Soret (B) band presents a different shape and is red shifted when compared to
its equivalent in Figure 2(f).Thus, these absorption spectra are very different from the typical absorption
profiles of TiO2 (Figure 2(a)) and pure ZnPc in the solid state (Figure 2(f))
and in very dilute liquid solutions [44]. The absorption spectrum of these
composites is characterized by an intense absorption band below 460 nm, and a large
and intense absorption band, above 475 nm, most probably due to an extense superposition
of the electronic states of TiO2 and ZnPc, indicating electronic
interaction between both species, resulting in several common electronic states
for both species. Szaciłowski et al. [46] reported in a recent work that the diffuse reflectance spectrum
of TiO2/prussian blue does not
correspond directly to the sum of the spectra of both components, indicating
electronic interaction between the particulate semiconductor and the polymeric
matrix.The spectrum shown in Figure2(e) is very similar to the absorption spectrum
for a flash-evaporated ZnPc thin film deposited on a glass substrate, as
reported by Senthilarasu et al. [47], in which the two energy bands characteristic
of phthalocyanines is evident, one in the region
between 500 and 900 nm, with an absorption peak at 690 nm, related to the Q
band, and the other, very intense, at 330 nm, attributed to Soret (B) band [48]. Mi et al. have reported a
similar absorption spectrum for thin films of Magnesium Phthalocyanine [49]. The
unstructured visible region band and the red shift of the Q band of these
composites can be attributed to the strong intermolecular interactions due to
ZnPc aggregation (ZnPcagg), resulting in exciton coupling effects of
the allowed transitions. This causes an important charge carrier mobility, as
suggested by Hoffmann [50], or to the lift of degeneracy increasing the band
splitting due to molecular distortion [49, 51, 52]. Based on the spectral characteristics of these composites, mainly in
the visible, we can conclude that the excitons formed by the aggregation of ZnPc
molecules behave as Frenkel’s J aggregates [53, 54].TheEg value
for ZnPc/TiO2 composites, estimated by diffuse
reflectance, is lower (2.7 eV) than the one observed for pure TiO2 (between
3.0 and 3.2 eV) [2, 3], and similar to the value for iron(II) phthalocyanine
excitons (2.6 eV) in FePc/TiO2 blends [55] and other metal phthalocyanine associated to semiconductor
oxides [16]. For ZnPc thin films, Senthilarasu et al. [47] have assigned an Eg of 1.97 eV, with a directly allowed optical
transition, near to the value estimated by us for the peak absorption Q-band
(2.25 eV) of pure ZnPc in the solid state (Figure 2(f)).TheEg for these composites might be related
to the electronic coupling between TiO2 and ZnPc, and its positive
implications. Similar to FePc/TiO2 blends [55] and ZnPc thin films [22, 47], the photoexcitation
of ZnPc aggregates results in the formation of e−/ZnPc+ pairs, parallel reactions from these species and the electron
injection from these ZnPc excitons to bulk TiO2, which explains
at least in part the improved photocatalytic activity observed for some of the ZnPc/TiO2 composites [11, 12]. The electronic coupling strength between donor and acceptor is one of
the critical factors for electron transfer to occur [22, 23, 47, 48]. Experiments based
on femtosecond pump-probe laser spectroscopy have provided valuable information
for understanding the electron transfer from excited dye molecules to
nanocrystalline TiO2 films or TiO2 nanoparticles [25, 56–58]. Sharma
et al. [55] observed a strong photoluminescence quenching
of FePc in FePc/TiO2 composite film, while the photosensitivity of a
device formed by FePc/TiO2 composite sandwiched between Al and ITO
is significantly enhanced when compared to pure FePc film. The authors
concluded that both effects arise from the charge transfer from FePc to TiO2 and charge separation after photoexcitation, resulting in FePc(h+) and
TiO2(e−). Additionally, they reported that the charge
transport and the current leakage through FePc films and the photogeneration
are due to the efficient dissociation of exciton at the donor-acceptor
interface of the bulk, and that the higher holes mobility in the organic
material layer, combined with lower conductance leakage, leads to the more
efficient collection of photogenerated carriers.The excited-state dynamics of metal phthalocyanines in
solution and thin filmshas been
extensively studied by transient absorption spectroscopy and time-resolved
photoluminescence measurements [47, 49, 59]. Despite the occurrence of several S1 state relaxation
processes in thin solid films (fluorescence, internal conversion, intersystem
crossing, ultrafast exciton-exciton annihilation, exciton-phonon coupling), the nonradiative processes tend to
be predominant [49, 60, 61]. The fluorescence quantum yields, for
example, tend to be extremely low in thin films owing to enhanced intersystem
crossing [61]. Sakakibara et al. in studies on photoluminescence properties of
phthalocyanine solid films, observed that fluorescence quantum yields at room
temperature were reduced to 10−5–10−4, much smaller than those of
the corresponding monomers (>0.5) [61]. In addition to these relaxation processes,
if these films are deposited onto bulk TiO2, the electron transfer
from metal phthalocyanine excitons has been reported [22, 55, 59].


## 3.2. Transmission Electron Microscopy

For composites containing 1%, 2.5%, and 5% m/m of
ZnPc, coatings of ZnPc with irregular thickness covering the TiO2 grains were observed (Figures 3(a)–3(c)).
For the first one, the coating thickness varies between 0 and 10 nm, whereas
for the composite containing 5% m/m of ZnPc, thicknesses between 0 and 25 nm
were observed.Transmission electron micrographs of granules of the different TiO2/ZnPc composites: (a) 1% ZnPc, (b) 2.5% ZnPc, and (c) 5% ZnPc.
(a)(b)(c)On the other hand, the composite containing 2.5% m/m of ZnPc presented the most regular coating thickness, ranging between 5 and
10 nm. Some uncoated surface areas also occur
but to a much lower degree than verified for the other compositions.ZnPc amounts exceeding about 2.5% lead to an
increasingly unhomogeneous distribution.
Microstructural investigations and energy dispersive X-ray (EDX) analyses of
the 5% m/m coated composite (Figure4) reveal two different regions: amorphous
areas consisting of aggregates rich in carbon, zinc and sodium and crystalline regions
formed by almost pure agglomerates of TiO2.
Signals corresponding to copper, from TEM support grids, can be also observed. Sodium
observed in the amorphous areas was probably incorporated during the
preparation of the samples.Figure 4
Transmission
electron micrography and X-ray dispersive energy of the composite containing 5%
of ZnPc.


## 3.3. Quantum Yield of Production of Hydroxyl Radicals

The following set of reactions [3, 17] can be
considered an approximate representation of the main reactions involved in the
photocatalytic process induced by TiO2 and composites on methanol
degradation:
(1)TiO2→hvTiO2/eCB−+TiO2/h+,(2)ZnPcagg→hv(1ZnPcagg)*,(3)(1ZnPcagg)*→isc(3ZnPcagg)*,(4)(ZnPcagg)*+O2→O2•−+(ZnPcagg)+,(5)(ZnPcagg)*→eCB−/ZnPcagg+,(6)eCB−/ZnPcagg++TiO2→ZnPcagg++TiO2/eCB−,(7)H2O+hVB+→HO•+H+,(8)H2O+O2•−→HO2•+HO−,(9)eCB−+O2→O2•−,(10)2O2•−+2H2O→H2O2+O2+2HO−,(11)eCB−+H2O2→HO•+HO−,(12)HO•+CH3OH→CH•2OH+H2O,(13)HO•+CH•2OH→HCHO+H2O,(14)CH2•OH+O2→O2•CH2OH→HCHO+HO2•,(15)HO−+hVB+→HO•,(16)TiO2/(ZnPcagg)+hVB+→TiO2/(ZnPcagg)+,(17)TiO2/(ZnPcagg)++HO−→TiO2/(ZnPcagg)+HO•,(18)(ZnPcagg)+HO•⇒oxidationproducts,(19)TiO2/(ZnPcagg)++CH3OH⇒oxidationproducts,(20)2HO•→H2O2,(21)H2O2+O2•−→HO•+HO−+O2,(22)H2O2+hVB+→HO2•+H+,(23)TiO2/(ZnPcagg)++H2O2→HO2•+H++TiO2/ZnPcagg,(24)2HO2•→H2O2+O2,(25)2HO2•+O2•−→HO•+HO−+2O2.The methodology used
to estimate the quantum yield of hydroxyl radicals production [35–37] can
be considered useful and safe since the reactions related to the generation of
hydroxyl radicals are sufficiently fast. For example, reaction (12) presents expressive
values for the rate constant, in the range between 8.3 and 9.7×108 L mol−1 s−1 [62, 63], whereas reaction (13), between the dehydro-methanol
radical and HO•, the
bimolecular rate constant is
in the limit for diffusion-controlled reactions
(109–1010 L mol−1 s−1). On the other hand, the trapping of
hydroxyl radicals by hydrogen peroxide, a possible side reaction, occurs at a
considerably slower rate (2.7×107 L mol−1 s−1) [62]. The primary steps that culminate in the formation of HO•
are also in the limit for
diffusion-controlled reactions.Figure5 shows the expected dependence between the quantum yield of hydroxyl
radicals generation (ΦHO•) and the amount of ZnPc adsorbed on TiO2 for experiments at
the laboratory scale (Section 2.4). Initially, an increase in the ΦHO• is observed, with a maximum value (ΦHO•=0.60) for a ZnPc concentration equals to 2.5% m/m. For pure TiO2,
ΦHO• is 0.06.A similar trend should
be expected during solar photocatalysis. However, since hydrogen peroxide is involved
as an additional source of hydroxyl radicals
[8–12], it is
expected that this must cause a
significant increase in ΦHO•. Sun and Bolton [37], investigating the generation
of hydroxyl radicals by aqueous suspensions of TiO2 (Anatase, Aldrich), reported
ΦHO• changed from 0.04 to 0.22 with the addition of hydrogen peroxide in the reactional medium. They also observed that when the H2O2 concentration in the
suspension is larger than 18 mM, a plateau for the ΦHO• values is reached.
The presence of H2O2 in the suspension might influence
the redox reactions occurring at the TiO2 particles surface, acting
as electron acceptor, competing with the O2, reactions (9) and (11), in
the redox reactions, and also producing additional HO• radicals, reactions (11), (21), (22), and (23). According to the data published by Sun and Bolton [37], the H2O2 concentration (about 9 mM)
used in our experiments under solar irradiation is capable of increasing ΦHO• at least 4.5 times.Figure 5
Dependence between the hydroxyl radicals quantum yield and the
amount of zinc phthalocyanine adsorbed on TiO2.In principle,
the light
intensity (1.1×10−6 einsteins s−1 cm−2) in the 295 to 710 nm range, capable to trigger the photocatalytic
processes mediated by the composites in the experiments at the laboratory scale,
also could accelerate the e−/h+ recombination, impairing
the production of HO• radicals, implying in ΦHO• values lower than desired
[35, 37]. However, it is probable that, at this wavelength range, nor all photons are capable to generate e−/h+ pairs in TiO2, or favor the electron injection
from ZnPc excitons to bulk TiO2. An estimate based on the spectral distribution of
the mercury lamp shows that the light intensitybetween 295 and 390 nm, capable to
photoexcite TiO2, generating e−/h+ pairs, is about 2.0×10−7 einsteins s−1 cm−2. The light intensity between 295 and 460 nm, 3.9×10−7 einsteins s−1 cm−2, should be capable to photoexcite TiO2/ZnPc composites, generating e−/h+ pairs in both species, and consequently favoring the
electron injection from ZnPc excitons to TiO2 conduction band (Scheme 1),
since in this wavelength range the superposition of the electronic states of
TiO2 and ZnPc implies in several common electronic states for both
species. The rest of
the emitted photons,between 460 and 710 nm (about 5.1×10−7 einsteins s−1 cm−2), should be related to other processes
directly mediated by the ZnPc excitons formed by the electronic excitation in
this wavelength range. This explains the reason by which the estimated ΦHO• value in our measurements using pure TiO2 (Degussa P25) is
equal to 0.06, 50% higher than the value estimated by Sun and Bolton [37] for aqueous suspensions of pure TiO2 (anatase), but equivalent to the value reported for TiO2 Degussa P25 (ΦHO•≈0.06) [64]. Several authors have shown that ΦHO• is inversely proportional to the
square root of the incident light intensity [35, 37, 65, 66], behavior attributed to an increase in the production rate of e−/h+ pairs, which favors the electron/hole recombination, thus decreasing the relative number of photoinduced
carriers taking part in the redox reactions at the catalyst surface [35, 37]. On the other
hand, as occurs in the solar photocatalytic process [8–12], the addition of H2O2 should contribute to reducing the recombination
of photoinduced electrons and holes [37], which must increase the ΦHO•.Scheme 1
Main processes involved in a photocatalytic process mediated by a TiO2/ZnPc composites.Based on these facts, considering the TiO2/ZnPc composites, the
participation of excitons formed by electronic excitation of the ZnPc aggregates is not
only restricted to the electron injection into the conduction band of TiO2,
which must occur only at wavelengths lower than 460 nm. Other processes should
be triggered by these charge carriers, especially after electronic excitation at
wavelengths higher than 460 nm, otherwise, we should observe a strong quenching
of the photocatalytic activity and not a synergic behavior involving TiO2 and ZnPcagg (observed in results to be published). This synergism explains
the higher ΦHO• values estimated for
three of the studied composites (Figure 5).Although the
values estimated by usΦHO• are not definite, they suggest an important trend, since they converge to
the results obtained in studies involving the photocatalytic degradation of
organic matter, mediated by solar radiation, that show some TiO2/ZnPc combinations
present significant increments in their photocatalytic activity. For example,
the ratio between ΦHO• for the composite containing 2.5% m/m of ZnPc and the estimated for
pure TiO2 shows an improvement of at least 1000% in the production of hydroxyl radicals.The comparison of the
photocatalytic degradation of the organic matter (LSA) present in a model
effluent [8–10], promoted by these composites and pure TiO2 in a solar photocatalytic process, confirms the better performance of the first
ones, especially the composite containing 2.5% m/m of ZnPc, to perform wastewater treatment using photocatalytic processes mediated by solar irradiation
[11, 12].Despite the fact that part of the adsorbed ZnPc is degraded during the photocatalytic
process, surprisingly, the photocatalytic efficiency of the recovered composite
does not decrease significantly when reused. Preliminary results (not shown)
suggest that a load of composite can be used at least five times before presenting
photocatalytic efficiency similar to pure TiO2. This can be due to, in
great part, the gradual ZnPc degradation, which occurs during the cycles of use
of the catalyst. In a minor extent, limited by its very low water
solubility, ZnPc also tends to migrate to the aqueous medium.Figure6 shows an increase in the
degradation rate, with significant changes in the degradation profile due to
the use of a recovered composite. This suggests that some reactions, less
probable to occur in virtue of ZnPc aggregates formation, become important for
the global process. For example, the production of singlet oxygen from 3ZnPc*
can be a plausible event if the level of
ZnPc aggregation is drastically lowered, which is consistent
with the fact that photoactive compounds like ZnPc, when aggregated, are
incapable to sensitize type I and II reactions [67, 68]. This behavior is attributed to the fact that aggregates in the
electronically excited state tend to be deactivated preferentially by
nonradiative processes [68]. On the other hand, the
better hydration of the catalyst surface, due to many cycles of use, should
favor reactions involving the holes (Scheme 1).Figure 6
Degradation of the organic load present in 50 L of an aqueous
model-effluent containing LSA (160 mg/L). Reactions done proceeded at pH 3,
using 100 mg of catalyst per liter of effluent, and addition of 1.00 mL of
hydrogen peroxide per liter of
effluent. Effluent treated using: (1) pure TiO2; (2) TiO2/ZnPc
2.5% composite; (3) a recycled TiO2/ZnPc 2.5% composite; (4) TiO2/ZnPc
2.5% in its second recycle. Inset: Pseudofirst-order
rate profiles for the effluent degradation under the above mentioned
conditions.For ZnPc concentrations above 2.5%,ΦHO• tends to decrease, most probably due to the occurrence of undesirable processes
related to the increasing size of the ZnPc aggregates, such as exciton-exciton
annihilation due to the excessive coating of the TiO2 surface. Essays
involving a ZnPc/SiO2 composite coated with 2.5% m/m of ZnPc reinforce,
as expected, that the observed process is not exclusively due to ZnPc or TiO2,
but to a synergism between these species, indeed when the composite is excited
by visible radiation (unpublished results).It is well known that the degradation of organic
matter induced by photocatalysis follows Langmuir-Hinschelwood kinetics [3, 8]. Assuming
the concentration of reactive species quickly reaches a stationary state at the
beginning of the photocatalytic process [8], the global rate law seems to follow
pseudofirst-order behavior with respect to the organic matter (Figure 6-Inset).The adsorption of the
organic matter at the surface of the catalyst in this situation occurs at a low
rate [8], since that LSA is constituted by fragments of large size and molecular
mass. The formation and desorption of reactive species,
derived from small molecules (water, H2O2, O2,
undegraded small fragments of LSA, etc.) should compensate the slow adsorption rate
of the large fragments, reacting with them in the catalyst/solvent interface. Considering
this aspect, we can consider that LSA degradation is a particular case in which
at least one of the steps does not follow the Langmuir-Hinshelwood mechanism.In the experiments using solar irradiation, to permit comparison with the
results obtained under different latitude and weather conditions, it is
necessary to use UV-A accumulated dose instead of the temporal variation data. Therefore, the
estimated parameters refer to
a constant related to the COD decrease rate relative to the variation of the accumulated
dose of UV-A radiation. These constants were calculated considering an
expression similar to the first-order reaction rate law in which the termd(COD)/dt was substituted by d(COD)/d(dose),
since d(dose)αdt. As
COD is dimensionally equivalent to the concentration of organic matter, the
calculated constant (now named degradation
constant, D) has the dimensions of the reciprocal of the radiation dose. The
monitoring of only the UV-A dose was adopted exclusively due to operational limitations.
However, this should have only a minor effect on the results since the spectral
pattern of the visible light between a set of experiments does not vary
significantly.Thedegradation constants, D, calculated
for the degradation of LSA mediated by TiO2 and TiO2/ZnPc
2.5% are presented in Table 1. Using pure
TiO2 or an unused sample of the composite as catalyst, the reaction
seems to occur in two steps being the second faster than the first one. The
observed increase in the degradation
constant of the second step is most probably due to the increasing
hydration of the surface of the catalyst and the participation of new reactive
species. These results also confirm previous results which show that the
apparent rate constant for reactions mediated by these composites are
systematically higher than the ones using pure TiO2 [8, 11, 12].Table 1
Degradation constants (D) for LSA degradation under different conditions.
CatalystD1 (m2J−1)D2 (m2J−1)D1(X)/D1(A)D2(X)/D2(A)Pure TiO2(A)5.6×10−72.0×10−61.01.0TiO2/ZnPc 2.5% (B)1.3×10−66.7×10−62.334Composite after
reuse(C)4.2×10−6—7.5—X=A,B,orC.Thedegradation constant for
the first step of the reaction mediated by the composite containing 2.5% m/m of
ZnPc, (D1), is about 130%
higher than the one for pure TiO2, whereas the corresponding value
for the second step is 3,300% higher. Although premature to present conclusions
on the involved processes, a comparison between these values and the estimated
increase in ΦHO• reinforces that the photocatalytic processes triggered by these
composites is much more complex than the process catalyzed by pure TiO2 [3], as suggests the set of reactions presented in Section 3.3.The two steps trend is also verified in the degradation mediated by reused
composite. However, in this caseD2 seems do be near zero. The
estimated value for the degradation
constant of the first step (D1) of these processes is valid until an accumulated UV-A dose of about 600 kJ m−2 (Figure 6-Inset) and is 650% higher than the observed value for the first step of the process
mediated by pure TiO2, and 230% higher when compared with the first
one for the composite used for the first time.As
a consequence of the reactions triggered by the composite containing 2.5% m/m
of ZnPc after the first time use,the final COD of the effluent after an
accumulated UV-A dose of 900 kJ m−2 (this value corresponds to about
3 hours of solar irradiation during a sunny day, or 5 to 6 hours during a day
with moderate to high nebulosity, in Uberlândia, MG, Brasil [12]) corresponds
to 4% of the initial value. For a reused composite, this value corresponds to
9%, whereas using pure TiO2 under similar conditions, the observed reduction
in COD was of 60%.Although
it is possible to perform the photocatalytic treatment until complete
mineralization of the organic matter, often it is economically more suitable to
use the photocatalytic process as an effluent pretreatment, completing the
treatment with othermore efficient processes. In a previous investigation [10], a
treated lignosulphonate wastewater was tested for its biochemical oxygen demand
(BOD). The results observed suggest that when a high bioavailability is reached
at the point where the performance of the photocatalytic treatment decreases, its
discontinuation and the beginning of a treatment based on the biological degradation
technology is more favorable.


## 3.4. Specific Surface Area

Table2 shows the specific surface
area (SSA) of pure TiO2 and of ZnPc-coated TiO2, at
different concentrations.Table 2
Specific surface area values for TiO2 and TiO2/ZnPc
composites at different m/m ratios.
CompositionSSA (m2g−1)P2552TiO2/FtZn 1%38TiO2/FtZn 2.5%35TiO2/FtZn 5%36The SSA of the composites is about 30% smaller
than the one of pure TiO2, suggesting
ZnPc adsorption reduces the TiO2 porosity. The changes in the SSA of the composites do not imply in distortions of
the TiO2 crystal structure. For pure TiO2 and composites,
the peak positions and lattice parameters of the anatase and rutile phases (not
shown) remain practically unchanged, supporting the structural integrity of the
TiO2 phases. The increase in the amount of dye results in a decrease
of the relative intensity of the peaks, evidencing the adsorption of the dye on
the semiconductor surface, and a partial absorption of the incident radiation
by the dye.X-ray powder diffraction patterns of
TiO2 and the different composites only show peaks due to the anatase
and rutile phases (figure not shown), suggesting ZnPc is adsorbed on the
semiconductor surface. This agrees with the results based
on scanning tunneling microscopy using different metal phthalocyanines, which concluded
that this class of compounds lies flat on the semiconductor surface [69].


## 3.5. Final Remarks

The spectral distribution of a 400 W high-pressure
(HP) mercury lamp, in
the 295 to 815 nm range, is shown in Figure7 .Figure 7
Spectral
distribution of the radiation emitted by a 400 W HPL-N mercury lamp in the 295 to
815 nm spectral range.The analysis of the spectral distribution shows that
the photonic flux in the 295 to 390 nm range, useful for the electronic
excitation of pure TiO2, ZnPc, and their aggregates (Figure 2),
is of 6.0×10−7 einsteins/s. The photonic
flux of 3.3×10−6 einsteins/s in the 295 to 710 nm range is 5.5
times higher. Considering an ideal situation in which 100% of the incident
photons furnished by HP, mercury lamps are able to trigger the photocatalytic
process, the use of these composites is very advantageous, since these
materials pick up 450% more photons than pure TiO2. This makes these composites very
attractive for use in processes induced by solar radiation, the main advantage inherent
to these composites, considering that more than 90% of the the incident
radiation on biosphere has wavelength above 390 nm. Hence an increase in the
population of excited ZnPc aggregates should imply in an electron/hole ratio higher than unity as observed for pure TiO2 [22, 50, 70], besides the other processes triggered by ZnPc excitons.Additional experiments using a metal vapor lamp to
excite the composites exclusively with visible radiation were done and
confirmed the effective participation of the visible radiation components in
the photocatalytic process (not shown).The increase in theΦHO• value indicates the important
role of ZnPc aggregates combined with TiO2 on the photocatalytic
processes triggered by these composites. On the other hand, the decrease of the photocatalytic efficiency observed
for composites containing ZnPc amounts higher than 2.5% m/m can be attributed
to an excessive ZnPc aggregation, which increases the chances of
exciton-exciton annihilation affecting the charge transport.One of the causes for the higher efficiency presented
by some of these composites, mainly the one containing 2.5% m/m of ZnPc, can be
attributed to the regular distribution and coating thickness of ZnPc on the TiO2 surface. Control of the coating thickness of the photosensitizing dye can
prevent/minimize the exciton-exciton annihilation, favoring electron injection in
the semiconductor oxide [22, 50, 59, 70] as well as other processes directly
mediated by ZnPc excitons.
Besides, good electronic coupling between this one and TiO2 is
fundamental for the electron transfer process from the exciton [22]. Terasaki et al. [60] using
femtosecond spectroscopy technique showed the excited-state dynamics of vanadyl
phthalocyanines in various molecular arrangements to be strongly dependent on
the molecular arrangements between the formed phases. The composites containing 2.5% m/m of ZnPc, who presented the best
photocatalytic performance, possesses the most homogeneous coatings with an
average thickness between 5 and 10 nm.At this point, the available informations are not yet sufficient
to propose a detailed mechanism involved in the enhancement of the photocatalytic
activity of these composites. However, previous studies give important
evidences. Based on the experimental facts reported here and the contribution
of other researchers, it is reasonable to assume that the reactions triggered
by these composites are based on the formation of electron/hole pairs from TiO2 and also from ZnPcagg. For these composites, the key steps which
lead to the formation of active species at the solid/solution interface are (a)
formation of e−/h+ pairs by the direct excitation of TiO2 (λexc≤390 nm); (b) formation of e−/ ZnPcagg+ by the photoexcitation of ZnPcagg; (c) electron transfer from ZnPc excitons to the TiO2 conduction band; (d) charge transport by ZnPcagg+ and its implications, and other reactions
triggered by e−/ ZnPcagg+ pairs.Ways to prevent/minimize the vulnerability of ZnPc during the photocatalytic process are under study, even though the results presented in this work show that, despite ZnPc
degradation, these composites are able to perform efficiently wastewater
treatment using solar radiation.



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*Source: 482373-2008-05-11.xml*