# Characterization of Conjugated Polymer Poly(fluorene-co-thiophene) and Its Application as Photosensitizer ofTiO2

Authors: Rongliang Qiu; Lin Song; Dongdong Zhang; Yueqi Mo; Eric Brewer; Xiongfei Huang
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

The copolymer poly(fluorene-co-thiophene) (PFT) has strong absorption in the visible light region. In this paper, PFTs with four different thiophene contents were evaluated for their suitability as photosensitizers of TiO2. All four of the PFTs were capable of being excited by light with wavelengths above 455 nm, and all had reductive potentials more negative than the conduction band potential of TiO2. Consequently, it was deemed that transfer of electrons from the excited PFTs to the conduction band of TiO2 was thermodynamically possible. PFTs with higher thiophene content had more electron transitions in the excited state, resulting in increased photocatalytic activity of PFT-sensitized TiO2. The method used to prepare the combined PFT/TiO2 photocatalyst affected its photocatalytic activity. Best results in this study were achieved by dropwise addition of a tetrahydrofuran solution of PFT to a stirred TiO2/ethanol suspension.


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## Body

## 1. Introduction

Photosensitization of the semiconducting oxideTiO2 is a method for extending its photoresponse in
the visible light region by physically combining it with another semiconductor
that acts as a photosensitizer. As p-type semiconductors, photosensitizers,
which are commonly organic dyes, have strong absorption in the visible light
region and can inject electrons into the conduction band of TiO2 when excited by visible light, giving rise to
a series of photochemical processes and allowing the photocatalytic reactions
mediated by TiO2 to operate under visible light irradiation.Conjugated polymers composed of repeated small aryl
ring molecules such as benzene, thiophene, and pyrrole are other types of
organic semiconductors. Although the energy level difference betweenπ-bonding and antibonding orbits in a single
aryl ring is large, it is found that as the number of repeated units increases, π orbits overlap each other, causing the energy
level difference between π-conjugated bonding and antibonding orbits to
decrease sharply and fall eventually to less than 4 eV. Energy level differences
of this magnitude make these polymers suitable for use as semiconductors. In
addition, some of these polymers have a number of optical properties, such as
electroluminescence and nonlinear optical behavior, that make them especially
useful as photovoltaic materials. At present, conjugated polymers such as
poly(3-methylthiophene) [1], poly(phenylene vinylene) [2], polyaniline [3], polypyrrole [4], poly(o-methoxy aniline)
[5],
poly(3-undecyl-2,2′-bithiophene) [6], and
poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] (PTEBs) [7] are being increasingly used
in the field of photovoltaics as solid electrolytes and electrode sensitizers
[8].Methods for sensitizing semiconductors are similar to
those for sensitizing electrodes of solar energy cells. However, little work
has been done to date on the use of conjugated polymer-sensitized
semiconductors to photocatalyze the degradation of organics under visible light
irradiation, in part because of limitations of the polymers themselves. One
problem is that the HOMO energy level of polymers is generally higher than
those of inorganic materials and dyes, so polymers tend to be unstable and
easily oxidized. In addition, the electron mobility of polymers is relatively
poor. Finally, it can be difficult to physically combine polymers and inorganic
semiconductors, because most polymers are not soluble in water or ethanol.
However, as research in this area continues, more and more polymers with
improved characteristics are being synthesized. For example, the wide band gap
polymer polyfluorene (PF) and the narrow band gap polymer polythiophene have
improved antioxidative ability. Moreover, electron transfer takes place easily
in thiophene-based polymers.In this paper, a series of conjugated copolymers
containing fluorene and thiophene [9] were investigated to evaluate their suitability as
photosensitizers of TiO2.


## 2. Experimental

### 2.1. Materials

A series of copolymer PFTs with thiophene contents of
5%, 15%, 30%, and 50% (so called PFT5, PFT15, PFT30, and PFT50, resp.) were
provided by College of Material Science and Engineering, South China University
of Technology; the chemical structure is presented in Figure1, TiO2 (Degussa P25: 80% anatase, 20% rutile; BET~50m2/g). Purified water was obtained from a
Millipore filtration system (Millipore ZLXS50020). All other chemicals were
analytical grade and used without further purification.Figure 1
Structure of
conjugated polymer poly(fluorene-co-thiophene).


### 2.2. Measurements and Procedures

An HP 8453 diode array UV-Vis spectrophotometer
(Agilent, USA) was used to directly record the UV-Vis spectra of the copolymer
PFTs using chloroform as solvent. The electrochemical properties of PFTs were
obtained with a CHI 660A electrochemical workstation. Phenol concentration was
monitored by UV-2450 spectrophotometer (Shimadzu, Japan) at a wavelength of
510 nm.Sensitization ofTiO2 was performed according to the literature
[10]. The
photocatalytic degradation experiment was carried out by adding 50 mg
photocatalyst into 50 mL phenol aqueous solution with 10 mg/L initial
concentration. A 250 W GaI3 lamp (DJG250, Shanghai Hualun Lighting Co. Ltd.,
China) with maximum radiant wavelength matched to the absorption wavelength of
PFT was used as the visible light source. UV and IR cutoff filters were placed
between the lamp and the glass reactor to ensure irradiation by visible light
(400–700 nm) only. Light intensity was measured by a UV-A ultraviolet meter
equipped with two probes with wavelengths of 420 nm and 365 nm, respectively. The
glass reactor was open to air to ensure enough oxygen in the reaction solution.
Prior to irradiation, the solution containing photocatalyst and phenol was
magnetically stirred in darkness for 30 minutes to establish
adsorption-desorption equilibrium. After a specified interval of irradiation
time, a 5 mL aliquot of the reaction mixture was centrifuged and filtered
through a 0.22 μm filter (Millipore), and the change in the
concentration of phenol was monitored spectrophotometrically.


## 2.1. Materials

A series of copolymer PFTs with thiophene contents of
5%, 15%, 30%, and 50% (so called PFT5, PFT15, PFT30, and PFT50, resp.) were
provided by College of Material Science and Engineering, South China University
of Technology; the chemical structure is presented in Figure1, TiO2 (Degussa P25: 80% anatase, 20% rutile; BET~50m2/g). Purified water was obtained from a
Millipore filtration system (Millipore ZLXS50020). All other chemicals were
analytical grade and used without further purification.Figure 1
Structure of
conjugated polymer poly(fluorene-co-thiophene).


## 2.2. Measurements and Procedures

An HP 8453 diode array UV-Vis spectrophotometer
(Agilent, USA) was used to directly record the UV-Vis spectra of the copolymer
PFTs using chloroform as solvent. The electrochemical properties of PFTs were
obtained with a CHI 660A electrochemical workstation. Phenol concentration was
monitored by UV-2450 spectrophotometer (Shimadzu, Japan) at a wavelength of
510 nm.Sensitization ofTiO2 was performed according to the literature
[10]. The
photocatalytic degradation experiment was carried out by adding 50 mg
photocatalyst into 50 mL phenol aqueous solution with 10 mg/L initial
concentration. A 250 W GaI3 lamp (DJG250, Shanghai Hualun Lighting Co. Ltd.,
China) with maximum radiant wavelength matched to the absorption wavelength of
PFT was used as the visible light source. UV and IR cutoff filters were placed
between the lamp and the glass reactor to ensure irradiation by visible light
(400–700 nm) only. Light intensity was measured by a UV-A ultraviolet meter
equipped with two probes with wavelengths of 420 nm and 365 nm, respectively. The
glass reactor was open to air to ensure enough oxygen in the reaction solution.
Prior to irradiation, the solution containing photocatalyst and phenol was
magnetically stirred in darkness for 30 minutes to establish
adsorption-desorption equilibrium. After a specified interval of irradiation
time, a 5 mL aliquot of the reaction mixture was centrifuged and filtered
through a 0.22 μm filter (Millipore), and the change in the
concentration of phenol was monitored spectrophotometrically.


## 3. Results and Discussion

### 3.1. Electrochemical Properties of PFTs

Having strong absorption in the visible light region
is the most important property of a photosensitizer. Figure2 shows the
UV-Vis spectra of the copolymers PF, PFT5, PFT15, PFT30, and PFT50 in
chloroform. These polymers all have strong absorption in the visible light
region. As expected, the intensity and maximum absorption peaks progressively
increased and were red-shifted with increasing content of thiophene in the
PFTs. The maximum absorption peaks are at 384 nm, 384 nm, 390 nm, 400 nm, and 433 nm,
respectively. From Figure 2 it is observed that the absorption peaks of PFT5
and PFT15 were little changed from that of PF, while their spectra were
slightly red-shifted toward the visible light region. The absorption peak of
PFT30 was red-shifted from 384 nm (PF) to 400 nm, and its absorption edge was
red-shifted to 480 nm. In the case of PFT50, the red shifting was more obvious;
the absorption peak and edge were red-shifted to 433 nm and 490 nm, respectively.
The increased red shifting that was associated with increased thiophene content
of the PFTs was due to strong absorption of visible light by the thiophene
units of the polymer. Compared with PF oligomers, the blocks of spatial
configuration between thiophene ring and benzene ring in PFTs are smaller than
those between benzene rings in PF, and twists between thiophene ring and
benzene ring take place more easily, leading to higher coplanarity and a larger
conjugated system in PFTs. In addition, the lone pair electrons of the sulfur
atom in thiophene units increase the electron cloud density in the PFT chain,
thereby increasing its ability to act as an electron donor and decreasing its
band gap. Thus, higher thiophene content of a PFT is associated with higher
coplanarity, lower band gap, and red shifting of its absorption spectrum.Figure 2
UV-Vis
spectra of the different copolymers.The second most important property of a
photosensitizer ofTiO2 is that its energy level matches that of  TiO2.
Only if the reductive potential of the photosensitizer is more negative than
the energy level of the conduction band of TiO2,
it is thermodynamically possible for excited electrons to be injected from the
conduction band of the photosensitizer to the conduction band of  TiO2.Table1 presents the wavelength at peak absorption
and electrochemical properties of the PFTs used in this study. The peak and
onset of absorption wavelength of PFTs were obtained from Figure 2, and the
band gaps (Eg) were estimated by
(1)
[11]:(1)Eg=1240/λonset,(2)Ered=(EgEox),(3)EHOMO=e(Eox+4.4),(eV)(4)ELUMO=e(Ered+4.4),(eV).Table 1
Wavelength at
peak absorption and electrochemical properties of PFTs.
CopolymersλabsmaxEoxEredEgEHOMOELUMO(nm)(V)(V)(eV)(eV)(eV)PTF54551.38−1.352.73−5.78−3.05PTF154701.31−1.332.64−5.71−3.07PTF304801.30−1.282.58−5.70−3.12PTF504901.22−1.312.53−5.62−3.09All oxidative potentials (Eox) of PFTs were measured by a CHI 660A
electrochemical workstation, using a platinum electrode as working electrode, a
saturated calomel electrode (SCE) as reference electrode, and a platinum wire
as assistant electrode in 0.1 mol/L of tetra-n-butylammonium tetrafluoroborate (Bu4NPF6)
in anhydrous acetonitrile as a supporting electrolyte with a scan rate of
50 mV/s. The reductive potentials (Ered), HOMO, and LOMO energy levels were
calculated by empirical equations (2)–(4) [12], respectively.The results in Table1 indicate that the band gaps
of these PFTs were less than or equal to 2.73 eV, which corresponds to
excitation by wavelengths at or above 455 nm. The reductive potentials were
approximately −1.30 V and were all more negative than the energy
level of the conduction band of  TiO2.
Thus, transfer of electrons from excited PFTs to the conduction band of TiO2 is thermodynamically possible. It was also
found that with increasing thiophene content in the main chain of the polymer,
the oxidative potential gradually decreased from 1.38 V (for PFT5) to 1.22 V (for
PFT50). These oxidative potentials are higher than those of many polymers, and
lead to stronger antioxidative ability.


### 3.2. Photodegradation of Phenol Using PFT-SensitizedTiO2

The photodegradation of phenol by PFT-sensitizedTiO2 is presented in Figure 3. During the
photodegradation experiment, the light intensity was E420=11.1×100 μw/cm2 and E365=1.4 μw/cm2.
The maximum absorption wavelength of phenol is about 270 nm and phenol has no
response for visible light. Our earlier experimental results showed that no
phenol decomposes under visible light irradiation. From Figure 3, all of the
PFTs were able to photosensitize TiO2.
The photocatalytic activity of sensitized TiO2 excited by visible light increased with
increasing thiophene content of the PFT. The relatively small increase in catalytic
activity between PFT30- and PFT50-sensitized TiO2 was attributed to two factors. First, addition
of 50% thiophene reduced the molecular weight of the PFTs from above 20 000 to
about 6000. As a result, there was less entwisting between PFT chains, so, the
polymer film was weaker and did not adsorb as well onto the TiO2 powder. Second, it was observed that PFT50 had
lower solubility in tetrahydrofuran (THF) than the other polymers, which
interfered with the physical process of combining it with the TiO2.
Thus, we used PFT30 as sensitizer in the following experiments.Figure 3
Photodegradation of phenol using PFT-sensitizedTiO2[phenol]i=10 mg/L,  [photocatalyst]=1 g/L, irradiation time =10 hours.Commonly, PF has a larger band gap than PFT, with
maximum absorption in the UV region [13, 14]. With increasing thiophene content in the main chain
of PF, its band gap decreases and its absorption spectrum is gradually
red-shifted. It was found that PFT with a higher thiophene content had more electron
transitions when excited by visible light, which resulted in greater
photocatalytic activity of PFT-sensitized TiO2.
Hara and coworkers [15] reported for the first time a new kind of sensitizers
based on introducing thiophene moieties into a coumarin backbone, and then
bonded it to the surface of TiO2 with COOH bonds. Using this new compound in
dye-sensitized solar cells, they achieved a remarkable solar
energy-to-electricity conversion efficiencies of up to 7.7% under simulated
solar irradiation (100mW⋅cm2), which was the highest efficiency obtained
among solar cells based on organic dye photosensitizers in the past decades.
Luo and coworkers [16], studying the properties of oligothiophene carboxylic
acid sensitized TiO2  films, found that oligothiophene carboxylic
acid could be used as a cosensitizer with N3 [Ru complex, Ru(II)L2(SCN)2, L=2,2′-bipyridyl-4,4′-dicarboxylate] dye, extending its spectrum
and improving its photoelectric properties. Thiophene has also been used to
improve the properties of porphyrin photosensitizers [17, 18]. Ha and coworkers [19] investigated singlet oxygen generation efficiencies
of porphyrin and thiophene-modified porphyrin using time-resolved and steady-state
spectroscopic methods. They found that the introduction of the sulfur atom from
thiophene into the porphyrin core greatly increased the intersystem crossing
rate due to spin-orbit coupling enhancement, resulting in enhanced triplet and
singlet oxygen quantum yields. These results suggested that thiophene can
improve the photophysical efficiency of photosensitizers. From the above, it is
clear that addition of thiophene directly affects the properties of combinative
photosensitizers.Additionally, PFTs showed higher stability than other
small molecular dyes such as rhodamine B and eosin Y under irradiation because
of the higher oxidative potentials. Reused after 10 hours irradiation, PFT30
still had efficient photosensitization toTiO2,
and the photocatalytic activity of PFT30-sensitized TiO2 slightly decreased less than 10% under the
same experiments conditions.


### 3.3. Effect of Photosensitization Method on Photodegradation of Phenol

In studies of organic pollution degradation using
dye-sensitizedTiO2,
the primary methods used for combining dye with TiO2 are simple adsorption [20, 21], chemical bonding [22], and LB (Langmuir-Blodgett)
film technology [23].
The combining of TiO2 with polymers depends primarily on adsorption
and wrapping by polymer long chains. The conjugated polymer PFTs used in this
paper have high solubility in aromatic solvents, begin to precipitate in
ethanol, and have very low solubility in water. Thus, in the combining method
that we designed, PFTs were first dissolved in THF, a good solvent, and then
the solution was added to ethanol, a worse solvent, in order to initiate a
gradual precipitation of PFT via wrapping of long chains. Finally, the solvents
were removed by a rotary evaporator (RE-100, QUICKFIT, England), leaving the
PFT coated on the surface of the TiO2.
The precipitation of PFT and its bonding to TiO2 are affected by the physical parameters of the
combining process, including the method of mixing the solvents, the mixing
time, and the method of adding the TiO2  powder. These parameters must be optimized in
order to avoid uneven deposition of PFT or premature agglomeration of TiO2,
both of which can adversely affect configuration and separateness of the
combinative photocatalyst and impair its photocatalytic activity. Figure 4
shows the results of an experiment in which the photodegradation of phenol was
catalyzed by PFT30-sensitized TiO2 prepared using four different methods. The
photodegradation experiments conditions were the same with Section
3.2 in this
paper.Figure 4
Photodegradation of phenol using PFT30-sensitizedTiO2 with different methods of photosensitization.
A: PFT30/THF solution added to TiO2/ethanol suspension quickly. B: TiO2/ethanol suspension added to PFT30/THF
solution quickly. C: PFT30/THF solution added to TiO2/ethanol suspension in two parts. D: PFT30/THF
solution added dropwise to TiO2/ethanol suspension.Figure4 indicates that addition of PFT30/THF
solution to a TiO2/ethanol suspension produced a more active
photocatalyst than addition of TiO2/ethanol suspension to the PFT30/THF solution.
The best results were obtained by dropwise addition of PFT30/THF solution to
the TiO2/ethanol suspension, which ensured that the
PFT precipitated slowly enough to evenly coat the surface of the TiO2.


## 3.1. Electrochemical Properties of PFTs

Having strong absorption in the visible light region
is the most important property of a photosensitizer. Figure2 shows the
UV-Vis spectra of the copolymers PF, PFT5, PFT15, PFT30, and PFT50 in
chloroform. These polymers all have strong absorption in the visible light
region. As expected, the intensity and maximum absorption peaks progressively
increased and were red-shifted with increasing content of thiophene in the
PFTs. The maximum absorption peaks are at 384 nm, 384 nm, 390 nm, 400 nm, and 433 nm,
respectively. From Figure 2 it is observed that the absorption peaks of PFT5
and PFT15 were little changed from that of PF, while their spectra were
slightly red-shifted toward the visible light region. The absorption peak of
PFT30 was red-shifted from 384 nm (PF) to 400 nm, and its absorption edge was
red-shifted to 480 nm. In the case of PFT50, the red shifting was more obvious;
the absorption peak and edge were red-shifted to 433 nm and 490 nm, respectively.
The increased red shifting that was associated with increased thiophene content
of the PFTs was due to strong absorption of visible light by the thiophene
units of the polymer. Compared with PF oligomers, the blocks of spatial
configuration between thiophene ring and benzene ring in PFTs are smaller than
those between benzene rings in PF, and twists between thiophene ring and
benzene ring take place more easily, leading to higher coplanarity and a larger
conjugated system in PFTs. In addition, the lone pair electrons of the sulfur
atom in thiophene units increase the electron cloud density in the PFT chain,
thereby increasing its ability to act as an electron donor and decreasing its
band gap. Thus, higher thiophene content of a PFT is associated with higher
coplanarity, lower band gap, and red shifting of its absorption spectrum.Figure 2
UV-Vis
spectra of the different copolymers.The second most important property of a
photosensitizer ofTiO2 is that its energy level matches that of  TiO2.
Only if the reductive potential of the photosensitizer is more negative than
the energy level of the conduction band of TiO2,
it is thermodynamically possible for excited electrons to be injected from the
conduction band of the photosensitizer to the conduction band of  TiO2.Table1 presents the wavelength at peak absorption
and electrochemical properties of the PFTs used in this study. The peak and
onset of absorption wavelength of PFTs were obtained from Figure 2, and the
band gaps (Eg) were estimated by
(1)
[11]:(1)Eg=1240/λonset,(2)Ered=(EgEox),(3)EHOMO=e(Eox+4.4),(eV)(4)ELUMO=e(Ered+4.4),(eV).Table 1
Wavelength at
peak absorption and electrochemical properties of PFTs.
CopolymersλabsmaxEoxEredEgEHOMOELUMO(nm)(V)(V)(eV)(eV)(eV)PTF54551.38−1.352.73−5.78−3.05PTF154701.31−1.332.64−5.71−3.07PTF304801.30−1.282.58−5.70−3.12PTF504901.22−1.312.53−5.62−3.09All oxidative potentials (Eox) of PFTs were measured by a CHI 660A
electrochemical workstation, using a platinum electrode as working electrode, a
saturated calomel electrode (SCE) as reference electrode, and a platinum wire
as assistant electrode in 0.1 mol/L of tetra-n-butylammonium tetrafluoroborate (Bu4NPF6)
in anhydrous acetonitrile as a supporting electrolyte with a scan rate of
50 mV/s. The reductive potentials (Ered), HOMO, and LOMO energy levels were
calculated by empirical equations (2)–(4) [12], respectively.The results in Table1 indicate that the band gaps
of these PFTs were less than or equal to 2.73 eV, which corresponds to
excitation by wavelengths at or above 455 nm. The reductive potentials were
approximately −1.30 V and were all more negative than the energy
level of the conduction band of  TiO2.
Thus, transfer of electrons from excited PFTs to the conduction band of TiO2 is thermodynamically possible. It was also
found that with increasing thiophene content in the main chain of the polymer,
the oxidative potential gradually decreased from 1.38 V (for PFT5) to 1.22 V (for
PFT50). These oxidative potentials are higher than those of many polymers, and
lead to stronger antioxidative ability.


## 3.2. Photodegradation of Phenol Using PFT-SensitizedTiO2

The photodegradation of phenol by PFT-sensitizedTiO2 is presented in Figure 3. During the
photodegradation experiment, the light intensity was E420=11.1×100 μw/cm2 and E365=1.4 μw/cm2.
The maximum absorption wavelength of phenol is about 270 nm and phenol has no
response for visible light. Our earlier experimental results showed that no
phenol decomposes under visible light irradiation. From Figure 3, all of the
PFTs were able to photosensitize TiO2.
The photocatalytic activity of sensitized TiO2 excited by visible light increased with
increasing thiophene content of the PFT. The relatively small increase in catalytic
activity between PFT30- and PFT50-sensitized TiO2 was attributed to two factors. First, addition
of 50% thiophene reduced the molecular weight of the PFTs from above 20 000 to
about 6000. As a result, there was less entwisting between PFT chains, so, the
polymer film was weaker and did not adsorb as well onto the TiO2 powder. Second, it was observed that PFT50 had
lower solubility in tetrahydrofuran (THF) than the other polymers, which
interfered with the physical process of combining it with the TiO2.
Thus, we used PFT30 as sensitizer in the following experiments.Figure 3
Photodegradation of phenol using PFT-sensitizedTiO2[phenol]i=10 mg/L,  [photocatalyst]=1 g/L, irradiation time =10 hours.Commonly, PF has a larger band gap than PFT, with
maximum absorption in the UV region [13, 14]. With increasing thiophene content in the main chain
of PF, its band gap decreases and its absorption spectrum is gradually
red-shifted. It was found that PFT with a higher thiophene content had more electron
transitions when excited by visible light, which resulted in greater
photocatalytic activity of PFT-sensitized TiO2.
Hara and coworkers [15] reported for the first time a new kind of sensitizers
based on introducing thiophene moieties into a coumarin backbone, and then
bonded it to the surface of TiO2 with COOH bonds. Using this new compound in
dye-sensitized solar cells, they achieved a remarkable solar
energy-to-electricity conversion efficiencies of up to 7.7% under simulated
solar irradiation (100mW⋅cm2), which was the highest efficiency obtained
among solar cells based on organic dye photosensitizers in the past decades.
Luo and coworkers [16], studying the properties of oligothiophene carboxylic
acid sensitized TiO2  films, found that oligothiophene carboxylic
acid could be used as a cosensitizer with N3 [Ru complex, Ru(II)L2(SCN)2, L=2,2′-bipyridyl-4,4′-dicarboxylate] dye, extending its spectrum
and improving its photoelectric properties. Thiophene has also been used to
improve the properties of porphyrin photosensitizers [17, 18]. Ha and coworkers [19] investigated singlet oxygen generation efficiencies
of porphyrin and thiophene-modified porphyrin using time-resolved and steady-state
spectroscopic methods. They found that the introduction of the sulfur atom from
thiophene into the porphyrin core greatly increased the intersystem crossing
rate due to spin-orbit coupling enhancement, resulting in enhanced triplet and
singlet oxygen quantum yields. These results suggested that thiophene can
improve the photophysical efficiency of photosensitizers. From the above, it is
clear that addition of thiophene directly affects the properties of combinative
photosensitizers.Additionally, PFTs showed higher stability than other
small molecular dyes such as rhodamine B and eosin Y under irradiation because
of the higher oxidative potentials. Reused after 10 hours irradiation, PFT30
still had efficient photosensitization toTiO2,
and the photocatalytic activity of PFT30-sensitized TiO2 slightly decreased less than 10% under the
same experiments conditions.


## 3.3. Effect of Photosensitization Method on Photodegradation of Phenol

In studies of organic pollution degradation using
dye-sensitizedTiO2,
the primary methods used for combining dye with TiO2 are simple adsorption [20, 21], chemical bonding [22], and LB (Langmuir-Blodgett)
film technology [23].
The combining of TiO2 with polymers depends primarily on adsorption
and wrapping by polymer long chains. The conjugated polymer PFTs used in this
paper have high solubility in aromatic solvents, begin to precipitate in
ethanol, and have very low solubility in water. Thus, in the combining method
that we designed, PFTs were first dissolved in THF, a good solvent, and then
the solution was added to ethanol, a worse solvent, in order to initiate a
gradual precipitation of PFT via wrapping of long chains. Finally, the solvents
were removed by a rotary evaporator (RE-100, QUICKFIT, England), leaving the
PFT coated on the surface of the TiO2.
The precipitation of PFT and its bonding to TiO2 are affected by the physical parameters of the
combining process, including the method of mixing the solvents, the mixing
time, and the method of adding the TiO2  powder. These parameters must be optimized in
order to avoid uneven deposition of PFT or premature agglomeration of TiO2,
both of which can adversely affect configuration and separateness of the
combinative photocatalyst and impair its photocatalytic activity. Figure 4
shows the results of an experiment in which the photodegradation of phenol was
catalyzed by PFT30-sensitized TiO2 prepared using four different methods. The
photodegradation experiments conditions were the same with Section
3.2 in this
paper.Figure 4
Photodegradation of phenol using PFT30-sensitizedTiO2 with different methods of photosensitization.
A: PFT30/THF solution added to TiO2/ethanol suspension quickly. B: TiO2/ethanol suspension added to PFT30/THF
solution quickly. C: PFT30/THF solution added to TiO2/ethanol suspension in two parts. D: PFT30/THF
solution added dropwise to TiO2/ethanol suspension.Figure4 indicates that addition of PFT30/THF
solution to a TiO2/ethanol suspension produced a more active
photocatalyst than addition of TiO2/ethanol suspension to the PFT30/THF solution.
The best results were obtained by dropwise addition of PFT30/THF solution to
the TiO2/ethanol suspension, which ensured that the
PFT precipitated slowly enough to evenly coat the surface of the TiO2.


## 4. Conclusion

Four copolymers, consisting of poly(fluorene-co-thiophene)
(PFT) with a range of thiophene content, were evaluated as photosensitizers ofTiO2.
All of the PFTs had strong absorption in the visible light region and had band
gaps less than or equal to 2.73 eV, which corresponds to excitation by
wavelengths above 455 nm. The reductive potentials of the PFT copolymers were
approximately −1.30 V, which was more negative than the energy
level of the TiO2 conduction band. Thus, transfer of electrons
from the excited PFTs to the conduction band of TiO2  is thermodynamically possible. All of the PFTs
were capable of photosensitizing TiO2. Photocatalytic activity of PFT-sensitized TiO2 increased with increasing thiophene content of
the PFT. The method of preparation of the combined PFT/TiO2 photocatalyst affected its photocatalytic
activity. The best method of photocatalyst preparation in our study was
dropwise addition of a solution of PFT in tetrahydrofuran to a stirred
suspension of TiO2 in ethanol.



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*Source: 164702-2007-11-28.xml*