Polymer dye-containing nanocomposites as photocatalysts

Article abstract of DOI:10.1080/15421406.2016.1255527

A significant improvement of photocatalytic efficiency is achieved by the synthesis of TiO₂/SiO₂ nanocomposites where silica matrix provides the transport of reagents to TiO₂ nanoparticles via porous structure, generation of the new active sites and thermal stability. The semiconductive films contained 10 or 30% of TiO₂ in silica matrix are synthesized by sol-gel method using concentrated anatase titania colloid. The complex composites consisted of i) the Acridine Yellow dye molecules and the polyepoxypropyl carbazole; ii) the azobenzene containing polymer covered onto the titania/silica films are obtained as photocatalytic materials. Photocatalytic activity of the composites is tested via the reduction of dichromate ions.

Full text of DOI:10.1080/15421406.2016.1255527

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Polymer dye-containing nanocomposites as
photocatalysts
Oksana Linnik & Oksana Nadtoka
To cite this article: Oksana Linnik & Oksana Nadtoka (2017) Polymer dye-containing
nanocomposites as photocatalysts, Molecular Crystals and Liquid Crystals, 642:1, 106-114, DOI:
10.1080/15421406.2016.1255527
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1255527
Published online: 08 Feb 2017.
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Date: 26 February 2017, At: 00:23

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Polymer dye-containing nanocomposites as photocatalysts
Oksana Linnika and Oksana Nadtokab
aChuiko Institute of Surface Chemistry, National Academy of Science of Ukraine, Kyiv, Ukraine; bTaras
Shevchenko National University of Kyiv, Kyiv, Ukraine
ABSTRACT
KEYWORDS
photocatalysis;
semiconductivity;
polyepoxypropyl carbazole;
azopolymer; dyes
A significant improvement of photocatalytic efficiency is achieved by
the synthesis of TiO2/SiO2 nanocomposites where silica matrix provides
the transport of reagents to TiO2 nanoparticles via porous structure,
generation of the new active sites and thermal stability. The semicon-
ductive films contained 10 or 30% of TiO2 in silica matrix are synthesized
by sol-gel method using concentrated anatase titania colloid. The com-
plex composites consisted of i) the Acridine Yellow dye molecules and
the polyepoxypropyl carbazole; ii) the azobenzene containing polymer
coveredontothetitania/silicafilmsareobtainedasphotocatalyticmate-
rials. Photocatalytic activity of the composites is tested via the reduction
of dichromate ions.
1. Introduction
The significance of the wastewater treatment, management and its disposal gradually
increases in the modern times and it becomes a major concern for public health scientific
interest. The low-cost and non-toxic semiconductor composites based on titania are perspec-
tive materials for solar energy utilization in the processes of soil and water purification as
well as photovoltaic materials. Titanium dioxide (TiO2), as an important semiconductor, has
been widely investigated in the photocatalytic field, due to its abnormal chemical and phys-
ical behavior. Interaction of the semiconductor particles with quantum of light leads to the
appearance of the active radicals that are able to the simultaneous oxidation and reduction
reactions. In our previously works we have showed the high catalytic activity of synthesized
mesoporous titania thin films in various redox processes [1]. Titania possesses optical prop-
erties at UV light trapping which are corresponded to its band energy. A significant improve-
ment of photocatalytic efficiency is achieved by the synthesis of TiO2/SiO2 nanocomposites
where silica matrix provides the transport of reagents to TiO2 nanoparticles via porous struc-
ture, generation of the new active sites and improved mechanical strength, thermal stability
and surface area of titania [2]. On the other hand, the high charge recombination rate in TiO2
considerably limits its photocatalytic application [3].
In recent times, some studies have been published on the combination of conductive poly-
mers and TiO2 to get better performance of UV light and sunlight activities [4–7]. Among
the conducting polymers polycarbazole (PCz) is attributed with good electroactivity, thermal,
CONTACT Oksana Nadtoka
oksananadtoka@ukr.net
This paper was originally submitted to Molecular Crystals and Liquid Crystals, Volumes –, Proceedings of the th
International Conference on Electronic Processes in Organic and Inorganic Materials (ICEPOM-).
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
107
electrical and photophysical properties [8]. The increasing interest in PCz is determined by
its role as a hole-transport material and an efficient photoluminescence unit upon visible light
excitation. Therefore, conjugated polymer PCz with large band gap inorganic semiconductors
is used for optical, electronic, photocatalytic and photoelectric conversion applications.
Different derivatives of azobenzene show the best advantage as photoactive fragments [9].
Illumination of these materials with exciting light stimulates reversible or irreversible trans-
cis isomerization of azochromophores. The role of the various excited states of azobenzene
in photoisomerization reaction has attached close attention. Excitation of the n-π∗ bands of
cis- and trans- azobenzene resulted in isomerization with quantum yields of 0.70 and 0.20,
respectively. However, π-π∗ excitation is also accompanied by isomerization, and this state
could be also involved. In the latter case, quantum yields of 0.44 and 0.10 are observed for the
cis- and trans-isomers respectively [10]. To create photocatalytic semiconductor dye-polymer
films the methacrylic azobenzene containing polymers were used too.
2. Experimental section
2.1. Synthesis and characterization of materials
Unless otherwise specified the reagents were obtained from Aldrich and used as received
without further treatment. Carbazole (analytical grade) was used after recrystallization. Poly-
(N-epoxypropyl carbazole) was synthesized by the reaction N-epoxypropyl carbazole with
anhydrous N, N-dimethylformamide and further anion polymerization according to the lit-
erature [11]. Methacrylic azopolymer were synthesized by free-radical copolymerization of
methacrylic azomonomer and methyl methacrylate.
Synthesis of N-epoxypropyl carbazole (EPC) Carbazole (0.167 g, 1 mmol), sodium methox-
ide (0.16 g, 3 mmol) and 23 mL of anhydrous N, N-dimethylformamide were added to a
three necked flask. The solution was heated to 40°C and dropwise added epichlorohydrin
(1.28 g, 13 mmol), then stirred for 1 h and was allowed to cool to room temperature. After
removal of the solvent, the remaining crude product was isolated by flash chromatography
(silica gel, hexane/dichloromethane: 5/1) to isolate 1 as a white solid in 53% yield. 1H NMR
(CDCl3, 400 MHz) δ (ppm) 8.12 (d, 2H); 7.48 (d, 4H); 7.24 (t, 2H); 4.66 (d, 1H); 4.40 (d, 1H);
3.35 (d, 1H); 2.81 (d, J = 4.0 Hz, 1H); 2.58 (t, J = 4.8 Hz, 1H).
Synthesis of poly-(N-epoxypropyl carbazole) (PEPC). Potassium hydroxide (0.0176 g,
0.3 mmol), 18-crown-6 (0.1170 g, 0.4 mmol) and 8 mL methanol were added to a flask. When
the solid dissolved, 8 mL toluene was added. The mixture was heated in vacuo until the vol-
ume of the distillate became half that of the volume of the original solution. Then a new por-
tion of toluene was added and distilled again. This procedure was repeated five times. The
solution was then cooled down to room temperature and filtered. Polymerization of EPC was
carried out in a flask equipped with nitrogen purge. 8 mL of the mentioned solution and EPC
(0.8741 g) were added to the flask. The solution was heated at 90°C for 12 h. The resulting
polymers were isolated by precipitation in hexane or in methanol and dried in vacuo at ambi-
ent temperature.
Synthesis of methacrylic azopolymer (AzoP). Copolymers of 4•−methacryloxyhexyloxy-4-
nitroazobenzene and methyl methacrylate were synthesized by free-radical polymerization
in toluene. The polymerization was carried out in 10 wt.% toluene solution of monomer with
AIBN as free radical initiator (10 wt.% of monomer) at 80°C over more than 30 hours. Poly-
mers were isolated from the reaction solution by precipitation into excess of methanol fol-
lowed by reprecipitation from toluene into methanol and then dried at 20°C overnight. The

108
O. LINNIK AND O. NADTOKA
Figure . Structural formulas of dye (AY), semiconductor polymer (PEPC) and Azopolymer (AzoP).
synthetic work is described in more details in a separate paper [12]. The synthesized azopoly-
mers were characterized by 1H NMR spectroscopy. The obtained results were in agreement
with the proposed structures.
Tetraethoxysilane Si(OC2H5)4, aqueous solution of hydrochloric acid, de-ionized water,
Pluronic P123 (BASF) were used to prepare silica sols. To control hydrolysis-condensation
reaction rates and to prevent oxides precipitation, acetylacetone was used as the complexing
agent. Silica-titania films (10 and 30 wt% TiO2) were obtained via addition of TiO2 in anatase
crystalline form (6–7 nm) [13] to SiO2 sol. Dip-coating procedure was used for film-forming
(withdrawal rate was 1.5 mm/s). The clean and dried glass substrates were used for this target.
Thermal treatment of films was performed at 350°C.
Acridine Yellow (AY) (1×10–6 mol/L) was applied onto the films for 24 hours for adsorp-
tion. After film drying on the air, the polyepoxypropylcarbazole (PEPC) (5% PEPC in ben-
zene) was twice covered onto adsorbed AY. The composites were left for 48 hours at the
room temperature. The obtained composites are signed as x%TiO2/SiO2//AY//PEPC. They
consist of 10 or 30 wt% of titania in silica mesoporous thin films, AY dye and PEPC
layers.
Methacrylic azopolymer (AzoP) was dissolved in dichloroethane up to the final concen-
tration of 1.5 w/w %. Obtained solution was used for dip-coating onto freshly synthesized
films. The systems were dried on the air for 48 hours. The synthesized composites are signed
as x%TiO2/SiO2//AzoP where x means the amount of titania in the silica matrix (10 or 30%).
The chemical structure of used substances is presented on Figure 1.
The composite film was immersed in 40 ml of an aqueous solution of potassium dichromate
(the initial concentration of dichromate ions was 2×10−4 M). The reducing agent (disodium
salt of ethylenediaminetetraacetic acid (Na2EDTA)) in the molar ratio 1:1 was adjusted to
pHࣙ2 by perchloric acid.
2.2. Observation and measurements
The XRD spectrum of TiO2 sol was recorded with DRON-4-07 diffractometer (CuKα radi-
ation). Calculation of apparent crystallite size for titania has been performed by Debye-
Scherrer formula β (2θ) = 0.94 λ/(Dcosθ), using (1 0 1) reflections employing the FWHM
procedure.
Optical spectra of the composites were recorded with a double beam spectrophotometer
(Lambda 35, PerkinElmer) within the wavelength range of (190–1200) nm.
The Cr(VI) ions concentration change was monitored by a Lambda 35 UV-Vis spectropho-
tometer (Lambda 35, PerkinElmer) every 20 min at λ = 350 nm.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
109
Figure . Absorption spectra of % TiO/SiO//PEPC () and % TiO/SiO//AY//PEPC () composites.
The Brunauer–Emmett–Teller (BET) surface area and pore size distribution of the thin
films were measured from hexane adsorption–desorption isotherms. Hydrophilic properties
of films were estimated by measurements of water contact angle using the sessile drop method
with a MIR-1 microscope (LOMO, St. Peterburg, Russia). Measurements were carried out at
room temperature (20 2°C).
3. Results and discussion
In the applied science, the photocatalysts in the film form are more convenient (suitable) com-
paring to the powders due to the avoidance of the technical stage such as the sedimentation
and/or ultrafiltration of the solid form from liquid phase.
Analyzing FTIR and XPS data of TiO2, SiO2 and TiO2/SiO2 we have concluded [14] that
molecular scale mixing for sol–gel derived titanosilicates with formation of Ti–O–Si bonds is a
key factor affecting surface properties such as level of surface hydroxylation and surface acid-
ity as well as the catalytic function of silica–titania. Freshly prepared TiO2/SiO2 films showed
high hydrophylic properties with the water contact angles being ca. 12 before and 5° after
UV irradiation. In addition, the enhanced adsorption of AY on SiO2/TiO2 film (1.5–2 time
higher) in comparison with that on the parent oxide: SiO2 or TiO2 film surfaces, coincides
with high acidity, surface area and hydrophilicity of synthesized films. Thus, strong adsorp-
tion of AY on specific surface sites of SiO2/TiO2 films is responsible for its high efficiency of
light absorption and its stability.
The semiconductor//dye//polymer composites were tested for the stability in several sol-
vents (water, ethanol/water mixtures and pure ethanol). No penetration of AY molecules
through PEPC layer in aqueous media was noted for the films with the amount of adsorbed AY.
The appeared maximum at 435 nm is signed to the adsorbed AY molecules onto TiO2/SiO2
surface (Fig. 2).
AzoP covered onto TiO2/SiO2 films influences on the red shift in the absorption region
of the composites (Fig. 3.) where two-layered films exhibit an optimum absorption shift (all
reported results are related to two-layered systems only). At the same time as shown on
Figure 3, no significant change in the absorption spectra is observed for the systems contained
10 and 30% of titania.
Photocatalytic activity of the films was assessed via tetracycline hydrochloride (TC) degra-
dation and dichromate ions reduction. TC is one of the most frequently prescribed groups
of antibiotics. Residues of TC and their metabolites were detected in eggs, meat and ani-
mals based on different exposure methods [15]. Their accumulation in human organism

110
O. LINNIK AND O. NADTOKA
Figure . Absorption spectra of cut-off-filter (bold curve), % TiO/SiO (), % TiO/SiO (), %
TiO/SiO//AzoP () and % TiO/SiO//AzoP () films.
can produce arthropathy, nephropathy, central nervous system alterations, spermatogenesis
anomalies, possible mutagenesis and photosensitivity in human beings. In turn, chromium
containing ions are recognized as a pollutant derived from the chrome-plating and electronic
industries.
When visible light was applied, the conversion percentage was increased slightly for 10%
TiO2/SiO2//AY//PEPC and doubled for 30% TiO2/SiO2//AY//PEPC comparing with the AY
free composites in dichromate reduction process (Fig. 4 A). The efficiency of the photocata-
lysts was enhanced in 2 and 4 times for 10% and 30% in TiO2/SiO2//AY//PEPC, respectively
during TC destruction (Fig. 4 B). It must be noted that the AY free composites were completely
Figure . Photocatalytic activity of the composites tested in the reduction of dichromate ions (A and
C) and degradation of TC (B and D):  − % TiO/SiO//PEPC,  −% TiO/SiO//AY//PEPC,  − %
TiO/SiO//PEPC,  − % TiO/SiO//AY//PEPC.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
111
Figure . The proposed mechanism of dichromate reduction (pathways ., ., . and .) and TC destruc-
tion(pathways.,.,.and.)undervisible(blue)andUV(red)lightoverTiO/SiO//AY//PEPCcomposite.
inert under visible light (Fig. 4 A and B, columns 1 and 3). Upon visible light, the exciting of
AY molecule takes place with the following charge carrier transfer from the excited (AY∗)
molecules to the CB of TiO2 known as dye-sensitization process (Eq. 1, 2 and Fig. 5, pathway
1.1). Further steps should implicate the reduction of the oxidized species (AY•+) by electron
donor such as EDTA or TC (Eq. 3; Fig. 5, pathways 1.3 and 1.5) and processes involving elec-
trons from the CB as namely dichromate or oxygen reduction (Eq. 5 and 7; Fig. 5, pathways
1.2 and 1.4).
When light with energy higher than 3.2 eV interacts with titania particles, an electron tran-
sition from the valence band (VB) of semiconductor to the conduction band (CB) occurs
(Eq. 4). As the photocatalytic actyvity of AY free TiO2/SiO2//PEPC enhances comparing with
the blank experiment and depends on TiO2 amount for both processes (Fig. 4 C, D columns
1, 3), it leads to the conclusion that the photogenerated electrons became to be trapped and/or
directly move through PEPC layer to the redox couple.
Dye − sensitization process
AY + hν (≤3.2eV) → AY∗ + e− → AY•+
(1)
(2)
(3)
ꢀ
ꢁ
TiO2 + e− → TiIVO2/Ti3+ e−
,
CB
AY•+ + TC/EDTA → AY + TC•+/EDTA•+
Direct photocatalysis
ꢀ
ꢁ
TiO2 + hν (≥3.2eV) → TiO2 e−CB + h+
(4)
VB
Photocatalytic reduction of dichromateions
Cr2O72− + 14H+ + 6e−
→ 2Cr3+ + 7H2O
(5)
(6)
CB/trapped
RCOOH (EDTA) + h+
→ RCOO• + H+ (H2O) → CO2 + R•
VB/trapped
Photocatalytic destruction of TC
O2 + e−
→ O2 → H2O2 + H+
(7)
(8)
•−
CB/trapped
TC + h+
→ TC•+ → CO2 + NH3 + H2O
VB/trapped

112
O. LINNIK AND O. NADTOKA
Figure . Absorption spectra of % TiO/SiO//AY//PEPC composites:  − before irradiation;  − after
 min of visible light exposure;  − after  min of full light irradiation.
Almost no influence on the photocatalytic performance under UV light was observed for
TiO2/SiO2//AY//PEPC. Even the decrease in conversion percentage of dichromate reduction
is noted (Fig. 4 C, column 4). While the TC conversion is slightly raised or unchanged for
TiO2/SiO2//AY//PEPC (Fig. 4 D, columns 2 and 4).
Figure 6 shows the change in the absorption spectra of 30% TiO2/SiO2//AY//PEPC com-
posites after light exposure. The intensity of adsorbed AY molecules is obviously lowered as
a result of AY destruction by UV light. It is assumed that the photoformed electrons in CB of
semiconductor are involved in the reaction with new formed AY residues making the path-
ways 2.1 impossible. However, the destruction process with the holes participation follows
Figure . Photocatalytic activity of the composites tested in the reduction of dichromate ions: a − %
TiO/SiO//PEPC (), % TiO/SiO//AY//PEPC (), % TiO/SiO//PEPC (), % TiO/SiO//AY//PEPC (); b
− % TiO/SiO (), % TiO/SiO//AzoP (), % TiO/SiO (), % TiO/SiO//AzoP ().

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
113
freely (Fig. 4 D columns 2 and 4; Fig. 5, pathways 2.2). Based on this, we assumed the fate of
the pathways 2.2 and 2.3 (Fig. 5).
The TiO2/SiO2//AzoP composites possessed much lower photocatalytic activity than
TiO2/SiO2 systems. It can be caused by insufficient electron transferring from the semicon-
ductor through the polymer layer to the liquid phase when UV or visible light is applied
(Fig. 7b). The azobenzene fragments broaden the absorption region of the photocatalytic
systems, however they may be subject to photodegradation [16] and perform as electron-
traps. In this case the photocatalysts should be modified by semiconducting polymers such
as PCz or others. So the regularities established may be used for formulation of the gen-
eral rules, which could allow predict and improve the photocatalytic activity of TiO2/SiO2
composites.
Conclusions
Hence, the complex composites based on the mesoporous TiO2/SiO2 thin films with adsorbed
AY dye and PEPC as well as azopolymer are designed. The double PEPC covering pre-
vents the penetration of AY molecules to the liquid phase when aqueous solution is
applied.
The presence of acidic surface sites in TiO2/SiO2 thin films, amount of adsorbed AY as
well the number of PEPC layers have different influence on the stability of the composites.
The TiO2/SiO2//AY//PEPC films can be used as the new type of photocatalytic systems in
Ecological Photocatalysis as the significant increase in the photoactivity under visible light
for reduction process is reported.
It was shown that TiO2/SiO2//AzoP composites possess much lower photocat-
alytic activity comparing with TiO2/SiO2//AY//PEPC. However, the further investi-
gation has to be done to find out the appropriate synthesis conditions to improve
their photocatalytic activity and stability to the more aggressive media as well as
to find out the new opportunities for the photochromic materials with azobenzene
moieties.
References
[1] Linnik, O., Zhukovskiy, M., Starukh, G. et al. (2015). J. Appl. Spectrosc., 81(6), 990.
[2] Kumar, D., Xavier, J., Shyla, J. et al. (2013). J. Mater. Sci., 48(10), 3700.
[3] Ochiai, T., & Fujishima, A. (2012). J. Photochem. Photobiol. C: Photochem. Rev., 13, 247.
[4] Muthirulan, P., Kannan Nirmala Devi, C., & Meenakshi Sundaram, M. (2013). J. Environ. Chem.
Eng., 1, 620.
[5] Hou, J., Cao, R., Zhu, H., & Kumar, R. V., (2011). Appl. Catal. B: Environ., 104, 399.
[6] Xu, S., Gu, L., Yang, K.H., Song, Y., Jiang, L., & Dan, Y. (2012). Sol. Energy Mater. Sol. Cells, 96,
286.
[7] Li, X. Y., Wang, D. S., Cheng, G. X., Luo, Q. Z., An, J., & Wang, Y. H. (2008). Appl. Catal. B. Environ.,
81, 267.
[8] Pavlova-Verevkina, O., Politova, E., & Nazarov, V. (1999). Colloid. J., 61, 359.
[9] Yaroshchuk, O., Bidna, T., Nadtoka, O. et al. (2005). Mol. Cryst. Liq. Cryst., 437, 1377.
[10] Taudi, H., Bernede, J., Valle, D. et al. (2001). J. Mater. Sci., 36(3), 631.
[11] Qin, L., Zhang, Sh., Xu, J., Lu, B., Duan, X., Zhu, D., & Huang, Y. (2013). Int. J. Electrochem. Sci.,
8, 5299.
[12] Nadtoka, O., Syromyatnikov, V., & Olkhovykh, L. (2005). Mol. Cryst. Liq. Cryst., 427, 259.

114
O. LINNIK AND O. NADTOKA
[13] Gregg, S. J., & Sing, K. S. W. (1982). Adsorption, surface area and porosity. Academic Press: London,
New York. 303.
[14] Andrulevicˇius, M., Tamulevicˇius, S., Gnatyuk, Yu., Vityuk, N., Smirnova, N., & Eremenko, A.
(2008). Mater Sci. (Medzˇiagotyra), 14, 8.
[15] Zurhelle, G., Muller-Seitz, E., & Petz, M. J. (2000). Chromatogr. B: Biomed. Sci. Appl., 739(1), 191.
[16] Zhiyong, Yu., Laub, D., & Bensimon, M., Kiwi, J. (2008). Inorganica Chimica Acta, 361(3), 589.