E. Aslan, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry390(2020)112301
dyes [15–20]. Ionic and neutral D-π-A dyes have been studied on
photocatalytic hydrogen evolution and explicated that the photo-
catalytic activity of ionic type dye is higher than a neutral dye [21]. In
our previous study, we have also investigated the photocatalytic HER
activities of two different D‐π‐A dyes and explained that the difference
of photocatalytic HER is changed by additional electron donating
groups on the donor part, π bridge between donor and acceptor groups,
and also spacer groups [22–24]. The photochemical HER activities of D-
π-A dyes have also been investigated as a sensitizer for covalently
functionalized graphene/Pt and p-type photocathodes [25–29]. D‐π‐A
conjugated microporous polymers were used on the photocatalytic HER
as an organic photocatalyst under UV–Vis light irradiation (λ > 300
nm) [30]. The photovoltaic performance and photocatalytic HER ac-
tivities of two D‐A‐π‐A dyes have been researched covered by limited
wavelength frame (780 nm > λ > 420 nm), which are showed both
high photoelectric conversion efficiency and photocatalytic HER ac-
tivity because of high molar extinction coefficient of the dyes [31]. The
photocatalytic HER by dye sensitized TiO2 photocatalyst have been
usually performed in the presence of co-catalyst to host active sites,
enhance photogenerated charge transport yield and stability of reac-
tions, reduce photocorrosion and charge recombination rates. Platinum
group metals (PGMs) like Pt, Pd, etc. are usually operated as a co-cat-
alyst for photocatalytic HER [9]. Hydrogenase-like metal sulfides such
as molybdenum/tungsten sulfides co-catalysts are promising candidate
as an alternative to PGMs [32,33]. The catalytic activities on HER of
metal sulfide co-catalysts could be increased by alloyed/doped struc-
tures on HER studies [22,34,35].
photoelectrodes were allowed to cool down and immersed in a glass
bottle include dye solution (10-2 mM in THF) for a day. Prepared
photoelectrodes washed out with THF and ethanol in order to eliminate
unbinding molecules of dye. Finally, dye sensitized TiO2 photoelec-
trodes and photocatalyst were dried at room conditions.
2.2. Photochemical HER experiments
The hydrogen production from photoelectrochemical process was
achieved by using conventional three-electrode cell, equipped with dye
sensitized TiO2-coated FTO (Dyesol MS 001630), Ag/AgCl and pla-
tinum wires used as a working electrode, reference and counter elec-
trodes, respectively. Two types electrochemical techniques were ap-
plied in the photoelectrochemical HER experiments, which is
chronoamperometry (CA) and linear sweep voltammetry (LSV) tech-
niques. The LSV was used to investigate the stability of dyes with a
potential range between -0.6 V and 0.6 V that has shown good stability
and the CA was used for determination of dye stability with a potential
of 0 V for 350 s light on/off periods of 50 s each. Photocatalytic activity
experiments were performed in n a Pyrex flask (135 ml) under the
visible light (Solar Light XPS-300™, λ ≥ 420 nm). (MK-2 or MK-8) /
TiO2 hybrid photocatalysts (10 mg), with and without Cu2WS4 or Pt co-
catalysts, and TEOA (0.33 M) sacrificial agent were added into the flask
under N2 atmosphere. After that the flask was sealed with silicon
septum prior to the photocatalytic HER. Ultrasonic treatment was ap-
plied to the solution to supply the uniform distribution of photo/cata-
lysts. Eventually, hydrogen production is evaluated by Shimadzu GC-
2010 Plus GC.
In this study, we have synthesized two novel photosensitizer D-π-A
organic dyes, which are named MK-2 and MK-8, to sensitize of TiO2 for
the absorption of visible light. The chemical structures of synthesized D-
π-A organic dyes have been explored by FT-IR, NMR and HRMS tech-
niques; optical and electrochemical properties have been clarified by
absorption/photoluminescence spectroscopies, and cyclic voltammetry
methods, respectively. The MK-2/MK-8 sensitized TiO2 have been used
first time in the literature as the visible light-driven photocatalysts in
3. Results and discussion
3,6-bis [4-methylthien-2-yl]-stetrazine (1) molecule was prepared
according to reported methods [36,37]. 4-[5-{6-[5-Bromo-4-(2-ethyl-
hexyll)-2-thienyl]-1,2,4,5-tetrazin-3-yl}-3-(2-ethylhexyl)-2-thienyl
-N,N-bis[4-(hexyloxy)phenyl]aniline (3), methyl-4[5-{6-[5-(4-{bis[4-
(hexyloxy)phenyl]amino}phenyl)-4-(2-ethylhexyll)-2-thienyl]-1,2,4,5-
tetrazin-3-yl)-3-(2-ethylhexyl-2-thienyl]benzoate (4) and 4-[5-{6-[5-(4-
{bis[4-(hexyloxy)phenyl]amino}phenyl)-4-(2-ethylhexyl)-2-thienyl]-
1,2,4,5-tetrazin-3-yl}-3-(2-ethylhexyl)-2-thienyl] benzaldehyde (5)
were synthesized by Suzuki-Miyaura reaction. Molecule (5) was con-
verted by Knoevenagel condensation reaction into MK-8 dye. The de-
tailed synthesis steps of MK-2 and MK-8 are given in the SI. Structural
characterizations of MK-2 and MK-8 are carried out by using NMR, FT-
IR and HRMS techniques (see SI). The synthetic pathways of dyes are
the
photochemical
hydrogen
evolution
reaction
(HER).
Photoelectrochemical features of the dye sensitized electrodes have
been investigated in the aqueous TEOA/Na2SO4 solution by linear
sweep voltammetry (LSV) and chronoamperometry (CA) techniques.
There are no changing transient photocurrent densities by dye/TiO2
electrodes during the light on/off cycles in the photoelectrochemical
experiments, which is indicated the stabilities of MK-2/MK-8 sensitized
TiO2 electrodes. Photocatalytic hydrogen evolution experiments have
been figured out in the aqueous sacrificial electron donor TEOA solu-
tion. In addition, ternary metal sulfide Cu2WS4 has been used as the co-
catalyst in the photocatalytic HER and its catalytic activity has been
compared to Pt. The photocatalytic HER by MK-2/MK-8 sensitized TiO2
shows good stabilities due to linearly increasing hydrogen evolution
during the photocatalytic reactions. Rates of HER have been explained
by molecular variation of D-π-A organic dyes. Finally, the hydrogen
evolution mechanism has been stated by energy levels of each con-
stituent, which determined electrochemically.
Binding ratios between dyes (MK-2 or MK-8) and TiO2 have been
investigated by using energy dispersive x-ray spectroscopy (EDX).
According to elemental analysis data obtained from EDX spectra (see SI,
Figure S1), N/Ti ratios have been calculated for the MK-2/TiO2, and
MK-8/TiO2 due to the common elements in the MK-2/MK-8 dyes sen-
sitized TiO2. N/Ti ratios were found out 2.96 × 10−3 and 4.01 × 10−3
for the MK-2/TiO2 and MK-8/TiO2, respectively (Table 1). These results
show that aggregate formation is figured out in the MK-8 on the surface
of TiO2 [38–41]. The effect of aggregate formation has been discussed
in the results of photoelectrochemical and photocatalytic HER.
2. Experimental Section
2.1. Dye sensitization process
UV-Vis absorption spectra have been carried out by using 10−5
M
dye solution in THF as shown in Fig. 1a. The sharp absorption peaks of
dyes are seen in 349 nm and 296 nm for MK-2; 343 nm and 298 nm for
MK-8. The first peaks about 300 nm and second peaks about 350 nm are
assigned to the localized π-π* transition and delocalized π-π* transi-
tion, respectively [12]. In comparison with MK-8, the maximum ab-
sorption wavelength of MK-2 shows a red-shift. This can be explained
by the addition of a cyano group, which increases the electron with-
drawing ability and decrease the electron density in MK-8 dye [42].
Molar extinction coefficients have been estimated MK-2 and MK-8 as
915 M-1 cm-1 (349 nm) and 2938 M-1 cm-1 (343) by Beer–Lambert Law
In a dye-sensitized system, to prepare the photocatalytic sample,
TiO2 (Degussa P25) powder was annealed in a furnace at 450 °C for 45
min in order to eliminate of some organic impurities and adsorbed
water. Followed by annealing step, calcined TiO2 was taken out; pre-
pared donor-π-acceptor dye species (10−5 M in THF) was added to the
TiO2 and magnetically stirred under dark conditions. After the TiO2
coating with dye, the filtered-out solution was typically washed on the
filter with THF and ethanol, respectively, for three times. TiO2 coated
FTO electrodes were sintered (450 °C for 1 h). Then, TiO2 coated
2
Aslan, Emre
