Donor-p-acceptor dye-sensitized
photoelectrochemical and photocatalytic hydrogen
evolution by using Cu2WS4
co-catalyst
Imren Hatay Patir
a,*, Emre Aslan
b, Gizem Yanalak
c, Merve Karaman
d,
Adem Sarilmaz
e, Mumin Can
e, Mustafa Can
d, Faruk Ozel
e,faSelcuk University, Department of Biotechnology, 42030, Konya, Turkey b
Selcuk University, Department of Chemistry, 42030, Konya, Turkey
cSelcuk University, Department of Biochemistry, 42030, Konya, Turkey
dIzmir Katip Celebi University, Faculty of Engineering and Architecture, Department of Engineering Sciences, Cigli,
35620 Izmir, Turkey
eKaramanoglu Mehmetbey University, Department of Metallurgical and Materials Engineering, 70200, Karaman,
Turkey
fKaramanoglu Mehmetbey University, Scientific and Technological Research and Application Center, 70200,
Karaman, Turkey
a r t i c l e i n f o
Article history:
Received 24 September 2018 Received in revised form 22 October 2018
Accepted 21 November 2018 Available online 15 December 2018 Keywords:
Hydrogen evolution Dye sensitization Donor-p-acceptor dyes
a b s t r a c t
Photoelectrochemical and photocatalytic hydrogen evolution reaction (HER) have been investigated by using metal free donor-acceptor (D-A) and donor-p-acceptor (D-p-A) dyes, which are abbreviated as MC-32 and MC-048, respectively, sensitized TiO2as a
photo-catalyst with or without Cu2WS4co-catalyst. This co-catalyst is synthesized by a low-cost
and simple hot injection method, under visible light illumination. The photoactivities of these dyes have been clarified according to their structural, optical and electrochemical properties. Photocatalytic activities have been slightly increased when added the Cu2WS4
co-catalyst (dye/TiO2/Cu2WS4). This catalytic activity is also compared to that of noble
metal Pt (dye/TiO2/Pt). It has been found that 121 mmolg1h1, 179 mmolg1h1,
348mmolg1h1, 212mmolg1h1, 422mmolg1h1and 1139mmolg1h1hydrogen have been
evolved by using MC-32/TiO2, MC-32/TiO2/Cu2WS4, MC-32/TiO2/Pt, MC-048/TiO2, MC-048/
TiO2/Cu2WS4and MC-048/TiO2/Pt, respectively.
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
Hydrogen is one of the promising renewable fuels due to the high energy density, reduced greenhouse gas emissions and depletion of fossil-fuel. Photocatalytic and photo-electrochemical hydrogen evolution reaction (HER) have
recently been attracted to solve the problem of solar energy storage. These processes have been generally carried out by using semiconductor oxides as photocatalysts due to the efficient way for the HER. TiO2,which is well-known
photo-catalyst for the HER, was used for the first time by Fujishima and Honda for the water splitting reaction under UV light illumination[1]. The obstacles of the TiO2are that it absorbs
* Corresponding author.
E-mail address:imrenhatay@gmail.com(I.H. Patir).
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https://doi.org/10.1016/j.ijhydene.2018.11.161
small percentages of solar spectrum due to the wide band gap (approximately 3.2 eV) and displays high recombination rates, which coupled of separated exited state electron and hole[2]. Photocatalytic activity of wide band gap TiO2 could be
comprehensively enhanced by dye sensitization process to absorb the visible light and adding co-catalyst to decrease recombination rates[3]. Xanthane type dye molecules have been generally used as the sensitizers in dye sensitization process for the photocatalytic and photoelectrochemical HER [4e10]. Recently, donor-p-acceptor (D-p-A) dyes have been drawn interest in dye sensitized photochemical (photo-catalytic and photoelectrochemical) hydrogen evolution sys-tems and dye sensitized solar cells (DSSC) for the visible light sensitization because of their intramolecular charge transfer and adjustable absorption properties[11e14]. Donor groups of these dyes have been generally included carbazole, coumarin, phenothiazine, merocyanine, triphenylamine, and their de-rivatives [11,15,16]. The research group of Kang has been investigated loaded dye amount, steric and hydrophilic effect of triphenylamine based D-p-A dyes on photocatalytic HER system by using TiO2/Pt as a photocatalyst [17,18]. The
amount of anchoring groups in the triphenylamine based D- p-A dyes have been explored for the dye sensitized photo-catalytic HER, environmental remediation[19,20]and photo-electrochemical HER[21]. Tiwari et al. have been designed the series of phenothiazine based donoreacceptor type organic dyes and used for the sensitization of TiO2 photocatalyst
to evolve hydrogen under solar light in the neutral media. The enhancement of photocatalytic activities of these dyes are explained by increasing the different substituents of methine chain on the nitrogen of the thiophenothiazine ring [22,23]. The photocatalytic HER from water by using two donoreacceptorepeacceptor (D-A-p-A) organic dyes have been studied under visible light illumination (420 nm<l < 780 nm), which are displayed vigorous photo-catalytic HER activity with methanol as an electron donor for 10 h[24]. Konieczna et al. have been investigated the pyr-idinium based ionic D-p-A dyes on photocatalytic HER and reported that ionic dyes shows more catalytic activities than neutral dyes[25]. Watanabe et al. have been explored photo-catalytic HER by using 3 type of metal-free D-p-A dyes on TiO2
to sensitize visible light and the differences of photocatalytic activities of these dyes have been explained by the spacer length of dyes[26]. D-p-A dyes have been also used as the sensitizer for p-type photocathodes on dye sensitized photo-electrochemical hydrogen evolution [16,27,28]. In addition, triphenylamine based D-p-A dyes, which are covalently functionalized graphene or Pt, were used for the photo-catalytic HER in the absence of photocatalyst, such as TiO2,
NiO, etc. It has been shown that photocatalytic activities for the HER have been substantially showed less activity than the presence of photocatalyst[29e31]. Dye sensitized HER have been mostly carried out in the presence of co-catalyst to in-crease photogenerated charge transport efficiency, host active sites, stability of reactions and to decrease photocorrosion and charge recombination rates. Platinum group noble metals (PGMs) is generally used as the co-catalyst for photocatalytic HER[13]. Well-known co-catalysts as an alternative to PGMs are metal sulfides such as MoS2and WS2[32]. The catalytic
activities of metal sulfides can be enhanced by doped metals
to metal sulfides or using ternary alloyed structures on pho-tocatalytic HER applications [33e36]. In our previous work, Cu2WS4 were used for the first time as the co-catalyst for
photocatalytic hydrogen evolution reaction sensitized by D- p-A dyes[35].
In this study, we have synthesized triphenylamine based donor-acceptor (D-A) and donor-p-acceptor (D-p-A) organic dyes, which are named as MC-32 and MC-048, respectively, to sensitize TiO2photocatalyst for the visible light. The
nomen-clatures of MC-32 and MC-048 are (3-(4-{bis[4-(hexyloxy) phenyl] amino} phenyl)-2-thioneacetic acid) and (3-[7-(4-{bis[4-(hexyloxy) phenyl]amino}phenyl)-2,3-dihydrothieno[3,4-b] [1,4] dioxin-5-yl] -2-cyanoacetic acid), respectively. Herein, photocatalytic and photoelectrochemical hydrogen evolution have been carried out for the first time by using these D-A and D-p-A dyes sensitized TiO2and Cu2WS4co-catalyst to increase the
cata-lytic activity of dye sensitized TiO2. The photoelectrochemical
responses of TiO2/MC-32 and TiO2/MC-048 electrodes have
been explored by linear sweep voltammetry (LSV) and chro-noamperometry methods. The photocatalytic HER rate of TiO2/MC-32, TiO2/MC-32/Cu2WS4, TiO2/MC-048 and TiO2
/MC-048/Cu2WS4have been investigated by using sacrificial
elec-tron donor agent triethanolamine (TEOA) under visible light illumination. Finally, prospected mechanism of photo-catalytic HER has been explained by electrochemical band levels of the each component of reaction.
Experimental section
Materials
All solvents and reagents were purchased as puriss quality. Copper (II) acetate dihydrate [Cu(CO2CH3)2, 99.99%],
tungsten (IV) chloride [WCl4, 99.998%], sulfur powder [99.98%],
ethanol [, 99.8%] and toluene [99.7%] were purchased from Sigma-Aldrich. Oleylamine [OLAM, %80e90] was ob-tained from Across organic. Dichloromethane, hexane, n-buthyllithium, 1,2-dimethoxyethane (DME), [1,10- bi (diphe-nylphospino) ferrocene]dichloropalladium (II) were supplied from Sigma-Aldrich. Potassium carbonate and potassium hydroxide were obtained from Riedel-de Haen. The 3,4-ethylenedioxythiophene-based compounds a and b were synthesized as the p-spacer by usual reactions. 4-[bis [4-(hexyloxy) phenyl]amino]phenyl]-boronic acid (c) was syn-thesized using previously published procedures[37,38]. The synthetic process of dyes is given in the SI in detail. Na2SO4
(anhydrous, 99.0%), TEOA (99.0%), tetrahidrofuran (THF), sodium hydroxide (NaOH, 97.0%) and hydrochloric acid, (HCl, 37.5%) were supplied by Merck. TiO2powder (Degussa
P25) and TiO2coated transparent electrodes were obtained by
Sigma and Dyesol, respectively. Synthesis of Cu2WS4
Cu2WS4 nanostructures were prepared by a hot-injection
synthesis method as follows. In a typical procedure; 1 mmol CuCl2$2H2O and 0.5 mmol WCl4 were dissolved in 10 ml of
OLAM and the solution was stirred and heated 300C under argon flow. After that, OLAM-sulfur mixture (64 mg/mL)
quickly injected (atz 180C) into the solution under argon flow. Finally, the mixed solution was allowed to react at 300C for 40 min by stirring. After cooling down to room temperature naturally, the black precipitate was collected and washed with ethanol and toluene mixtures, then dried at 70C.
Optical and electrochemical experiments
Optical and electrochemical properties of dyes and co-catalyst have been explored by UVeVis absorption spectra (Shimadzu UV1800 UVeVis absorption spectrophotometer) and cyclic voltammetry methods (CH Instruments 760D electrochemical working station), respectively. UVeVis absorption spectrums of dyes have been measured by using 105M dye solution in THF and normalized by usinglmax peaks. Molar extinction
coefficients of dyes have been calculated by using BeereLambert law [39]. Cyclic voltammograms have been investigated by dye solutions in the acetonitrile, which included 0.1 M tetrabutylammoniumhexafluorophosphate (Bu4NPF6) as a supporting electrolyte. Herein, platinum wire,
Ag/AgCl and glassy-carbon electrodes (GCE) were played a role as the counter, reference and working electrodes, respec-tively. Electrochemical energy levels of dyes have been also figured out by oxidation and reduction potentials[40]. Dye sensitization of TiO2
Powdered TiO2(Degussa P25) and TiO2coated electrodes have
been calcined at 450C for 45 min before sensitized by MC-32 and MC-048 dyes to eliminate adsorbed water and some organic contaminants on the surface[35]. Then dye solution (102mM) in THF have been added the calcined TiO2
/elec-trodes. TiO2powders have stirred overnight under dark
con-ditions and filtered to obtain photocatalyst. The obtained photocatalyts have been rinsed by THF and ethanol three times to remove unbinding dye molecules. Similarly, the calcined TiO2 coated electrodes have been kept in the dye
solutions (102mM) in THF overnight and rinsed by THF and ethanol three times to remove unbinding dye molecules. After these process, TiO2/electrodes have been dried and used in the
photocatalytic/photoelectrochemical hydrogen evolution system.
Photoelectrochemical and photocatalytic hydrogen evolution experiments
Photoelectrochemical hydrogen evolution experiments have been performed by using dye sensitized electrodes as a working electrode in the aqueous TEOA/Na2SO4solution with
standard three electrode voltammetry method under the LED light illumination, which is given visible region, light switched on/off[35]. Photocatalytic hydrogen evolution studies have been performed under the visible light irradiation (with cut-off filter l 420 nm, Solar Light XPS-300™). Herein, dye sensitized TiO2, electron donor and each co-catalyst (Pt or
Cu2WS4) have been put in the photocatalytic reaction cell in
the oxygen-free glovebox. This reaction cell has been sealed under nitrogen atmosphere with rubber septa and put the cell out from glovebox. The sealed reaction cell have sonicated and put in the view of solar simulator with stirring
magnetically for homogenously continuous photocatalytic HER. Eventually, the amount of evolved hydrogen has been determined by gas chromatography (Shimadzu GC2010Plus).
Results and discussion
Structural, optical and electrochemical characterization of Cu2WS4
Fig. 1a and (b) show the low resolution transmission electron microscope (TEM) and scanning electron microscope (SEM) micrographs of the Cu2WS4nanostructures at different
mag-nifications, respectively. It can be seen that the particles assemble slightly polydispersed, square and rectangular par-ticles with most of the nanocrystals falling in the range of 200e500 nm. The lower magnification of SEM image has been shown that Cu2WS4 nanocubes are homogeneously
distrib-uted inFig. S1. The SEM images and its elemental mapping have been investigated by using physically added Cu2WS4
co-catalyst on the dye sensitized TiO2, dye sensitized TiO2and
Cu2WS4phases have been shown clearly inFig. S2. High
res-olution TEM (HR-TEM) and the corresponding Fast Fourier Transform (FFT) are shown inFig. 1c and (d) in order to study the detailed crystalline feature of the Cu2WS4nanostructures.
A representative HR-TEM lattice and FFT image demonstrates the high crystallinity and single crystal features of Cu2WS4.
Lattice fringes shown inFig. 1c have an average spacing of d¼ 5.20 A corresponding to (002) lattice planes of the tetrag-onal I-Cu2WS4structure. No significant lattice distortions in
the Cu2WS4 structures were detected by HR-TEM, which
demonstrates that the samples had high crystallinity. Powder X-Ray Diffraction (PXRD) analyses have been studied to determine the phase structures of the Cu2WS4, as
shown inFig. 1e. All located peaks match well with the pure tetragonal phase of I- Cu2WS4, which is coincident with that in
the previously reported works [41,42]. The sample of the Cu2WS4shows sharp diffraction peaks and these peaks are
originated by big size of the nanostructures. Moreover, the strong and intense diffraction peaks in the pattern show that the as-obtained product is well crystallized.
The valance states of the constituent elements in the Cu2WS4 nanostructures have been studied by X-ray
photo-electron spectroscopy (XPS) analysis.Fig. 1f and (g) and 1(h) show the XPS spectra of the Cu 2p, W 4f and S 2p of the nanostructures. The 2p3/2- 2p1/2signals at binding energies of
931 eV (2p3/2) and 951 (2p1/2) can be fitted into two chemical
states for Cuþmetal cations[34,43]. The peaks of W 4f located at 30.9 eV (W 4f7/2) and 33.9 eV (W 4f5/2) was assigned to the
value of W6þ[44,45]. The sulfur spectrum can be assigned to the presence of sulfides at binding energies of 159.7 (2p3/2) and
162.1 eV (2p1/2) with a splitting peak at 2.4 eV, which are
consistent with the 160e164 eV range of the sulfur element in the sulphide phases[46].
Turning now to optical properties, room temperature UVeVisible absorption spectroscopy as shown inFig. 2a has been used to analyze the optical properties of the as-synthesized Cu2WS4. It shows that the cubic-like Cu2WS4
have a strong absorption at higher energies and absorb light in the whole visible range (Fig. 2a), which results in a black color
of the material. The optic band gap (Eg) of the cubic-like
Cu2WS4has been obtained by extrapolating the linear part
of the curves to intercept the photon energy axis. The ab-sorption onset reveals a bandgap of 1.8 eV[35,42]. The elec-trochemical properties of the Cu2WS4used as co-catalyst were
explored by using cyclic voltammetry method in the Bu4NPF6
solution as the supporting electrolyte in acetonitrile. Electro-chemical band levels (valance and conduction bands) of Cu2WS4have been figured out from cyclic voltammogram as
shown inFig. 2b. Conduction band of Cu2WS4has found at
0.163 V vs. NHE. Increasing current density at positive po-tentials, which is about þ1.5 V, may be indicated as the valence band level [35]. The electrochemical bandgap of Cu2WS4has been calculated as ~1.66 eV and it have been
turned out to be close value to acquire from Tauc Plot calculations.
Synthesis route, optical and electrochemical properties of organic dyes
The synthetic route to the dyes MC-32 and MC-048 is given in Scheme 1. 4-[bis [4-(hexyloxy) phenyl]amino]phenyl]-boronic acid, have been synthesized according to literature [37,38]. The electron donating triarylamine moiety have been attached to 7-bromo-2,3-dihydrothieno [3,4-b][1,4] dioxine-5-carbaldehyde via a Suzuki coupling reaction to obtain donor acceptor molecule d. Donor acceptor molecule d have been converted to the MC-048 dye by Knoevenagel condensation. Fig. 1e (a) TEM image, (b) SEM image, (c) HR-TEM image, (d) corresponding FFT image (e) XRD pattern, (f,g,h) XPS spectrums and (I) Corresponding Crystal Structure with different directions.
Fig. 2e (a) UVeVis absorption spectra of Cu2WS4. The inset shows the calculated band gap of Cu2WS4, Tauc Plot. (b) Cyclic
The synthetic process of dyes is given in the SI in detail. The chemical structures of the MC dyes have been characterized by FT-IR and1H NMR spectroscopy. In H NMR spectrum of
MC-32, alkene proton is at 7.99 ppm as a singlet, aromatic protons are at between 7.78 and 6.85 ppm as a doublet, O-CH2protons
are at 3.97 ppm as a triplet and aliphatic protons are at be-tween 1.80 and 1.00 ppm as expected (see SI section S3). In the FT-IR spectrum of MC-32 dye, 2958.09 cm1- 2869.86 cm1, 2212.20 cm1, 1703.21 cm1, 1439 cm1 - 1500.96 cm1and 1146.98 cm1corresponds to aliphatic C-H, C^N, C]O, C]C (benzene), (C-O) vibrations, respectively (see SIFig. S9a). In H NMR spectrum of MC-048, alkene proton is at 8.14 ppm as a singlet, aromatic protons are at between 7.93 and 6.76 ppm as a dublet, O-CH2 protons are at 4.46 ppm as a triplet and
aliphatic protons are at between 1.70 and 0.88 ppm (see SI section S3). 2926.29 cm1 - 2857 cm1, 2209.94 cm1, 1700.41 cm1, 1502.69 cm1, 1442.78 cm1, 1232.18 cm1shows aliphatic C-H, C^N, C]O, C]C (benzene), C-O vibrations, respectively (see SIFig. S9b).
The maximum UVeVis absorption peak of dyes has been located in 425 nm and 486 nm for MC-32 and MC-048, respectively as shown in Fig. 3a. Absorption peaks result fromp-p* transition and may be ascribed to the intermolec-ular charge-transfer from the donor moeity (hexyloxy-triphenylamine) to the acceptor (carboxylic acid) [47]. Compared to MC-32, the maximum UVevis absorption band of MC-048 have been shifted to 486 nm due to extended p-conjugated structure of the MC-048. Addition of more electron donating group (EDOT) on dye causes extendedp-conjugation [48]. Molar extinction coefficients of dyes have been found
4710 M1cm1(425 nm) ve 7540 M1cm1(486 nm) for MC-32 and MC-048, respectively. Increasing molar extinction coeffi-cient of MC-048 is provided that more light absorption and originating more photoexcited electrons under the same illu-mination [35,48]. The electrochemical features of the dyes have been investigated by the cyclic voltammetry method as given inFig. 3b. The oxidation/reduction potentials of MC-32/ MC-048 have been found at 0.86/-1.38 V and 0.76/-1.33 V (vs. NHE), while HOMO and LUMO levels have been calculated as 5.26/5.16 eV and 3.02/3.07 eV, respectively. Oxidation potential of MC-048 is decreased by the addition of EDOT group (p group) resulted an increased HOMO level value of the MC-048 dye.
Photoelectrochemical and photocatalytic hydrogen evolution properties of dye-sensitized TiO2with Cu2WS4co-catalyst Photoelectrochemical (PEC) hydrogen evolution studies have been realized in the Na2SO4(0.1 M) and TEOA (0.3 M) solution
playing as the electrolyte and electron donor, respectively, under nitrogen flow oxygen-free media. PEC hydrogen evolu-tion studies have been performed by linear sweep voltam-metry (LSV) and chronoamperovoltam-metry methods under visible light source by on/off. Firstly, LSV experiments have been performed from 0.6 V to0.6 V, which have obtained good stability (Fig. 4a). Chronoamperometric experiments have been carried out at the 0 V potential after LSV experiments during 350 s with 50 s light on and 50 s light off. The bare FTO electrode has not been produced transient photocurrent under the irradiation. However, TiO2coated FTO electrode has
been presents weak transient photocurrent density under the visible light illumination and also this transient photocurrent density value have been in good agreement with the literature [49]. However, dye sensitized electrodes show increasing transient photocurrent density under the visible light illumi-nation. The transient photocurrent density is also related to photogenerated electron-hole separation efficiency [50]. As shown inFig. 4b, the transient photocurrent densities have been increased by using dye sensitized TiO2electrodes when
compared to only TiO2 electrodes. Transient photocurrent
densities have been reached 25mA cm1and 360mA cm1by
sensitized MC-32 and MC-048 electrodes, respectively. The differences of transient photocurrent densities can be explained by intramolecular charge transfer efficiencies of donor-p-acceptor dyes[51,52]. Recently, it is approved that the increased dipole moment in a donor-p-acceptor dye initiates the intramolecular charge transfer in the excited state[11].
Photocatalytic hydrogen evolution reactions have been performed by using 0.33 M triethanolamine (TEOA) in water as an electron donor solution under the light of solar simulator with>420 nm cut-off filter (using only visible region). Firstly, optimal pH has been found out for the photocatalytic HER as 9 (see SIFig. S10), which is in accordance with previously pub-lished studies[19,35,53e55]. The hydrogen evolution amounts against time have been carried out in the TEOA solution at pH 9 with suspended photocatalyst and/or catalyst under visible light illumination. The amount of hydrogen gas has been
normalized to the amount of photocatalyst. The HER rate of TiO2/MC-32, TiO2/MC-32/Cu2WS4, TiO2/MC-048 and TiO2
/MC-048/Cu2WS4 have been found 121 mmolg1h1,
179 mmolg1h1, 212 mmolg1h1 and 422 mmolg1h1,
respectively. The produced hydrogen amount of TiO2/MC-32,
TiO2/MC-32/Cu2WS4, TiO2/MC-048 and TiO2/MC-048/Cu2WS4
have been found 1472mmolg1, 1980mmolg1, 1632mmolg1
and 2580mmolg1, respectively, after 8 h visible light
illumi-nation. There is no hydrogen gas detected without the dye sensitization (using only TiO2, Cu2WS4 or TiO2/Cu2WS4) in
this photocatalytic HER system. Moreover, the catalytic ac-tivity of Cu2WS4has been compared to photodeposited noble
metal catalyst Pt, which was obtained by reducing H2PtCl6,
under the same system. The HER rates of TiO2/MC-32/Pt
and TiO2/MC-048/Pt have been found 348 mmolg1h1 and
1139mmolg1h1, respectively. After 8 h of photocatalytic HER,
TiO2/MC-32/Pt and TiO2/MC-048/Pt have been produced
3120 mmolg1 and 7730 mmolg1hydrogen gas, respectively
(Fig. 5). In addition, photocatalytic reusability experiments were carried out with the TiO2/MC-32 and TiO2/MC-048 in the
same conditions and they were almost displayed the same catalytic activity and good stability (Fig. S11). The amount of evolved hydrogen by adding co-catalysts (Cu2WS4and Pt) have
been found out higher than in the absence of co-catalyst. The differences of HER rates between MC-32 and MC-048 have been originated by molecular structures of dyes. MC-048 has been obtained by addition of EDOT group to MC-32 and this Fig. 3e (a) UVeVis spectra and (b) cyclic voltammograms of MC-32 and MC-048 dyes.
Fig. 4e Photoelectrochemical response of MC-32 and MC-048 dyes by using linear sweep voltammetry (LSV) (a) and chronoamperometry methods (b).
structure has supplied the increasing intramolecular charge transfer and molecular absorption coefficient. The higher photocatalytic hydrogen evolution activity of MC-048 can be attributed to electrochemical and optical properties of dyes.
The photocatalytic HER activity of some D-p-A dyes on TiO2with or without co-catalyst has been also compared to
each other as shown inTable 1. It can be seen that MC-048 and MC-32 dyes have competitive photocatalytic HER results with different systems of D-p-A dye sensitized TiO2in the presence
or absence co-catalyst.
The proposed hydrogen evolution mechanism by using dyes and Cu2WS4 co-catalyst have been illustrated by
Fig. 5e The comparison of photocatalytic activity: (a) TiO2/MC-32, TiO2/MC-32/Cu2WS4, TiO2/MC-32/Pt and (b) TiO2/MC-048,
TiO2/MC-048/Cu2WS4, TiO2/MC-048/Pt (10 mg TiO2/dye; electron donor: 20 ml TEOA (0.33 M), pH¼ 9).
Table 1e The photocatalytic activity comparison of some different systems.
Name of sample Electron donor pH HER rates References
DEO1/TiO2 EDTA (10 mM) 3 733mmolg1h1 [18]
DEO2/TiO2 EDTA (10 mM) 3 800mmolg1h1
DEO3/TiO2 EDTA (10 mM) 3 533mmolg1h1
D1@Pt/TiO2 TEOA (10 mM) 10 1000mmolg1h1 [19]
D2@Pt/TiO2 TEOA (10 mM) 10 3200mmolg1h1
D3@Pt/TiO2 TEOA (10 mM) 10 3000mmolg1h1
YFT-1@Pt/TiO2 TEOA (16.6%) e 78mmolg1h1 [20]
YFT-2@Pt/TiO2 TEOA (16.6%) e 104mmolg1h1
UP1@Pt/TiO2 TEOA (10%) 7 64.6mmolg1h1 [22]
UP2@Pt/TiO2 TEOA (10%) 7 93.8mmolg1h1
UP3@Pt/TiO2 TEOA (10%) 7 104.8mmolg1h1
DN1@T TEOA (10%) 7 790mmolg1h1 [23] DN2@T TEOA (10%) 7 1160mmolg1h1 DN3@T TEOA (10%) 7 1190mmolg1h1 DN4@T TEOA (10%) 7 1220mmolg1h1 DN5@T TEOA (10%) 7 1550mmolg1h1 AQ Methanol e 120mmolg1h1 [24] AP Methanol e 110mmolg1h1
TiO2/1/Pt TEOA (10%) 7 29mmolg1h1 [26]
TiO2/2/Pt TEOA (10%) 7 302mmolg1h1
TiO2/3/Pt TEOA (10%) 7 405mmolg1h1
MZ-341/TiO2 TEOA (0.33 M) 9 661mmolg1h1 [35]
MZ-341/TiO2/Cu2WS4 TEOA (0.33 M) 9 1406mmolg1h1
MZ-341/TiO2/Pt TEOA (0.33 M) 9 2900mmolg1h1
MZ-235/TiO2 TEOA (0.33 M) 9 531mmolg1h1
MZ-235/TiO2/Cu2WS4 TEOA (0.33 M) 9 943mmolg1h1
MZ-235/TiO2/Pt TEOA (0.33 M) 9 1900mmolg1h1
TiO2/MC-32 TEOA (0.33 M) 9 121mmolg1h1 This work
TiO2/MC-32/Cu2WS4 TEOA (0.33 M) 9 179mmolg1h1
TiO2/MC-32/Pt TEOA (0.33 M) 9 348mmolg1h1
TiO2/MC-048 TEOA (0.33 M) 9 212mmolg1h1
TiO2/MC-048/Cu2WS4 TEOA (0.33 M) 9 422mmolg1h1
electrochemical band levels of component, as shown inFig. 6. Dyes on TiO2are absorbed light and electrons are excited by
photons from HOMO levels to LUMO levels of organic sensi-tizer dyes. These photoexcited electrons are transferred to TiO2owing to the more positive conduction band levels of
TiO2than LUMO levels of dyes[35,56]. In the absence of
co-catalyst (Pt or Cu2WS4), water can be reduced by
photoex-cited electrons to evolve hydrogen gas owing to more positive redox potential of H2O/H2(0 V vs. NHE) than conduction band
levels of TiO2 [2]. In the presence of co-catalyst (Pt or
Cu2WS4), photoexcited electrons on the TiO2 can be
trans-ferred to co-catalyst and hydrogen gas can be taken place on the co-catalyst surface. This reaction is thermodynamically favorable because the conduction band level of Cu2WS4 is
located between conduction band level of TiO2 and redox
potential of H2O/H2 [35]. Finally, sacrificial electron donor
TEOA (TEOAþ/TEOA is 0.82 V vs. NHE) can be injected the electrons to dyes to regenerate the photocatalytic HER in our system.
Conclusions
Two triphenylamine based organic dyes (MC-32 and MC-048) in the absence and presence of EDOT group as thep bridge have been used to sensitize TiO2photocatalyst in the
photo-electrochemical and photocatalytic HER. The differences of photocatalytic and photoelectrochemical HER activity of dyes have been attributed to the structural, optical and elec-trochemical properties of these dyes. The HER activities of dye/TiO2 have been explored with or without additional
Cu2WS4as the co-catalyst and H2evolution performance have
been raised softly in the presence of Cu2WS4. The catalytic
activity of Cu2WS4 has been compared to the noble metal
photodeposited Pt, which was obtained by reducing H2PtCl6.
This study leads to the novel way for investigating of con-verting solar energy into hydrogen by using sensitizer/TiO2/
alloyed metal sulfide hybrid photocatalyst system, which have stability, non-expensive, simple and highly efficient of hydrogen producing, in the presence of metal free dyes, and in the absence of noble metal co-catalysts.
Acknowledgment
The Scientific and Technological Research Council of Turkey (TUBITAK) (215M309) supports financially this study. Imren Hatay Patir is also intimately thanked to research support of Turkish Academy of Sciences via a TUBA-GEBIP fellowship and UNESCO-Loreal for Woman in Science program. This study is prepared from a section of Ph.D thesis by Emre Aslan, which is also supported by Selcuk University Scientific Research Foundation (17201020).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.11.161.
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