Organic dye-sensitized TiO2 for the redox conversion of water pollutants under visible light

Article abstract of DOI:10.1039/b924829c

Titania nanoparticles sensitized with metal-free organic dye exhibit high visible-light activities for the redox conversion of water pollutants in a wider pH range in comparison with TiO₂ sensitized with ruthenium bipyridyl complexes. The Royal Society of Chemistry.

Full text of DOI:10.1039/b924829c

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Organic dye-sensitized TiO2 for the redox conversion of water pollutants
under visible lightw
Yiseul Park,a Su-Hyun Lee,b Sang Ook Kangb and Wonyong Choi*a
Received (in Cambridge, UK) 25th November 2009, Accepted 12th February 2010
First published as an Advance Article on the web 5th March 2010
DOI: 10.1039/b924829c
Titania nanoparticles sensitized with metal-free organic dye
exhibit high visible-light activities for the redox conversion of
water pollutants in a wider pH range in comparison with TiO2
sensitized with ruthenium bipyridyl complexes.
Table 1 Spectroscopic and electrochemical parameters of OD and
RuL3
a
a
lmax
nm
/
emax
/
E0(dye/dyeꢂ+)/ E0(dye*/dyeꢂ+)/
Dye
OD
Mꢁ1 cmꢁ1 DEb/V VNHE
VNHE
445
24 500
19 500
2.45
2.20
1.35
1.39
ꢁ1.0
c
Dye-sensitized TiO2 has been frequently investigated to utilize
solar light for chemical conversion. In particular, ruthenium
bipyridyl derivatives have been extensively studied as an
efficient sensitizer of TiO2 for visible light photocatalysis
(e.g., the degradation of water pollutants, production of
hydrogen).1–3 However, Ru-based dyes are not environmentally
and economically acceptable because ruthenium is an expensive
and toxic element. Therefore, metal-free organic dye sensitizers
with low production costs, higher visible light absorption, high
stability, and facile molecular design have attracted attention.4–7
Although organic dye-sensitized TiO2 has been successfully
applied for dye-sensitized solar cells, its applications in environ-
mental photocatalysis are few.8,9 Organic dyes for sensitized-
TiO2 photocatalysts should have (1) strong visible light
absorption, (2) suitable energetics for electron injection and
regeneration, and (3) high affinity for the TiO2 surface and
stability in water over a wide pH range. In this study, we
designed and synthesized an organic dye with such characteristics
and tested its performance as a sensitizer of TiO2 for the redox
conversion of water pollutants. The organic dye-sensitized
TiO2 demonstrated superior performance in comparison with
the common Ru-dye sensitized TiO2.
RuL3 465
ꢁ0.81
c
UV-vis absorption spectra in ESI. HOMO–LUMO gap. ref. 12.
a
b
methodw). The dye molecule consists of a diphenylamino-
phenyl-2,20-bithiophene (donor part where HOMO electron
density is localized), a cyanoacrylic acid (acceptor part where
LUMO electron density is localized), and ethylene-oxide
chains (hydrophilic part that increases the solubility).
RuII(4,4-bpy(COOH)2)3 was also prepared according to the
previous studies1,2 and compared as a reference sensitizer of
TiO2 (see ESIw). The organic and Ru-dye sensitizers will
be referred as OD and RuL3, respectively throughout the
communication. They are compared for their spectroscopic
and electrochemical parameters in Table 1. The optical
absorption and energy levels of OD are similar to those of
RuL3.
OD was dissolved in an acetonitrile–tert-butyl alcohol
mixture (1 : 1) and RuL3 in water (pH 3). The dissolved dyes
were adsorbed on the surfaces of TiO2 particles (Degussa P25;
size 20–30 nm) and the dye-adsorbed TiO2 was recovered from
the slurry by filtration. The dye loading was 10 mmol
dye/(gTiO2). Comparison of the lmax absorption of the dye
solution before and after the adsorption on TiO2 confirmed
that the adsorption of the dye was almost quantitative. Dyes
adsorbed on TiO2 remained stable without desorption
in aqueous suspension. The visible-light activities of the
dye-sensitized TiO2 catalyst were tested for the oxidation of
4-chlorophenol (4-CP) and arsenite (AsIII to AsV (less toxic
form)) and the reduction of chromate (CrVI to CrIII (less toxic
form)) in water.10
The synthesized organic dye was ((E)-3-(5-(5-(4-(bis(4-
((2-(2-methoxyethoxy)ethoxy)methyl)phenyl)amino)phenyl)-
thiophen-2-yl)thiophen-2-yl)-2-cyanoacrylic acid) and its
molecular structure is shown in Fig. 1 (refer to ESI for synthetic
Fig. 2 shows the time profiles of the photocatalytic removal
of 4-CP, AsIII (the production of AsV monitored) and CrVI in
aqueous suspensions of OD/TiO2, RuL3/TiO2 and bare TiO2
under visible light illumination. OD/TiO2 exhibited higher or
comparable photocatalytic activities in comparison with
RuL3/TiO2 for all cases. The apparent photonic efficiencies
[(substrates removed)/(incident photons) ꢀ 100] determined
for the photocatalytic conversion under visible light are 0.6,
0.09, and 1.3% for 4-CP, AsIII, and CrVI, respectively (see
ESIw). OD/SiO2 was also prepared as a control sample since
the excited OD cannot inject the electron into SiO2. OD/SiO2
showed a negligible photoactivity for 4-CP degradation
(Fig. 2a), which implies that the photoactivity of OD/TiO2
Fig. 1 Molecular structures of OD and RuL3.
a School of Environmental Science and Engineering, Pohang University
of Science and Technology (POSTECH), Pohang, 790-784,
South Korea. E-mail: wchoi@postech.edu; Fax: +82-54-279-8299
b Department of Materials Chemistry, Sejong Campus, Korea
University, Chung-Nam, 339-700, South Korea
w Electronic supplementary information (ESI) available: Synthesis
and catalyst preparation, photocatalytic activity test details, photo-
catalyst characterization and UV-vis absorption spectra of OD and
RuL3. See DOI: 10.1039/b924829c
ꢃc
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Fig. 3 The effect of oxygen and ferric ion (alternative electron
acceptor) on the conversion of 4-CP and CrVI in an OD/TiO2 system.
Experimental conditions: [4-CP]0 = 100 mM, [CrVI
] = 200 mM,
0
[Fe3+] = 1 mM, [TBA] = 0.1 M, [catalyst] = 1 g Lꢁ1, pHi = 3,
dye loading 10 mmol/(gTiO2), l > 420 nm, air-equilibrated or
continuous N2 purged.
generated after 30 min irradiation while the dark control test
showed no production of O2.
Fig. 2 Visible light-induced oxidation of (a) 4-CP and (b) AsIII and
(c) reduction of CrVI in aqueous suspensions of OD/TiO2, RuL3/TiO2
and bare TiO2. The concentration at time zero indicates the
equilibrium concentration after adsorption on the catalyst surface.
In the overall photocatalysis, the role of O2 is essential as an
electron acceptor (eqn (4)). Fig. 3 shows that the photo-
catalytic oxidation of 4-CP on OD/TiO2 is negligible in the
absence of O2 (N2-saturated) since the injected electron
recombines with the dye cation (dyeꢂ+) in the absence of
suitable electron acceptors. The addition of an alternative
electron acceptor (Fe3+) markedly enhanced the removal rate,
which demonstrated the importance of electron acceptors. The
addition of tert-butyl alcohol (TBA: OH radical scavenger)
had little influence on the removal rate. This implies that the
degradation of 4-CP is mainly mediated by direct electron
transfer (eqn (3)), not by the OH radical. On the other hand,
CrVI reduction was significantly retarded in the presence of the
electron acceptor (Fe3+) because of the competition for
electrons between CrVI and Fe3+ (eqn (2)). This confirms
that CrVI reduction proceeds by accepting electrons from the
dye-sensitized TiO2.
Experimental conditions: [4-CP]0 = [AsIII
] ] =
0 0
= 100 mM, [CrVI
200 mM, [catalyst] = 1 g Lꢁ1, pHi = 4.5 (a), 3 (b, c), dye loading
10 mmol/(gTiO2), l > 420 nm, air-equilibrated.
should be based on the photoinduced electron transfer from
OD to TiO2.
The general action of dye-sensitized TiO2 photocatalysts is
initiated by the electron injection from an excited dye to the
TiO2 conduction band (CB) (eqn (1)) and completed through
the subsequent electron transfer reactions.1,2 The substrates
(S, S0) may be converted upon direct electron transfer (eqn (2),
(3)) or indirectly transformed through the reaction with
reactive oxygen species (eqn (4)) (e.g., superoxide).
Dye–TiO2 + hn(visible) - Dyeꢂ+–TiO2(ecb
)
(1)
(2)
ꢁ
ꢁ
ecb + S - Sred
The visible light-induced electron transfer on OD/TiO2 was
directly monitored by collecting photocurrents in the aqueous
catalyst suspension as shown in Fig. 4. The redox couple
of Fe3+/Fe2+ serves as an electron shuttle that transfers
electrons from TiO2 to a Pt collector electrode immersed in
the illuminated catalyst suspension.11 The appearance of
photocurrents under visible light indicates that electrons are
injected from the adsorbed sensitizers to TiO2. OD/TiO2
generated higher photocurrent than RuL3/TiO2, which is
Dyeꢂþ ꢁ TiO2 þ S0 ! Dye ꢁ TiO2 þ S0ox
ð3Þ
ꢁ
ꢁ
ꢂ
ecb + O2 - O2
(4)
The dye should be regenerated to be catalytic after the
electron injection. To reduce 200 mM of CrVI to CrIII
(Fig. 1b), 600 mM of OD is needed, provided that the dye is
a one-electron donor. Since the total concentration of OD
contained in the suspension is only 10 mM, the dye should be
recycled and the corresponding turnover number is 60. As for
the oxidation of 100 mM 4-CP, the turnover number is 10 on
the basis of one-electron oxidation. The main electron donor
that reduces the oxidized dye should be either the substrate to
be oxidized (eqn (3)) or water molecule (eqn (5)) (E0(O2/H2O) =
1.05 VNHE at pH 3 vs. E0(dye/dyeꢂ+) = 1.35 VNHE).
Dyeꢂ+–TiO2 + 0.5H2O - Dye–TiO2 + H+ + 0.25O2 (5)
The in situ generation of O2 through eqn (5) was monitored
using an oxygen electrode in the deaerated suspension of
OD/TiO2 under visible light. An alternative electron acceptor,
S2O82– (10 mM), was added in excess to inhibit both the back
electron transfer in eqn (1) and the immediate consumption of
O2 through eqn (4). About 0.1 ppm of dissolved oxygen was
Fig. 4 Photocurrent (Iph) collected via Fe3+/Fe2+ redox couple in
suspensions of dye/TiO2 catalysts. Experimental conditions: [Fe3+] =
1 mM, [acetate] = 0.2 M (electron donor), [catalyst] = 1 g Lꢁ1, dye
loading 10 mmol/(gTiO2), pHi = 2, applied potential = 0.7 VAg/AgCl
,
Ag/AgCl (RE), graphite rod (CE), Pt (WE), l > 420 nm, continuous
N2 purged.
ꢃc
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surface charge on TiO2 because RuL3 is negatively charged.
Above pHzpc E 6 of TiO2, the negative surface charge on TiO2
electrostatically repels the anionic sensitizer (deprotonated
RuL3; pKa o 3)12 with inhibition of its adsorption. The
electrostatic repulsion among the adsorbed sensitizers may
undermine the stability of RuL3/TiO2 as well. On the other
hand, OD adsorbed on TiO2 is little affected by the TiO2
surface charge. The sensitizer molecule consists of the
carboxylate group that binds to the surface, the chromophoric
aromatic portion, and the hydrophilic ethylene-oxide group
that increases the compatibility of the dye at the water/TiO2
interface. The hydrogen bonding between the cyano group (or
the ethylene-oxide group) and the surface hydroxyl group add
to the OD/TiO2 binding strength.13,14
Fig. 5 pH-dependent photoactivities of OD/TiO2 and RuL3/TiO2 for
the degradation of 4-CP. The experimental conditions were the same
as for Fig. 2a.
In summary, we have prepared metal-free organic
dye-sensitized TiO2 and compared it with the popular
Ru-complex sensitized TiO2 system for its visible light photo-
catalytic activity. The synthesized OD is similar in comparison
with RuL3 as a sensitizer and has a higher visible light
absorptivity. OD/TiO2 showed better or comparable activities
for a wider pH range and higher stability in water. The
proposed OD/TiO2 can serve as a model of a green photo-
catalyst working under visible light.
Fig. 6 DRUVS of dye/TiO2 coated electrode before and after
(a) basic solution treatment (pH 10, 2 h) and (b) visible light
irradiation (1 h).
This work was supported by the KOSEF NRL program
(No. R0A-2008-000-20068-0), KOSEF EPB center (Grant No.
R11-2008-052-02002), and the Korean Center for Artificial
Photosynthesis (KCAP: Sogang Univ.) funded by MEST
through NRF (NRF-2009-C1AAA001-2009-0093879).
consistent with the higher photocatalytic activity of OD/TiO2
and the higher absorption of visible light of OD (see Table 1).
The most marked difference between the OD/TiO2 and
RuL3/TiO2 systems is the pH dependence of the photoactivity
as shown in Fig. 5. The visible light activity of RuL3/TiO2 is
maintained only in the acidic region because the adsorption of
RuL3 on TiO2 is limited only in the acidic pH region.2 On the
other hand, the photoactivity of OD/TiO2 remained constant
over a wide pH range, which indicates that the binding of OD
onto the TiO2 surface is little affected by pH.
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Both OD and RuL3 should be anchored onto the surface
through the carboxylate linkage. However, the binding of
RuL3 should be sensitively affected by the pH-dependent
ꢃc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 2477–2479 | 2479