Improved photocatalytic efficiency of a WO3 system by an efficient visible-light induced hole transfer

Article abstract of DOI:10.1039/c2cc31162c

Amorphous Cu(ii) nanoclusters grafted WO₃ particles were coated on a smooth TiO₂ film, and site selective depositions of PbO ₂ and metal Ag particles by photocatalytic processes were observed on TiO₂ and WO₃ due to transfer of holes to TiO₂, and accumulation of electrons in WO₃ respectively. As a result, the photocatalytic activity of TiO₂ modified Cu(ii)-WO₃ increased ～3.5 fold higher than that of Cu(ii)-WO₃. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc31162c

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Improved photocatalytic efficiency of a WO3 system by an efficient
visible-light induced hole transferw
Srinivasan Anandan and Masahiro Miyauchi*
Received 16th February 2012, Accepted 8th March 2012
DOI: 10.1039/c2cc31162c
Amorphous Cu(II) nanoclusters grafted WO3 particles were
coated on a smooth TiO2 film, and site selective depositions of
PbO2 and metal Ag particles by photocatalytic processes were
observed on TiO2 and WO3 due to transfer of holes to TiO2, and
accumulation of electrons in WO3 respectively. As a result, the
photocatalytic activity of TiO2 modified Cu(II)–WO3 increased
B3.5 fold higher than that of Cu(II)–WO3.
from VB of WO3 to TiO2 upon visible-light illumination has
been observed by thermodynamic and kinetic analysis. And
the visible-light activity was improved much more than the
reported Cu(II)–WO3.
We constructed a Cu(II)–WO3/TiO2 thin-film by dispersing
Cu(II)–WO3 particles on a smooth TiO2 film (thickness E
200 nm) (see Fig. S1 and S2, ESIw), which is an ideal setup to
observe charge transfer experimentally by an electron microscope
analysis. We photocatalytically oxidized 1 mM Pb(NO3)2 solution
by employing the Cu(II)–WO3/TiO2 thin-film under visible-light
irradiation, and tried to observe PbO2 which formed by the
oxidation of lead6 (Pb2+ + 2H2O + 2h+ - PbO2 + 4H+)
by holes. The surface morphology of photo-irradiated
Cu(II)–WO3/TiO2 is measured by an Auger electron, and scanning
electron microscope coupled with an energy dispersive X-ray
spectroscope (SEM-EDX), and the results are shown in Fig. 1.
Fig. 1a and b show that PbO2 particles (diameter E 400 nm)
generate on both surfaces of TiO2 and WO3. TiO2, WO3 and PbO2
are indicated in Fig. 1b as blue, green, and red color respectively.
Although N-doped TiO2 is considered as an efficient visible-light
photocatalyst;1a its quantum yield (QE) under visible light is 1–2
orders of magnitude smaller than those under UV light due to
slower hole mobility and lower oxidation power of holes.1b
Recently, Irie et al.2 succeeded in fabricating an efficient visible-
light responsive photocatalyst through high oxidation power of
holes in the valence band (VB) of photocatalysts induced by
interfacial charge transfer (IFCT) and multi-electron reduction
via Cu(II)-ions. The efficiency of Cu(II)-grafted WO3 photo-
catalysts is B16 times higher than that of N-doped TiO2. Based
on this report, extensive research studies3 have been carried out
such as (i) varying co-catalysts,3a,b or (ii) varying other metal
oxides,3c–e or (iii) using nanotubes/nanosheets,3f,g to improve the
performance. These previous works optimized the conduction
band (CB) side to improve its reduction reaction of electrons,
but the performance becomes saturated. The oxidation pathway
is also very important in WO3 based photocatalysts, but the
optimization of the VB side for Cu(II)–WO3 is still challenging.
Recently in water splitting systems, RuO2 or IrO2 act as a
co-catalyst for oxygen evolution.4 Previous studies also reported
heterogeneous semiconductor systems like CdS–TiO2,5a TiO2–
SnO2,5b TiO2–ZnO,5c TiO2–WO3,5d but the UV light irradiation
to excite the TiO2 side was indispensable to proceed photocatalytic
reactions. Especially under the indoor environment, a light source
like an incandescent lamp or LED does not emit any UV light,
thus the visible-light-sensitive photocatalyst is very important for
the practical use in the indoor environment. Herein, we report a
simple and effective strategy to improve the efficiency of photo-
catalysts (Cu(II)–WO3) by the modification of TiO2. Hole transfer
Department of Metallurgy and Ceramic Science, Graduate School of
Science and Engineering, Tokyo Institute of Technology,
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan.
E-mail: mmiyauchi@ceram.titech.ac.jp
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc31162c
Fig. 1 (a) Auger electron image, (b) Auger elemental mapping, (c) SEM
image, and (d) EDX patterns of PbO2 deposited Cu(II)–WO3/TiO2. (Photo-
catalytic oxidation conditions: 1 mM Pb (NO3)2 = 5 ml; light source: xenon
lamp with Y-43 and c-50s cut-off filters; adsorption in dark = 1 hour;
irradiation time = 1 hour.).
c
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It is interesting to note that PbO2 mainly appears on the TiO2
surface near the interface between TiO2 and WO3, though some
PbO2 particles deposit on WO3. In contrast, PbO2 particles were
not observed in the bare TiO2, even after long-time photo-
irradiation of Pb(NO3)2 solutions (see Fig. S3, ESIw), revealing
that TiO2 cannot be excited by visible-light illumination. SEM
observation at various portions of the Cu(II)–WO3/TiO2 thin film
reveals the uniform deposition of PbO2 on the surface of the TiO2
film (see Fig. S4, ESIw). SEM-EDX results of the Cu(II)–WO3/
TiO2 thin film are shown in Fig. 1c and d. The EDX patterns
represented in red, blue, and green in Fig. 1d correspond to the
spectrum I, II, and III of Fig. 1c respectively. The EDX pattern for
point I reflects the presence of TiO2 alone, while point II displays
the existence of TiO2 and PbO2. Point III exhibits the presence of
not only WO3, but also TiO2 and PbO2. These results instruct that
the holes generated initially on WO3 are responsible for the
deposition of PbO2 on WO3 particles, while migration of holes
from WO3 to TiO2 is the reason for the formation of PbO2 on
TiO2. We compared the peak intensities of PbO2 (Fig. 1d) on the
WO3 with that on the interface between TiO2 and WO3. Then, the
PbO2 particles deposited more on the interface than on WO3.
Moreover, we investigated the SEM-EDAX analysis on several
portions of particles, then a similar trend has been observed. The
grazing angle X-ray diffraction (GXRD) pattern (see Fig. S5,
ESIw), and X-ray photoelectron spectroscopic (XPS) results
(Fig. S6, ESIw) of photo-irradiated Cu(II)–WO3/TiO2 revealed
the presence of crystalline PbO2 particles.7
(see Fig. S7, ESIw) confirm the presence of metallic Ag on
photo-deposited Cu(II)–WO3/TiO2. These results indicate the
accumulation of electrons in WO3 particles. It is also important
to consider the electron transfer from the CB of WO3 to Cu(II)
ions. As reported by Irie et al.,2 a visible-light excited electron in
CB of WO3 is consumed by Cu(II) ions, which reduces oxygen
through multi-electron reduction.
The CB potential of WO3 is +0.3 to +0.5 V (vs. SHE,
pH = 08a) more positive than that of TiO2 (0.04 V vs. SHE,
pH = 0) and it is not sufficient for single-electron reduction of an
oxygen molecule (O2 + H+ + eÀ - HO2, À0.046 V vs. SHE at
pH = 0).8b The VB potential of WO3 is 3.1–3.2 V (vs. SHE,
pH = 08c) positive than that of TiO2 (3.04 V vs. SHE, pH = 08b).
When visible-light is irradiated on Cu(II)–WO3/TiO2 the charge
carriers (holes and electrons) are generated on WO3 as shown in
Fig. 3. Holes in the VB of WO3 transfer to TiO2, since the VB
positions of TiO2 is more negative than that of WO3.9 The holes
in the VB of TiO2 have sufficient life-time to initiate photo-
catalytic oxidation either in solution (oxidation of Pb2+ in to
Pb4+) or under aerobic conditions (oxidation of acetaldehyde
into carbon dioxide) as shown in Fig. 3. The electron in the CB of
WO3 transfers to either Cu2+ or Ag+ ions as per thermodynamic
consideration, since their redox potentials {Cu2+/Cu+ (E0
=
0.16 V); Ag+/Ag (0.77 V)} are more positive than that of CB of
WO3. The electron transfer to Cu(II) ions may reduce oxygen into
hydrogen peroxide through multi-electron reduction reaction.9
Due to this charge transfer, holes and electrons generated in WO3
efficiently separated, so that Cu(II)–WO3/TiO2 can utilize
visible-light efficiently to decompose organic pollutants.
Based on our experimental analysis for the visible-light induced
charge transfer between WO3 and TiO2, a highly active visible-light
sensitive powder system has been developed. We prepared TiO2
modified Cu(II)–WO3 by physical mixing of Cu(II)–WO3 and
commercial TiO2 particles. Since small size particles are beneficial
for hole mobility,10 we used TiO2 nanoparticles (MT-150A, grain
size 15 nm, TAYCA, Japan) for the modification of Cu(II)–WO3.
We investigate the photocatalytic efficiency for the decomposition
of gaseous acetaldehyde under visible-light illumination (l 4
400 nm), and the changes in generated CO2 concentration by
oxidation reaction are shown in Fig. 4. For comparison, the results
of TiO2 and Cu(II)–WO3 are also included in Fig. 4. Pure TiO2
shows negligible visible-light activity due to lack of visible-light
absorption. Cu(II)–WO3 showed visible light activity, whereas pure
WO3 without Cu(II) co-catalysts was inactive.3a In contrast, it is
noted that the addition of TiO2 particles onto Cu(II)–WO3 abruptly
Next, to investigate reduction reaction, Cu(II)–WO3/TiO2 in
AgNO3 solution was irradiated by visible-light, and the results of
microscopic analysis are shown in Fig. 2. Fig. 2a and b show
SEM images of the surfaces of the Cu(II)–WO3/TiO2 thin-film
after Ag deposition. Ag particles (inside black line in Fig. 2a and b)
having sizes of about 2–5 nm deposit only on WO3 rather than on
TiO2. The EDX pattern (Fig. 2d) represented in green and red
corresponds with points I and II of Fig. 2c, revealing that photo-
reduction of Ag+ ions takes place only on WO3. XPS results
Fig. 2 (a and b) FE-SEM images, (c and d) SEM-EDX analysis of Ag
deposited Cu(II)–WO3/TiO2. (Photocatalytic reduction conditions: 1 mM
AgNO3 = 5 ml; light source: xenon lamp with Y-43 and c-50s cut-off
filters; adsorption in dark = 1 hour; irradiation time = 1 hour.)
Fig. 3 Mechanism of charge separation over TiO2 modified Cu(II)–WO3.
The aerobic phase reaction and the solution phase reaction take place
independently in this system.
c
4324 Chem. Commun., 2012, 48, 4323–4325
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has been confirmed by site-selective photo-deposition of PbO2 on
the surface of TiO2 by the excitation of WO3 alone. This charge
transfer ensures the efficient separation of charge carriers generated
on WO3. TiO2 nanoparticles modification drastically improved the
visible-light activity of Cu(II)–WO3. Its visible light activity
showed B3.5 fold higher reaction rate than Cu(II)–WO3, which
was reported as one of the best efficient visible-light photocatalysts
to date. Here TiO2 plays an important role as a co-catalyst and is
used for the accumulation of holes from WO3. We have investigated
the effect of TiO2 addition to Cu(II)–WO3 for other commercial
TiO2 particles, then this commercial TiO2 also enhanced the photo-
catalytic activity of Cu(II)–WO3 (see Fig. S9, ESIw). The simple
strategy, i.e. modification of TiO2 onto Cu(II)–WO3 used in this
study promises to be very valuable for designing more efficient
visible-light-active photocatalysts using other semiconductors.
This work is supported by the New Energy and Industrial
Technology Development Organization (NEDO) in Japan. We
would like to thank the Japan Society for the Promotion of Science
(JSPS) for providing the JSPS fellowship. This work was initiated
by the discussion with Mr K. Yotsugi and Mr S. Yanai in
Sekisuijushi Technical Research Corporation.
Fig. 4 Initial reaction rate of CO2 generation over TiO2, Cu(II)–WO3,
and TiO2 modified Cu(II)–WO3.
enhances the photocatalytic activity of WO3. It shows B3.5 fold
higher reaction rates than Cu(II)–WO3. The photocatalytic activity
as a function of TiO2 loading in TiO2 modified Cu(II)–WO3 can be
found in the ESIw (Fig. S8). Since we carried out the photocatalytic
reactions under light-limited conditions,11 the photocatalytic
efficiency largely depends on the charge separation. High visible-
light sensitivity of TiO2 modified Cu(II)–WO3 is due to its efficient
charge separation for both holes and electrons. TiO2 nanoparticles
act as co-catalysts for oxidation reaction by the extraction of holes
from WO3, while the Cu(II) nanoclusters act as cocatalysts for
reduction reaction through the multi-electrons reduction for
adsorbed oxygen molecules.9 Kinetic fluorescence lifetime analysis
(Fig. 5), revealing that TiO2 modified Cu(II)–WO3 shows a slow
decay curve with long-lived (382 ps) charge carriers, whereas Cu(II)–
WO3 exhibits fast decay with short-lived (188 ps) carriers. The high
photocatalytic activity of TiO2 modified Cu(II)–WO3 is assigned to
its slow recombination rate. The trade off relation observed between
photocatalytic activity and fluorescence analysis is consistent with
the previous studies12 (detailed discussion can be found in ESIw).
Long-lived photo-generated charge carriers in the present study are
mainly due to the addition of TiO2 on Cu(II)–WO3.
Notes and references
1 (a) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga,
Science, 2001, 293, 269–271; (b) H. Irie, Y. Watanabe and
K. Hashimoto, J. Phys. Chem. B, 2003, 107, 5483–5486.
2 H. Irie, S. Miura, K. Kamiya and K. Hashimoto, Chem. Phys.
Lett., 2008, 457, 202–205.
3 (a) R. Abe, H. Takami, N. Murakami and B. Ohtani, J. Am. Chem.
Soc., 2008, 130, 7780–7781; (b) H. Yu, H. Irie, Y. Shimodaira,
Y. Hosogi, Y. Kuroda, M. Miyauchi and K. Hashimoto, J. Phys.
Chem. C, 2010, 114, 16481–16487; (c) X. Qiu, M. Miyauchi, H. Yu,
H. Irie and K. Hashimoto, J. Am. Chem. Soc., 2010, 132, 15259–15267;
(d) S. Anandan, N. Ohashi and M. Miyauchi, Appl. Catal., B, 2010, 100,
502–509; (e) S. Anandan and M. Miyauchi, Phys. Chem. Chem. Phys.,
2011, 13, 14937–14945; (f) Z. G. Zhao and M. Miyauchi, Angew. Chem.,
Int. Ed., 2008, 47, 7051–7055; (g) A. Nakajima, Y. Akiyama,
S. Yanagida, T. Koike, T. Isobe, Y. Kameshima and K. Okada, Mater.
Lett., 2009, 63, 1699–1701.
4 M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. X. Mi,
E. A. Santori and N. S. Lewis, Chem. Rev., 2010, 110, 6446–6473.
5 (a) N. Serpone, E. Borgarello and M. Gratzel, J. Chem. Soc.,
Chem. Commun., 1984, 342–344; (b) H. Tada, A. Hattori,
Y. Tokihisa, K. Imai, N. Tohge and S. Ito, J. Phys. Chem. B,
2000, 104, 4585–4587; (c) G. Marci, V. Augugliaro, M. J. Lopez-
Munoz, C. Martin, L. Palmisano, V. Rives, M. Schiavello, R. J. D.
Tilley and A. M. Venezia, J. Phys. Chem. B, 2001, 105, 1026–1032;
(d) M. Miyauchi, A. Nakajima, T. Watanabe and K. Hashimoto,
Chem. Mater., 2002, 14, 4714–4720.
In conclusion, the visible-light induced charge transfer is success-
fully investigated in the Cu(II)–WO3/TiO2 thin-film. Hole transfer
6 (a) K. Tanaka, K. Harada and S. Murata, Sol. Energy, 1986, 36,
159–161; (b) J. M. Hermann, J. Disdier and P. Pichat, J. Catal.,
1988, 113, 72–81.
7 T. Ohno, K. Sarukawa and M. Matsumura, New J. Chem., 2002,
26, 1167–1170.
8 (a) G. R. Bamwenda, K. Sayama and H. Arakawa, J. Photochem.
Photobiol., A, 1999, 122, 175–183; (b) T. Torimoto, N. Nakamura,
S. Ikeda and B. Ohtani, Phys. Chem. Chem. Phys., 2002, 4,
5910–5914; (c) M. Miyauchi, A. Nakajima, K. Hashimoto and
T. Watanabe, Adv. Mater., 2000, 12, 1923–1927.
9 Y. Nosaka, S. Takahashi, H. Sakamoto and A. Y. Nosaka,
J. Phys. Chem. C, 2011, 115, 21283–21290.
10 J. M. Coronado, A. J. Maira, J. C. Conesa, K. L. Yeung,
V. Augugliaro and J. Soria, Langmuir, 2001, 17, 5368–5374.
11 H. Gerischer, Electrochim. Acta, 1993, 38, 3–9.
12 (a) H. N. Ghosh and S. Adhikari, Langmuir, 2001, 17, 4129–4130;
(b) Z. Zhang, Q. Lin, D. Kurunthu, T. Wu, F. Zuo, S. T. Zheng,
C. J. Bardeen, X. Bu and P. Feng, J. Am. Chem. Soc., 2011, 133,
6934–6937.
Fig. 5 Time resolved fluorescence decay analysis of Cu(II)–WO3, and
TiO2 modified Cu(II)–WO3.
c
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Chem. Commun., 2012, 48, 4323–4325 4325