Influence of Cu distribution on photocatalytic activity of Cu-doped titania prepared using metal alkoxides

Article abstract of DOI:10.1007/s11164-015-2321-x

Cu-doped titania was prepared from sols containing titanium tetraisopropoxide (TTIP) and copper(II) isopropoxide (CIP). The Cu distribution in the titania was controlled by varying the reaction time of the TTIP at which the CIP was added to the sol. The p

Full text of DOI:10.1007/s11164-015-2321-x

Res Chem Intermed (2016) 42:4813–4825
DOI 10.1007/s11164-015-2321-x
Influence of Cu distribution on photocatalytic activity
of Cu-doped titania prepared using metal alkoxides
Hiromasa Nishikiori1 Tomoaki Ikeda1
•
•
Yuki Izaki1 Ryuhei Katayama2 Yuichiro Shimizu2
•
•
Received: 9 July 2015 / Accepted: 8 October 2015 / Published online: 31 October 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Cu-doped titania was prepared from sols containing titanium tetraiso-
propoxide (TTIP) and copper(II) isopropoxide (CIP). The Cu distribution in the
titania was controlled by varying the reaction time of the TTIP at which the CIP was
added to the sol. The photocatalytic activities of the samples were evaluated by the
hydroxyl radical production during UV and visible irradiations. The influences of
the Cu distribution in the titania and the electronic interaction between the Ti and
Cu on the photocatalytic activity were discussed. The sample prepared by adding
CIP after some reaction time of TTIP exhibited a higher activity than that prepared
by initially mixing TTIP and CIP. The impurity states of Cu2? on the titania surface
effectively trapped the electrons in the conduction band of the titania and received
the electrons in the valence band upon photoexcitation, and consequently, sup-
pressed charge recombination of the electrons and holes. In the sample, the high Cu
distribution on only the titania surface rather than inside the titania bulk led to its
high photocatalytic activity.
Keywords Sol–gel method Á Metal alkoxide Á Copper Á Titania Á Photocatalysis
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2321-x)
contains supplementary material, which is available to authorized users.
&
Hiromasa Nishikiori
nishiki@shinshu-u.ac.jp
1
2
Department of Environmental Science and Technology, Faculty of Engineering, Shinshu
University, 4-17-1 Wakasato, Nagano 380-8553, Japan
Shinko Electric Industries Co., Ltd., 36 Kita-Owaribe, Nagano 381-0014, Japan
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Introduction
Hazardous organic substances should be completely decomposed and mineralized
by chemical or physical techniques. Photocatalysts are promising materials for the
detoxification of such substances utilizing photoenergy while saving thermal energy
[1, 2]. Titania is a typical photocatalyst, but requires UV light energy higher than
visible light energy, which is available from the sun and household lights [2].
Visible-light-driven titania photocatalysts consisting of uniform nanosized crys-
talline particles can be prepared by metal doping using the sol–gel method [3–7].
Cu-doping is valid for performing visible light activity because it leads to the
formation of impurity states at a potential level between the valence and conduction
bands and direct electron transition from the valence band to the Cu impurity states
[8, 9]. In addition, trapping electrons in the conduction band suppresses the
electron–hole charge recombination. The electron transfer from the titania bands to
the Cu states enhances the photocatalytic activity. This interaction is influenced by
the structural distribution of titania and Cu [7]. However, the atomic Cu distribution
is difficult to be visually observed by general analyses.
In our previous study [7], Cu-doped TiO2 was prepared by the refluxing of a
homogeneous mixture of copper and titanium alkoxides [10–12]. The refluxing
improved the Cu2? dispersion in the TiO2 and formed effective Ti–O–Cu bonds.
The Ti–O–Cu bonds and electronic interaction between Ti and Cu should
effectively promote the electron trapping. The Cu-doped titania containing
0.1 mol% Cu exhibited the highest photocatalytic activity for methylene blue
degradation (Supporting Information). The higher amount of Cu decreased the
photocatalytic activity due to the decreases in the specific surface area,
hydrophilicity, and light permeation [7]. The Cu2? distribution in the titania is
also important for the photocatalytic activity because the electron transfer from the
reduced Cu? to the reactant molecules, such as oxygen, is an indispensable process
as shown in Fig. 1. The Cu2? sites should be accessible from the surroundings such
Suppressing charge
recombination
e−
C.B.
-0.5 V
Electron trap
O2
e −
Cu2+ / Cu+
0.16 V
3.2 eV
2Cu+ + O2 + 2H+ → 2Cu2+ + H2O2
4Cu+ + O2 + 4H+ → 4Cu2+ + 2H2O
Direct excitation
2.54 eV
H2O
2.7 V
h+
V.B.
TiO2
H+ + •OH
Fig. 1 Energy diagram of photocatalytic reaction on Cu-doped titania
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Influence of Cu distribution on photocatalytic activity…
4815
as on the particle surface. The electron consumption is expressed by the following
equations [8, 9]:
Cu2þ þ eÀ ! Cuþ
2Cuþ þ O2 þ 2Hþ ! 2Cu2þ þ H2O2
4Cuþ þ O2 þ 4Hþ ! 4Cu2þ þ 2H2O
In the absence of Cu2?, the electrons in the titania conduction band directly reduce
oxygen molecules. However, the electrons easily recombine with the holes because
the efficiency of this process is low.
In this study, we tried to control the Cu distribution in the Cu-doped titania by
varying the reaction conditions of the sols containing titanium tetraisopropoxide
(TTIP) and copper(II) isopropoxide (CIP), i.e., mixing TTIP and CIP before
initiating the reaction or adding CIP after some reaction time of TTIP. The UV and
visible photocatalytic activities of the samples were evaluated by the hydroxyl
radical production. The influences of the Cu distribution in the titania and the
electron transfer from the titania to the Cu2? on the photocatalytic activity are
discussed.
Experimental
Materials
TTIP, CIP, ethanol, hydrochloric acid (35 %), 8-hydroxylquinoline (HQ), barium
sulfate, and sodium hydroxide (S or reagent grade) were obtained from Wako Pure
Chemicals and used without further purification. Terephthalic acid (reagent grade)
was obtained from Tokyo Kasei and used without further purification. The dry
nitrogen gas was obtained from Okaya Sanso. The water was deionized and distilled
by a distiller (Yamato WG23).
Preparation of photocatalysts
The sol–gel systems used to prepare the Cu-doped TiO2 consisted of 6.8 cm3 of
TTIP, CIP (0.10 mol% of the Ti amount), 40.0 cm3 of ethanol, and 0.21 cm3 of
hydrochloric acid as a catalyst for the sol–gel reaction. The three preparation
procedures are explained in detail as follows.
First, TTIP and CIP were mixed and agitated in a glove box filled with dry
nitrogen gas at ambient temperature. This mixture was refluxed at 120 °C for 3 h
(1-step reflux). Ethanol and hydrochloric acid were added to the refluxed mixture.
The resulting sol was labeled Sol-CT-r3h. The sol containing TTIP without CIP,
Sol-T-r3h, was also prepared. The gelation of these systems occurred 10–14 days
after starting the reaction at 60 °C.
Second, TTIP was refluxed at 120 °C for 0.5 h. CIP was mixed with the TTIP
and agitated at ambient temperature. This mixture was refluxed at 120 °C for 2.5 h
(two-step reflux). Ethanol and hydrochloric acid were added to the refluxed mixture.
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H. Nishikiori et al.
The resulting sol was labeled Sol-CT-r0.5h2.5h. The gelation of these systems
occurred 9–10 days after starting the reaction at 60 °C.
Third, TTIP, ethanol, and hydrochloric acid were mixed and agitated in a glove
box filled with dry nitrogen gas at ambient temperature. This composition was made
in order to steadily produce the sol–gel reaction. The mixture was heated at 120 °C
for 0.5 h. CIP was mixed with the heated sol at ambient temperature, and then
heated at 120 °C for 2.5 h (two-step heating). The resulting sol was labeled Sol-CT-
h0.5h2.5h. The sol containing TTIP without CIP, Sol-T-h0.5h2.5h, was also
prepared. The gelation of these systems occurred 7–8 days after starting the reaction
at 60 °C.
All the resulting dry gels were treated with 50 cm3 of water at 80 °C, heated at
100 °C for 1 h, and then heated at 500 °C for 2 h. The photocatalyst powder
samples prepared from Sol-T-r3h, Sol-CT-r3h, Sol-CT-r0.5h2.5h, Sol-T-h0.5h2.5h,
and Sol-CT-h0.5h2.5h were labeled P-T-r3h, P-CT-r3h, P-CT-r0.5h2.5h, P-T-
h0.5h2.5h, and P-CT-h0.5h2.5h, respectively. The nomenclature of the sol and
photocatalyst powder samples is shown in Table 1.
Characterization of photocatalysts
The prepared photocatalyst samples were characterized by SEM observations using
a field emission scanning electron microscope (Hitachi SU8000), by XRD analysis
during Cu-Ka radiation using an X-ray diffractometer (Rigaku SmartLab), and by
XPS during Al-Ka radiation using an X-ray photoelectron spectrophotometer
(ULVAC PHI 5600). The size of the crystallites of each sample was estimated from
the full-width at half-maximum of the 25.3° peak in the XRD pattern using
Scherrer’s equation, D = 0.9k/bÁcos h. The specific surface areas, average pore
sizes, and total pore volumes of the samples were estimated by the BET and BJH
methods from the adsorption isotherms of nitrogen gas using a volumetric gas
adsorption instrument (BEL Japan, BELSORP-mini) [13, 14]. The spectroscopic
properties of the samples diluted with barium sulfate powder were analyzed by UV–
Vis diffuse reflectance spectroscopy using a UV–Vis near-IR spectrophotometer
(Shimadzu UV3150). The FTIR spectra of the samples were observed using an
FTIR spectrophotometer (Shimadzu IRPrestige-21).
Each 5.0-mg photocatalyst sample was added to 50 cm3 of an HQ ethanol
solution (1.0 9 10-5 mol dm-3) and agitated in the dark for 12 h. It was confirmed
Table 1 Labels of the prepared photocatalyst samples
Refluxed
Heated
1 Step (3 h)
2 Steps (0.5 h–2.5 h)
2 Steps (0.5 h–2.5 h)
TiO2
*-T-r3h
–
*-T-h0.5h2.5h
Cu–TiO2
*-CT-r3h
*-CT-r0.5h2.5h
*-CT-h0.5h2.5h
* = Sol: sol sample, * = P: powder sample
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Influence of Cu distribution on photocatalytic activity…
4817
that the adsorption of HQ had equilibrated. The amount of the HQ adsorbed on each
sample was estimated from the equilibrium concentration in the HQ ethanol solution
using the UV–Vis near-IR spectrophotometer.
Evaluation of photocatalysts
Each 5.0-mg photocatalyst sample was added to 50 cm3 of terephthalic acid
(3.0 9 10-3 mol dm-3) and sodium hydroxide (1.0 9 10-2 mol dm-3). The
suspensions were agitated in a glass beaker during near-UV light irradiation
(300–400 nm, peak at 352 nm) from a 4-W black light bulb (Toshiba FL4BLB) and
visible light irradiation from a 150-W xenon lamp (Hamamatsu Photonics C2577)
using 420-nm-short cutoff filter. The reaction between the terephthalic acid and
hydroxyl radical resulting from the photocatalytic water oxidation produces
(a)
(c)
(d)
(b)
300 nm
300 nm
300 nm
(e)
300 nm
300 nm
Fig. 2 SEM images of the photocatalyst powder samples. a P-T-r3h, b P-CT-r3h, c P-CT-r0.5h2.5h, d P-
T-h0.5h2.5h and e P-CT-h0.5h2.5h
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H. Nishikiori et al.
2-hydroxy terephthalic acid [15, 16]. The fluorescence spectra of the 2-hydroxy
terephthalic acid in the centrifuged solutions upon 312-nm excitation were obtained
as a function of the light irradiation time using a fluorescence spectrophotometer
(Shimadzu RF5300).
Results and discussion
Characterization of the photocatalysts
Figure 2 shows the SEM images of the prepared photocatalyst samples. The titania
and Cu-doped titania powders consisted of about 30–100-nm-sized particles, which
were aggregates of smaller primary particles. The particle size was similar to that
observed in a previous study [7], in which the particles prepared by the sol–gel
method were easily aggregated and formed the secondary particles. The crystallite
size was estimated to be 19–23 nm from the XRD patterns of the samples as shown
in Fig. 3 and Table 2. The differences in the particle size properties of the present
titania and Cu-doped titania samples were not significant. The specific surface area
and pore properties of the powder samples were also close as shown in Table 3. The
preparation procedure did not significantly influence the particle properties of the
samples.
The surface chemical property was examined by observing the hydrophilic
groups and water molecules. Figure 4 shows the FTIR spectra of the powder
samples in the range of the O–H stretching vibration. A broad band was observed in
the titania samples at around 3300 cm-1 assigned to the O–H stretching of the TiOH
group and adsorbed H2O [17–20]. This O–H band is not generally observed in the
crystalline titania. The Cu-doped titania samples exhibited a stronger O–H
stretching band at the higher wavenumber side assigned to the water molecules
physically adsorbed on the particle surface [18]. This was significant especially in
Anatase
5
4
3
2
1
20
25
30
35
40
45
50
2θ / degree
Fig. 3 XRD patterns of the photocatalyst powder samples. 1 P-T-r3h, 2 P-CT-r3h, 3 P-CT-r0.5h2.5h,
4 P-T-h0.5h2.5h and 5 P-CT-h0.5h2.5h
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Influence of Cu distribution on photocatalytic activity…
4819
Table 2 Crystallite sizes of the prepared catalysts estimated by XRD analysis
Sample
P-T-r3h
P-CT-r3h
P-CT-r0.5h2.5h P-T-h0.5h2.5h P-CT-h0.5h2.5h
22.7 2.4 19.5 1.8
Crystallite size (nm) 21.3 1.3 20.6 2.9 22.7 1.8
Table 3 Pore characteristics of the prepared catalysts estimated by BET and BJH methods from the
adsorption isotherms of nitrogen gas measured by the volumetric gas adsorption method
Sample
P-T-r3h P-CT-r3h P-CT-r0.5h2.5h P-T-h0.5h2.5h P-CT-
h0.5h2.5h
Specific surface area (m2g-1
Average pore size (nm)
)
96
94
90
85
83
6.7
0.14
6.1
0.14
6.0
0.13
6.5
0.12
6.1
Total pore volume (cm3 g-1
)
0.13
0.45
0.4
5
4
3
0.35
0.3
2
1
0.25
0.2
3900
3700
3500
3300
3100
2900
Wavenumber / cm-1
Fig. 4 FTIR spectra of the photocatalyst powder samples. 1 P-T-r3h, 2 P-CT-r3h, 3 P-CT-r0.5h2.5h, 4
P-T-h0.5h2.5h and 5 P-CT-h0.5h2.5h
the Cu-doped titania prepared by adding CIP after some reaction time of TTIP, i.e.,
P-CT-r0.5h2.5h and P-CT-h0.5h2.5h. The Cu doping increased the surface
hydrophilicity of the titania due to the increase in the number of hydroxyl groups
on the surface based on the XPS analysis in our previous study [7]. It is suggested
that the Cu2? was readily distributed on the titania particle surface in the Cu-doped
titania prepared by adding CIP after some reaction time of TTIP rather than by
simultaneously adding CIP and TTIP. This is because the titania particle precursors
were initially formed by the TTIP polymerization during the sol–gel process.
Figure 5 shows the UV–Vis diffuse reflectance spectra of the samples. The titania
samples exhibited a UV absorption at a wavelength shorter than ca. 400 nm,
whereas the Cu-doped titania samples exhibited UV and visible absorptions at a
wavelength shorter than ca. 600 nm. Therefore, the Cu-doped titania probably more
efficiently utilizes visible light than the titania. This result indicated that Cu2? was
definitely doped into the titania and formed impurity states just below the edge level
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H. Nishikiori et al.
0.6
0.4
0.2
0
1
2
3
4
5
350
400
450
500
550
600
Wavelength / nm
Fig. 5 UV–Vis diffuse reflectance spectra of the photocatalyst powder samples. 1 P-T-r3h, 2 P-CT-r3h,
3 P-CT-r0.5h2.5h, 4 P-T-h0.5h2.5h and 5 P-CT-h0.5h2.5h
of the conduction band of the titania because the shifts in the absorption edge for the
samples correspond to the amount of the doped Cu2? [6, 8, 9].
The Cu-doped titania prepared by simultaneously adding CIP and TTIP, i.e.,
P-CT-r3h, exhibited a stronger absorption at around 400 nm due to the oxygen
defects as reported in a previous paper [7]. It is suggested that the Cu-doped titania
prepared by adding CIP after some reaction time of TTIP, i.e., P-CT-r0.5h2.5h and
P-CT-h0.5h2.5h, had a lower number of oxygen defects because CIP reacted only
on the surface of the titania nanoparticles. The visible absorption mainly originates
from the electron transition from the valence band to the Cu impurity states.
Figure 6 shows the XPS spectra related to the binding energy of the Ti 2p, O 1s,
and Cu 2p electrons of the samples. The relative Cu amount in the Cu-doped titania
samples was confirmed to be around 0.1 % of the Ti amount. The Ti 2p spectra of
the titania sample exhibited peaks at 464.3 and 458.7 eV, assigned to the typical
Ti4? [7, 21]. The Ti 2p peak was slightly shifted to the higher energy side by the Cu
doping except for P-CT-r3h because the Ti–O–Cu bonds were formed and caused an
electronic interaction between Ti4? and Cu2?, whose Pauling electronegativities are
1.54 and 1.90, respectively [22–25]. The Cu doping effect was low for P-CT-r3h
using this preparation condition with this Cu content as shown in a previous study
[7]. As Cu2? is homogeneously dispersed in TiO2 and substituted for Ti4? in the
networks, a charge transfer from the Ti4? to the Cu2? is expected to occur. This is
significantly enhanced on the particle surface whereas the charges are delocalized in
the bulk. The O 1s peaks were observed at 529.8 eV in the titania samples. This
peak was also slightly shifted to the higher energy side by the Cu doping except for
P-CT-r3h. The shift is because the increase in the amount of oxygen defects,
compared to that of the lattice oxygen, caused the formation of hydroxyl oxygen and
the adsorption of oxygen [22]. This species was easily formed on the particle
surface of the Cu-doped titania. The Cu 2p peaks were observed at 934, 943, 955,
and 962 eV in only the samples containing 10 mol% Cu from the previous study
[7]. These peaks were assigned to the Cu2? species [26]. In the present study, the
123

Influence of Cu distribution on photocatalytic activity…
4821
(a)
1
2
3
4
5
470
465
460
455
450
Binding enrgy / eV
(b)
1
2
3
4
5
534
533
532
531
530
529
528
527
526
Binding enrgy / eV
(c)
1
2
3
4
5
965
955
945
935
925
Binding enrgy / eV
Fig. 6 XPS spectra of a Ti 2p, b O 1s, and c Cu 2p electrons for the photocatalyst powder samples.
1 P-T-r3h, 2 P-CT-r3h, 3 P-CT-r0.5h2.5h, 4 P-T-h0.5h2.5h and 5 P-CT-h0.5h2.5h
weak Cu 2p peak was observed at only around 934 eV in the Cu-doped titania
samples due to the low amount of Cu. The XPS spectra did not make a significant
difference in the structure and electronic properties of the samples prepared by
reflux and heating, P-CT-r0.5h2.5h and P-CT-h0.5h2.5h. It was checked that the
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H. Nishikiori et al.
XPS spectra did not significantly change during the photocatalytic experiments. The
Cu2? was not dissolved into the water phase. The XPS measurements indicated that
the Cu-doped titania prepared by adding CIP after some reaction time of TTIP
dispersed Cu2? as an impurity on the titania surface, on which the Ti–O–Cu bonds
were formed.
Photocatalytic activity
It is difficult to visually observe the distribution of Cu in the titania at an atomic
level by general analyses. We tried to estimate the Cu distribution on the titania
particle surface by the metal complex formation of HQ. The deprotonated ion,
quinolate, is a typical ligand and readily forms a metal complex not only in
solutions, but also on particle surfaces [27–32]. Figure 7 shows the UV–Vis diffuse
reflectance spectra of the samples adsorbing HQ. The absorption band at around
380 nm originated from the metal complex of quinolate, i.e., the Ti and Cu
complexes [31, 32]. The Cu-doped titania prepared by adding CIP after the longer
reaction time of TTIP, i.e., P-CT-r0.5h2.5h and P-CT-h0.5h2.5h, clearly adsorbed a
higher amount of the metal quinolate complex. The adsorption amount of HQ is
shown in Table 4. This result indicated that the Cu2? on the titania particle surface
was strongly bonded to the quinolate whereas that in the bulk slightly interacted
with quinolate due to the electron affinity or negativity of Cu being higher than that
of Ti [31].
Figure 8 shows the time course of the fluorescence intensity of 2-hydroxy
terephthalic acid with the UV and visible irradiations using the samples in order to
examine the produced OH radicals. The OH radical production rate during the UV
and visible light irradiations for the Cu-doped titania prepared by simultaneously
adding CIP and TTIP, i.e., P-CT-r3h, was higher than that for the titania. This
indicated that the efficient electron transfer from the titania to the Cu2? enhanced
the photocatalytic activity due to the high Cu2? dispersion in the TiO2 and
0.2
1
2
3
0.1
4
5
0
300
400
500
600
Wavelength / nm
Fig. 7 UV–Vis diffuse reflectance spectra of the photocatalyst powder samples. 1 P-T-r3h, 2 P-CT-r3h,
3 P-CT-r0.5h2.5h, 4 P-T-h0.5h2.5h, and 5 P-CT-h0.5h2.5h, adsorbing HQ
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Influence of Cu distribution on photocatalytic activity…
Table 4 HQ adsorption amount of the prepared catalysts
4823
Sample
P-T-r3h
P-CT-r3h
P-CT-r0.5h2.5h P-T-h0.5h2.5h P-CT-h0.5h2.5h
Adsorption amount 25.1 4.6 29.2 6.1 37.4 .8.0
(10-6 mol g-1
27.7 5.5
42.0 10.0
)
(a)
1
2
3
4
5
600
400
200
0
0
20
40
60
80
100
120
Irradiation time / min
10
8
(b)
1
2
3
4
5
6
4
2
0
0
20
40
60
80
100
120
Irradiation time / min
Fig. 8 Time course of the fluorescence intensity of 2-hydroxy terephthalic acid in order to detect
hydroxyl radicals produced during a UV and b visible light irradiations using the photocatalyst powder
samples. 1 P-T-r3h, 2 P-CT-r3h, 3 P-CT-r0.5h2.5h, 4 P-T-h0.5h2.5h, and 5 P-CT-h0.5h2.5h
formation of effective Ti–O–Cu bonds as shown in our previous study [7]. The Cu-
doped titania prepared by adding CIP after the longer reaction time of TTIP, i.e.,
P-CT-r0.5h2.5h and P-CT-h0.5h2.5h, clearly exhibited a much faster reaction rate
and higher activity for the OH radical production.
The impurity states of Cu2? on the titania surface effectively trapped the
electrons in the conduction band of the titania during the UV light irradiation and
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H. Nishikiori et al.
suppressed charge recombination of the electrons and holes. The visible light
irradiation induced the direct electron transition from the valence band to the Cu
impurity states. In the sample, the high Cu distribution on only the titania surface
rather than inside the titania bulk led to its high photocatalytic activity because
reduction of the oxygen species is enhanced on the surface adsorption sites. This
reduction readily consumed the electrons, prevented the charge recombination, and
efficiently produced OH radicals.
Conclusions
Cu-doped titania was prepared from the sols containing TTIP and CIP by a sol–gel
method. The Cu distribution in the titania was controlled by varying the reaction
time of TTIP at which CIP was added to the sol. The Cu distribution on the titania
particle surface was estimated by the metal complex formation of HQ. The UV and
visible photocatalytic activities of the samples were examined by hydroxyl radical
production. A higher distribution of the Cu on the titania surface was achieved in the
sample prepared by adding CIP after some TTIP reaction time. The high distribution
of Cu on the titania surface enhanced the electron transfer from the titania to the
Cu2? and the photocatalytic activity. The impurity states of Cu2? on the titania
surface effectively trapped the electrons in the conduction band of the titania and
received the electrons in the valence band upon photoexcitation, and consequently,
suppressed charge recombination of the electrons and holes. In the sample, the high
Cu distribution on only the titania surface rather than inside the titania bulk led to its
high photocatalytic activity due to its accessibility from the surroundings.
References
1. A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C Photochem. Rev. 1, 1 (2000)
2. D. Chatterjee, S. Dasgupta, J. Photochem. Photobiol. C Photochem. Rev. 6, 186 (2005)
´
´
3. G. Colon, M. Maicu, M.C. Hidalgo, J.A. Navıo, Appl. Catal. B Environ. 67, 41 (2006)
4. E. Celik, Z. Gokcen, N.F.A. Azem, M. Tanoglu, O.F. Emrullahoglu, Mater. Sci. Eng. B 132, 258
(2006)
5. I.H. Tseng, J.C.S. Wu, H.Y. Chou, J. Catal. 221, 432 (2004)
´
´
6. R. Lopez, R. Gomez, M.E. Llanos, Catal. Today 148, 103 (2009)
7. H. Nishikiori, T. Sato, S. Kubota, N. Tanaka, Y. Shimizu, T. Fujii, Res. Chem. Intermed. 38, 595
(2012)
8. H. Irie, S. Miura, K. Kamiya, K. Hashimoto, Chem. Phys. Lett. 457, 202 (2008)
9. H. Irie, K. Kamiya, T. Shibanuma, S. Miura, D.A. Tryk, T. Yokoyama, K. Hashimoto, J. Phys. Chem.
C 113, 10761 (2009)
10. R.C. Mehrotra, J. Non-Cryst. Solids 100, 1 (1988)
11. T. Monde, H. Kozuka, S. Sakka, Chem. Lett. 17, 287 (1988)
12. T. Satoh, M. Ogawa, M. Kondo, J. Ceram. Soc. Jpn. 106, 1151 (1998)
13. S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60, 309 (1938)
14. E.P. Barrett, L.G. Joyner, P.H. Halenda, J. Am. Chem. Soc. 73, 373 (1951)
15. T. Hirakawa, Y. Nosaka, Langmuir 18, 3247 (2002)
16. T. Hirakawa, K. Yawata, Y. Nosaka, Appl. Catal. A-Gen. 325, 105 (2007)
17. M.L. Galzada, L. Delolmo, J. Non-Cryst. Solids 121, 413 (1990)
18. S. Ben Amor, G. Baud, J. P. Besse, M. Jacquet, Mater. Sci. Eng. B 47, 110 (1997)
123

Influence of Cu distribution on photocatalytic activity…
4825
´
´
´
19. M. Ivanda, S. Music, S. Popovic, M. Gotic, J. Mol. Struct. 480481, 645 (1999)
20. J. Trimboli, M. Mottern, H. Verweij, P.K. Dutta, J. Phys. Chem. B 110, 5647 (2006)
21. J. Yu, X. Zhao, Q. Zhao, Mater. Chem. Phys. 69, 25 (2001)
22. B. Xin, P. Wang, D. Ding, J. Liu, Z. Ren, H. Fu, Appl. Surf. Sci. 254, 2569 (2008)
23. G. Li, N.M. Dimitrijevic, L. Chen, T. Rajh, K.A. Gray, J. Phys. Chem. C 112, 19040 (2008)
24. Q. Fang, M. Meier, J.J. Yu, Z.M. Wang, J.Y. Zhang, J.X. Wu, A. Kenyon, P. Hoffmann, I.W. Boyd,
Mater. Sci. Eng. B 105, 209 (2003)
25. M. You, T.G. Kim, Y.M. Sung, Cryst. Growth Des. 10, 983 (2010)
26. W. Zhang, Y. Li, S. Zhu, F. Wang, Catal. Today 93, 589 (2004)
27. W.D. Johnston, H. Freiser, J. Am. Chem. Soc. 74, 5239 (1952)
28. W.E. Ohnesorge, J. Inorg. Nucl. Chem. 29, 485 (1967)
29. A.V. Karyakin, T.S. Sorokina, L.I. Anikina, T.G. Akimova, M.G. Ezernitskaya, Dokl. Phys. Chem.
Translation Phys. Chem. Sect. Dokl. Akad. Nauk SSSR 241, 667 (1979)
30. H. Nishikiori, K. Teshima, T. Fujii, Photochem. Photobiol. Sci. 11, 1164 (2012)
31. H. Nishikiori, K. Todoroki, D. Natori, R.A. Setiawan, K. Miyashita, T. Fujii, Chem. Lett. 42, 556
(2013)
32. H. Nishikiori, K. Todoroki, R.A. Setiawan, K. Teshima, T. Fujii, H. Satozono, Langmuir 31, 964
(2015)
123