Journal of Photochemistry and Photobiology A: Chemistry 163 (2004) 3744

Comparison of Ag deposition effects on the photocatalytic activity of

nanoparticulate TiO2 under visible and UV light irradiation
Hyung Mi Sung-Suh, Jae Ran Choi, Hoe Jin Hah, Sang Man Koo , Young Chan Bae
Department of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, South Korea
Received 11 February 2003; received in revised form 21 May 2003; accepted 20 October 2003

Abstract
We investigated the photocatalytic degradation of rhodamine B (RB) dye in the aqueous suspensions of TiO2 (17 nm) and Ag-deposited
TiO2 nanoparticles under visible and UV light irradiation in order to evaluate and distinguish various effects of the Ag deposition on the
TiO2 photocatalytic activity. The TiO2 and AgTiO2 photocatalysts were characterized by XRD, TEM, XPS, UV-visible absorption and
photon correlation spectroscopy. For comparison, the RB photodegradation was carried out in Degussa P25 TiO2 and Ag-deposited P25
suspensions under the same condition. In the RB/AgTiO2 system, Ag deposits significantly enhanced the RB photodegradation under
visible light irradiation whereas the RB photodegradation under UV irradiation was slightly enhanced. The significant enhancement in the
AgTiO2 photoactivity under visible light irradiation can be ascribed to simultaneous effects of Ag deposits by both acting as electron
traps and enhancing the RB adsorption on the AgTiO2 surface.
2004 Elsevier B.V. All rights reserved.
Keywords: TiO2 ; Ag deposition; Rhodamine B; Charge separation; Photodegradation; Visible light

1. Introduction
Photocatalytic degradation and mineralization of organic
and inorganic pollutants on the semiconductor TiO2 have
been extensively studied in order to solve environmental
problems relating to wastewaters and polluted air [18].
Among various metal oxide semiconductors, TiO2 has been
the focus of photocatalysts under UV irradiation because of
its physical and chemical stability, low cost, ease of availability, non-toxicity, and electronic and optical properties.
However, there are still basic problems to be solved for
improving the photocatalytic activity of TiO2 . Because the
semiconductor TiO2 has a high band gap (Eg > 3.2 eV), it
is excited only by UV light ( < 388 nm) to inject electrons
into the conduction band and to leave holes in the valence
band [9]. Thus, this practically limits the use of sunlight or
visible light as an irradiation source in photocatalytic reactions on TiO2 [10]. In addition, the high rate of electronhole
recombination on TiO2 particles results in a low efficiency
of photocatalysis [11]. For the purpose of overcoming these
Corresponding authors. Tel.: +82-2-2290-0527
(S.M. Koo)/+82-2-2290-0529 (Y.C. Bae); fax: +82-2-2281-4800
(S.M. Koo)/+82-2-2296-0568 (Y.C. Bae).
E-mail addresses: clarkhah@ihanyang.ac.kr, sangman@hanayng.ac.kr
(S.M. Koo), ycbae@hanayng.ac.kr (Y.C. Bae).

1010-6030/$ see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/S1010-6030(03)00428-3

limitations of TiO2 as a photocatalyst, numerous studies

have been recently performed to enhance electronhole
separation and to extend the absorption range of TiO2 into
the visible range. These studies include doping metal ions
into the TiO2 lattice [12,13], dye photosensitization on
the TiO2 surface [1418], and deposition of noble metals
[1923].
In particular, noble metal-modified semiconductor
nanoparticles become of current importance for maximizing
the efficiency of photocatalytic reactions. The noble metals
such as Pt [19,20] and Au [21,22] deposited or doped on
TiO2 have the high Schottky barriers among the metals
and thus act as electron traps, facilitating electronhole
separation and promotes interfacial electron transfer process [2427]. Most studies of noble metal-modified TiO2
photocatalysts have focused on the details of the photoinduced electron transfer from the conduction band of
UV-irradiated TiO2 to noble metals for improving the photocatalytic activity of TiO2 under UV irradiation. Only a
few studies have been reported on visible light-induced
photocatalytic reactions using noble metal-modified TiO2
[28,29].
It is expected noble metals deposited on TiO2 may show
different effects on the photocatalytic activity depending on
the wavelength of light illuminating photoreaction systems
because the photocatalytic mechanism under UV irradiation

38

H.M. Sung-Suh et al. / Journal of Photochemistry and Photobiology A: Chemistry 163 (2004) 3744

is generally different from that under visible light irradiation. For example, in the photocatalytic degradation of dye
on noble metal-deposited TiO2 in the presence of O2 , TiO2
acts as a photosensitizer as well as a photocatalyst under
UV irradiation, but a dye acts as a photosensitizer as well
as a degraded substrate under visible light irradiation. The
charge separation within TiO2 particles and subsequent electron transfer to O2 for producing active oxygen radicals (e.g.
O2 , OOH, OH) [1517] are important to the efficiency
of the dye photodegradation under both UV and visible light
irradiation. The charge separation and subsequent electron
transfer may be enhanced by noble metal deposition on TiO2
particles, thereby improving the TiO2 photocatalytic activity
under both UV and visible light irradiation. Moreover, surface plasmon resonances of noble metal particles, which can
be excited by visible light, increase the electric field around
metal particles and thus enhance the surface electron excitation and electronhole separation on noble metal-deposited
TiO2 particles [3032].
Consequently, noble metals doped or deposited on TiO2
are expected to show various effects on the photocatalytic
activity of TiO2 by the different mechanisms as follows
that may act separately or simultaneously depending on
the photoreaction conditions: noble metals (i) enhance the
electronhole separation by acting as electron traps, (ii) extend the light absorption into the visible range and enhance
surface electron excitation by plasmon resonances excited
by visible light, and (iii) modify the surface properties of
photocatalyst.
Most studies of noble metal deposition on TiO2 have
been focused on group VIII metals using UV-irradiated photodegradation. Besides, some studies have reported contradictory results of the photodegradation of organic molecules
on TiO2 modified by Pt [19,20,33,34] or Ag [23,31] under
UV irradiation. This is probably due to the type of TiO2 used,
photoreaction medium, the nature of organic molecules and
their redox processes, and the metal content and dispersion
[19]. Very few studies have concerned effects of the Ag deposition on the visible light-induced photocatalysis on TiO2
[35]. Moreover, there has been no report to systematically
investigate and distinguish the different roles of Ag deposits
in the TiO2 photocatalytic behavior under UV and visible
light irradiation.
In this study, we examined the photocatalytic degradation
of the rhodamine B (RB) dye in the aqueous suspensions of
TiO2 and Ag-deposited TiO2 nanoparticles under visible and
UV light irradiation in order to evaluate and distinguish the
various effects of Ag deposits on the TiO2 photocatalytic activity. The TiO2 nanoparticles (17 nm in size) was synthesized by the peptization method, and Ag metals were loaded
on TiO2 by photocatalytic deposition process. The activities
of the synthesized TiO2 and Ag-deposited TiO2 were also
compared to those of the commercial Degussa P25 TiO2 and
Ag-deposited P25 in the same photocatalytic condition. The
experimental results are discussed by the different roles of
Ag deposits in the RB photodegradation on TiO2 photocat-

COOH

H3CH2C

N
CH2CH3

ClN+ CH2CH3
CH2CH3

Fig. 1. The molecular structure of rhodamine B dye.

alyst through different mechanisms under visible and UV

light irradiation.

2. Experimental
2.1. Materials
Titanium isopropoxide (+97%) and silver nitrate
(AgNO3 , analytical grade) were purchased from Aldrich
and used as titanium and silver sources for the preparation of TiO2 and Ag/TiO2 photocatalysts. A commercial
form of TiO2 (P25, ca. 80% anatase, 20% rutile; BET
area, ca. 50 m2 /g; primary size 2530 nm, agglomerate size 100 nm) from Degussa was used for the comparison of the photocatalytic activity. Rhodamine B dye
(N,N,N ,N -tetraethylrhodamine, RB) obtained from Junsei
was of analytical reagent grade and used without further
purification. The structure of RB is shown in Fig. 1. Deionized and doubly distilled water was used for the preparation
of all solutions.
2.2. Preparation of TiO2 and AgTiO2 photocatalysts
The nanoparticulate TiO2 suspension (denoted as TiO2
nanosol) was prepared by the peptization method. First, titanium isopropoxide (7.38 ml) was added dropwise to excess water with vigorous stirring at room temperature. The
resulting gel was kept stirring for 1 h, and the solvent was
then removed by filtration to leave a coagulated TiO2 powder. In order to peptize the surface of the coagulated TiO2
[36], the dry powder was added to water (40 ml) with vigorous stirring as adjusting pH to 3 by the addition of HNO3 .
The mixture was then heated and aged at 60 C for 6 h. The
obtained TiO2 nanosol (5 wt.% in TiO2 , pH 3) was transparent and stable for several months. The nanosol was further
diluted with water to desired concentrations for the photocatalytic experiments. The TiO2 concentration was 0.4 wt.%
for the RB photodegradation under visible light irradiation
and 0.04 wt.% under UV irradiation.
The suspension of metallic silver-deposited TiO2 nanoparticles (AgTiO2 nanosol) was prepared by a photocatalytic
deposition process [37]. A desired volume of aqueous
AgNO3 solution (1.17 102 M) was added to the diluted

H.M. Sung-Suh et al. / Journal of Photochemistry and Photobiology A: Chemistry 163 (2004) 3744

TiO2 nanosol (50 ml). The mixture solution was then irradiated with UV light (100 W mercury lamp from Philips)
for 30 min. The atomic ratio of Ag/Ti was calculated to be
in a range of 110%.

2.4. Measurements of photocatalytic activities

To investigate the effects of silver deposits on the photocatalytic activity of TiO2 , the oxidative photodegradation
of RB was carried out in the aerated TiO2 and AgTiO2
nanosols under visible and UV light irradiation, respectively.
A 100 ml pyrex beaker was used as a batch photoreactor. The
TiO2 or Ag/TiO2 nanosol (50 ml) containing RB (105 M)
was transferred into the photoreactor, and aerated with
stirring for 30 min in the dark. The RB/nanosol was then
irradiated with the lamp located above the reactor. The light
sources purchased from Philips were a 200 W halogen lamp
for visible light and a 100 W mercury lamp for UV. At given
irradiation time intervals, a 1 ml-aliquot was taken from the
RB/nanosol and analyzed by UV-visible absorption spectroscopy to monitor the degree of the RB photodegradation.
The RB concentration was determined from the absorbance
at a wavelength of 554 nm (max ). To compare the effects of
silver deposits, the RB photodegradation was also performed
in the commercial Degussa P25 TiO2 and Ag-deposited
P25 (Ag-P25) suspensions at the same experimental
condition.

Intensity

The crystalline phase of the synthesized TiO2 nanoparticles was analyzed by X-ray powder diffraction (XRD)
pattern using a Rigaku D/RAD-C diffractometer with Cu
K radiation ( = 1.5418 nm) at 40 kV and 100 mA. The
average particle size of TiO2 in the synthesized nanosol
was measured by photon correlation spectroscopy (PCS)
with an argon ion laser (Lexel Laser Inc. Model 95-2) operated at 514.4 nm and 200 mW. The TiO2 nanosol was
appropriately diluted with water for the light scattering
experiment. The light scattered by particles was detected
at 90 angle in respect to the incident beam. The optical
property of the TiO2 and Ag/TiO2 nanoparticles was studied by UV-Vis absorption spectroscopy using a SCINCO
S-2150 spectrophotometer. During the RB photodegradation, the RB concentration was also determined by
the absorption spectroscopy. The Ag/TiO2 particles were
characterized by transmission electron microscopy (TEM)
using a JEOL EM-2000EX II transmission electron microscope by applying a drop of the nanosol sample to the
carbon-coated copper grid. To verify the presence of metallic silver deposited on TiO2 , a film made of the AgTiO2
sample was examined by X-ray photoelectron spectroscopy
(XPS) using a VG Scientific ESCALAB MKII spectrometer with Mg K line at 15 kV and 10 mA. The binding
energy scale was calibrated to 284.6 eV for the main C 1s
peak.

A
A

20

30

40

50

60

70

2
Fig. 2. XRD pattern of the synthesized TiO2 sample. A: anatase phase.
The peak marked with an arrow corresponds to the brookite phase.

3. Results and discussion

3.1. Characterization of TiO2 and AgTiO2 photocatalysts
The aqueous TiO2 nanosol synthesized in this experiment
was transparent and stable for several months. The crystalline phase of the TiO2 sample was analyzed by XRD, and
its XRD pattern is shown in Fig. 2. The TiO2 sample consisted of mainly anatase with minor brookite phase [38] as
indicated in the XRD pattern. The particle size of the TiO2
sample was measured by PCS, and its particle size distribution is shown in Fig. 3. The TiO2 sample had an average
particle size of 17 nm with a narrow size distribution, indicating the TiO2 nanoparticles were well dispersed in the
synthesized nanosol although there were minor aggregates
of ca. 50 nm, as can be seen in Fig. 3.
The valence state of silver in the AgTiO2 sample was
examined by XPS. The XP spectrum in Fig. 4 shows the
characteristic Ag 3d5/2 peak that has a binding energy of
368 eV with a 6 eV splitting of the 3d doublet [39]. XPS
peaks corresponding to Ag+ ion were not found. This result
confirms the presence of metallic silver deposits on the TiO2
(17 nm)

Intensity

2.3. Analytical methods

39

50

500

5000

Diameter of Particles (nm)

Fig. 3. The size distribution of the synthesized TiO2 nanoparticles determined by photon correlation spectroscopy.

40

H.M. Sung-Suh et al. / Journal of Photochemistry and Photobiology A: Chemistry 163 (2004) 3744
1.5

Ag 3d

RB (max)

Absorbance

Intensity (a.u.)

5000

4000

3000

1.0

0
2
4
6
8
10 h

TiO2

0.5

2000

380

375

370

365

360

0.0
300

Binding Energy (eV)

400

500

600

Wavelength (nm)
Fig. 4. XP spectrum for UV-irradiated AgTiO2 sample.

100

RB Degradation (%)

surface of the AgTiO2 sample irradiated with UV light. The

size of Ag deposits on TiO2 was determined from the TEM
image shown in Fig. 5. The size varied with the Ag content
in the TiO2 nanosol. For the 2 at.% AgTiO2 sample, Ag
deposits were well dispersed on the TiO2 particles with an
average particle size of 24 nm as shown in Fig. 5. At higher
silver content, formation of large Ag particles (>100 nm)
was observed in the TEM image as similar to the previous
report [23]. The Ag content and the particle size of Ag
deposits may affect the photocatalytic activity of AgTiO2
as discussed in the following section.

Fig. 6. Absorption spectral changes of RB in the TiO2 nanosol as a

function of irradiation time (visible light). The initial concentration (C0 )
of RB was 1 105 M, and the TiO2 content was 0.4 wt.%.

3.2. Comparison of photocatalytic activities of TiO2 and

AgTiO2

60
40
20
0
0

10

12

Ag/Ti (atomic %)
Fig. 7. Photocatalytic degradation of RB in AgTiO2 nanosols as a
function of the Ag content under visible light (4 h irradiation). The initial
concentration (C0 ) of RB was 1 105 M, and the TiO2 content was
0.4 wt.%.

1.0

0.8

C/C0 of R B

3.2.1. RB photodegradation under visible light irradiation

Fig. 6 shows the spectral changes of RB in the TiO2
nanosol under visible light irradiation. A decrease in the
absorbance at 552 nm reflects the degradation of RB on
the TiO2 photocatalyst, thereby used as a measure of the
photocatalytic activity. Compared to the pure TiO2 , the
AgTiO2 nanosol exhibited a significant increase in the RB
photodegradation rate as shown in Figs. 7 and 8. It was
found that the 2% Ag content was optimum to achieve the
highest efficiency of the RB photodegradation for the TiO2

80

0.6

TiO2

0.4

P25
Ag/P25

0.2

Ag/TiO2
0.0
0

Irradiation Time (h)

Fig. 5. TEM micrograph of 2 at.% AgTiO2 sample.

Fig. 8. Comparison of the RB photodegradation in TiO2 , AgTiO2 ,

P25, and Ag-P25 nanosols under visible light irradiation. The TiO2 and
AgTiO2 nanosols were synthesized in this study. Ag-P25 was prepared
from the commercial Degussa P25 titania. The initial concentration (C0 )
of RB was 1 105 M. The TiO2 content was 0.4 wt.%. The Ag contents
was 2 at.%.

H.M. Sung-Suh et al. / Journal of Photochemistry and Photobiology A: Chemistry 163 (2004) 3744

Absorbance

1.5

1.0

max

irradiation
0
2
4
6
8h

0.5

0.0

450

(a)

500

550

600

Wavelength (nm)
1.5

Absorbance

nanosols (Fig. 7). More Ag content could be detrimental

to the photodegradation efficiency. It may be explained
that at the Ag content below its optimum, the Ag particles
deposited on the TiO2 surface can act as electronhole separation centers [2527]. The electron transfer from the TiO2
conduction band to metallic silver particles at the interface
is thermodynamically possible because the Fermi level of
TiO2 is higher than that of silver metals [19]. This results in
the formation of Schottky barrier at metalsemiconductor
contact region, which improves the charge separation and
thus enhances the photocatalytic activity of TiO2 . On the
contrary, at the Ag content above its optimum, the Ag particles can also act as recombination centers, thereby decreasing the photocatalytic activity of TiO2 . It has been reported
that the probability for the hole capture is increased by the
large number of negatively charged Ag particles on TiO2
at high Ag content, which reduces the efficiency of charge
separation [19,23,29].
Under visible light irradiation, the AgTiO2 sample
showed a 30% increase in the RB photodegradation as compared to the pure TiO2 (Fig. 8). In addition, the AgTiO2
sample showed a higher photodegradation rate than both
P25 and Ag-P25, although the pure TiO2 sample had a
slightly lower rate than P25 as shown in Fig. 8. By contrast,
Ag-P25 exhibited only a 10% increase in the photodegradation as compared to P25. Accordingly, the Ag deposition
was more beneficial to the photocatalytic activity of the
synthesized TiO2 than to P25 for the visible light-irradiated
RB photodegradation. This probably results from a smaller
particle size (17 nm) of the TiO2 sample as compared
the size (37 nm) of P25 analyzed by TEM and XRD
[40]. The synthesized TiO2 nanosol with smaller particles
size and well dispersion is expected to provide a larger
surface area for the RB adsorption and Ag dispersion on
the TiO2 surface, resulting in the higher photocatalytic
activity of the AgTiO2 sample as compared to that of
Ag-P25.
The enhanced adsorption of RB on the AgTiO2 surface can be inferred from a blue shift in max of the RB
absorption spectrum during its photodegradation as shown
in Fig. 9. The AgTiO2 sample showed a large decrease
in the absorbance simultaneously with a large blue shift in
max during visible light irradiation. According to the previous results reported by Watanabe et al. [41] and Zhao and
coworkers [1517], the blue shift in max of RB is caused
by de-ethylation of RB occurring in competition with the
degradation of the RB chromophore ring under visible light
irradiation in CdS or TiO2 suspensions. De-ethylation of
RB is mainly a surface occurring reaction, whereas the
RB degradation is predominantly a solution bulk process.
RB is the N,N,N ,N -tetraethylated rhodamine molecule
showing max at 552 nm. N,N,N -Trietylated rhodamine has
max at 539 nm, N,N -diethylated rhodamine at 522 nm,
and N-ethylated rhodamine at 510 nm [41]. Accordingly,
the large blue shift in max of RB in the AgTiO2 nanosol
in Fig. 9(b) results from the significant de-ethylation of

41

1.0

max

0.5

0.0
450

(b)

irradiation
0
1
2
3
4h

500

550

600

Wavelength (nm)

Fig. 9. Absorption spectral changes of RB in the (a) TiO2 nanosol and

(b) 2 at.% AgTiO2 nanosol as a function of irradiation time (visible
light). The dotted arrows indicate the blue shift in max of RB during the
photodegradation. The initial concentration (C0 ) of RB was 1 105 M,
and the TiO2 content was 0.4 wt.%.

RB occurring on the surface of AgTiO2 simultaneously

with the degradation of the RB chromophore ring during
visible light irradiation. This faster de-ethylation of RB on
the AgTiO2 surface may indicate that RB molecules can
be more adsorbed on the AgTiO2 surface than the TiO2
surface, since de-ethylation of RB has been reported to be
mainly a surface occurring reaction [16].
To compare the adsorption of RB molecules in TiO2
and AgTiO2 nanosols, the change in RB absorption spectra was monitored before and after adding the TiO2 and
AgTiO2 samples in the dark as shown in Fig. 10. The decrease of the RB absorbance in Fig. 10 indicates that RB
molecules acting as a photosensitizer under visible light irradiation are more pre-adsorbed on the Ag/TiO2 surface than
on the TiO2 surface. About 5.4% of RB molecules were adsorbed on Ag/TiO2 nanoparticles, whereas only 2.5% of RB
molecules were adsorbed on TiO2 nanoparticles. Therefore,
under visible light irradiation, electron transfer from excited
RB molecules to TiO2 particles becomes more efficient in
the Ag/TiO2 nanosol, leading to the higher photocatalytic
activity of AgTiO2 .

42

H.M. Sung-Suh et al. / Journal of Photochemistry and Photobiology A: Chemistry 163 (2004) 3744

1.4

1.0

1.40

(a)

1.36

(b)

1.32

(c)

(a)
(b)
(c)

0.8
C/C0 of RB

1.2

Absorbance

1.0

1.44

1.28

0.8

1.24

0.6

1.20
548

552

556

560

0.6
0.4

Visible

0.4

0.2

0.2

UV
0.0
200

250

300

350

400

450

500

550

600

Wavelength (nm)

Fig. 10. Absorption spectra of RB before and after adsorption in TiO2 and
AgTiO2 nanosol for 12 h in the dark: (a) initial RB solution (1105 M),
(b) after adsorption on TiO2 and (c) after adsorption on AgTiO2 . The
TiO2 content was 0.4 wt.%. The Ag content was 2 at.%.

3.2.2. RB photodegradation under UV irradiation

Under UV irradiation, only 10% more RB photodegradation was achieved in the AgTiO2 sample as compared
to that in the TiO2 sample (Fig. 11). Ag-P25 showed also
10% enhancement in the RB photodegradation than P25.
The AgTiO2 sample still showed a lower photodegradation
rate than both P25 and Ag-P25, which is contrary to the
result under visible light irradiation. The photocatalytic activities of both TiO2 and AgTiO2 were significantly lower
than that of P25, probably resulting from the lower crystallinity of the TiO2 sample that was not calcined in this experimental condition. This means that the Ag deposition on
the TiO2 sample does not overcome the lower crystallinity
of the TiO2 sample, thus slightly enhancing the RB photodegradation under UV irradiation.
Consequently, the beneficial effects of Ag deposition on
the photocatalytic activity of the TiO2 sample was more sig1.0

C/Co of RhB

0.8
0.6
TiO2

0.4

Ag/TiO2
P25

0.2

Ag/P25
0.0
0

10

20

30

0.0

40

50

UV Irradiation Time (min)

Fig. 11. Comparison of the RB photodegradation in TiO2 , AgTiO2 , P25,
and Ag-P25 nanosols under UV irradiation. The initial concentration (C0 )
of RB was 1 105 M. The TiO2 content was 0.04 wt.%. The Ag content
was 2 at.%.

Irradiation time (h)

Fig. 12. Comparison of the RB photodegradation in the TiO2 and 2 at.%
AgTiO2 nanosols under UV and visible light irradiation. The initial
concentration (C0 ) of RB was 1 105 M.

nificant in the RB photodegradation under visible light irradiation than under UV irradiation, as can be seen in Fig. 12.
Also, the Ag deposition was more beneficial to the synthesized TiO2 sample than to P25 in the RB photodegradation
under visible light irradiation (Fig. 9). These results may be
ascribed to the different roles of Ag deposits in affecting the
photocatalytic behaviors of TiO2 under UV and visible light
irradiation as discussed in the following.
3.3. Roles of Ag nanoparticles deposited on TiO2
The photocatalytic activity of TiO2 for the oxidative
degradation of RB may be enhanced by the Ag deposition
through the following mechanisms:
(1) Ag nanoparticles deposited on TiO2 act as electron traps,
enhancing the electronhole separation and the subsequent transfer of the trapped electron to the adsorbed
O2 acting as an electron acceptor [2427].
(2) More RB molecules are adsorbed on the surface of
AgTiO2 than on the TiO2 surface, enhancing the photoexcited electron transfer from the visible-light sensitized RB to the conduction band of and subsequently increasing the electron transfer to the adsorbed O2 (Figs. 9
and 10).
(3) The surface plasmon resonance of Ag particles is excited
by visible light, facilitating the excitation of the surface
electron and interfacial electron transfer [3032].
In addition to these different roles of Ag deposits, the
RB photodegradation in aqueous TiO2 suspension has been
shown to follow the different photocatalytic pathways under UV and visible light irradiation as shown in Fig. 13
[16,17,42]. Thus, we can estimate and distinguish various
effects of Ag deposits on the TiO2 photoactivity using the
different mechanisms for the roles of Ag deposits and RB
photocatalytic degradation.

H.M. Sung-Suh et al. / Journal of Photochemistry and Photobiology A: Chemistry 163 (2004) 3744

Fig. 13. Different photocatalytic pathways for the RB photodegradation

on AgTiO2 particles. (a) RB self-photosensitization pathway under visible light irradiation and (b) TiO2 -photosensitization pathway under UV
irradiation.

3.3.1. Effects of Ag nanoparticles under visible light

irradiation
The significantly enhanced photodegradation of RB in the
AgTiO2 nanosol under visible light irradiation (Fig. 12)
may be ascribed to the cooperative roles of Ag deposits
according to the three mechanisms mentioned above.
In the self-photosensitization pathway for the RB photodegradation on TiO2 under visible light irradiation as
shown in Fig. 13(a), RB, not TiO2 , is activated into its
excited state at > 470 nm, injecting an electron into the
conduction band (and/or surface states) of the TiO2 semiconductor, whereas RB is converted to the cationic radical
(RB+ ). In turn, the injected electron on the TiO2 particle reacts with adsorbed oxidants (usually O2 ) to produce
reactive oxygen radicals (e.g. O2 , OOH, OH). Subsequently, RB+ is degraded or mineralized by these oxygen
radicals. Under visible light irradiation, the semiconductor
TiO2 acts only as an electron-transfer mediator and the
oxygen as an electron acceptor leading to an efficient separation of the injected electron and RB+ (acting as a hole).
Also, the rate-determining step in photocatalytic oxidations
is believed to be the electron transfer from the TiO2 surface
to the adsorbed O2 [23,43]. Consequently, the RB adsorption as well as charge separation is essential for the RB
photodegradation on TiO2 under visible light irradiation.
For the RB photodegradation on AgTiO2 under visible
light irradiation, the Ag particles on the TiO2 surface can act
as electron traps facilitating the electronhole separation and

43

subsequent electron transfer to the adsorbed O2 according

to the mechanism (1) [2427].
For the mechanism (2), more RB molecules are adsorbed on the AgTiO2 surface than on the TiO2 surface, thus leading to more injection of the photoexcited
electron from RB to the conduction band of TiO2 in
self-photosensitization pathway under visible light irradiation as shown in Fig. 13(a). The enhanced RB adsorption
on the AgTiO2 surface can be supported by both the significant blue shift in max of RB due to faster de-ethylation
of RB on AgTiO2 during the RB photodegradation (Fig. 9)
and a decrease in the RB absorbance (max ) after contact
with AgTiO2 particles in the dark (Fig. 10).
For the mechanism (3), it has been reported that the surface plasmon resonance of Ag metals on TiO2 is excited by
visible light, enhancing the surface electron excitation and
electronhole separation [3032]. Although we did not obtained the significant absorption band of the plasmon resonance in the AgTiO2 sample due to the low Ag content
and scattering of AgTiO2 in this experimental condition,
it is expected that the excitation of plasmon resonance may
contribute to the enhancement in the photocatalytic activity
under visible light irradiation, as reported for AuTiO2 [29]
and PtTiO2 [28].
In summary, we suggest that the cooperative effects of
Ag deposits according to the three mechanisms lead to the
significant enhancement in the TiO2 photocatalytic activity
under visible light irradiation as can be seen in Fig. 12.
3.3.2. Effects of Ag particles under UV irradiation
For the RB photodegradation on AgTiO2 under UV irradiation, the Ag deposition slightly increases the TiO2 photoactivity (Fig. 12). This may be ascribed to the effect of
Ag deposits according to only the mechanism (1) under UV
irradiation.
In a TiO2 -sensitization pathway for the RB photodegradation under UV irradiation as shown in Fig. 13(b), the valence
electrons of TiO2 particles are excited to the conduction band
by UV light and after various other events, electrons on the
TiO2 particle surface are scavenged by the present molecular oxygen to produce reactive oxygen radicals, whereas the
valence hole become trapped as the surface-bound OH radicals on oxidation of either the surface OH group and/or
the surface H2 O molecules. Therefore, the charge separation
on TiO2 is a crucial factor to affect the efficiency of the RB
photodegradation in TiO2 under UV irradiation.
For the RB photodegradation on AgTiO2 under UV irradiation, Ag metals act as electron traps according to the
mechanism (1) thereby enhancing the charge separation. For
the mechanism (2), however, it seems that the enhanced RB
adsorption on the Ag/TiO2 does not significantly contribute
to enhancing the TiO2 photoactivity under UV irradiation as
much as under visible light irradiation, since RB molecules
are not excited by UV light. Also, the effect of Ag deposits
according to the mechanism (2) is not applicable under UV
irradiation because the Ag surface plasmon resonance is not

44

H.M. Sung-Suh et al. / Journal of Photochemistry and Photobiology A: Chemistry 163 (2004) 3744

excited by UV light. Consequently, only the mechanism (1)

is applicable to the effect of Ag deposits thus leading to the
slight increase in the AgTiO2 photoactivity under UV irradiation (Fig. 12).
4. Conclusions
We have investigated the different roles of Ag deposits
in enhancing the TiO2 photocatalytic activity under UV
and visible light irradiation. For comparison, the RB photodegradation was carried out in Degussa P25 titania and
Ag-deposited P25 suspensions in the same photoreaction
condition. The AgTiO2 nanosol showed a 30% increase
in the RB photodegradation under visible light irradiation,
as compared to the pure TiO2 . The significant de-ethylation
of RB occurs on the AgTiO2 surface simultaneously with
the degradation of its aromatic ring structure under visible
light irradiation, indicating the enhanced RB adsorption on
the AgTiO2 surface. Under UV irradiation, however, the
Ag/TiO2 sample revealed only 10% more RB photodegradation as compared to the pure TiO2 . Under visible light
irradiation, the significant enhancement in the AgTiO2
photoactivity can be ascribed to simultaneous effects of Ag
deposits by both acting as electron traps and enhancing the
RB adsorption on the AgTiO2 surface. Under UV irradiation, however, Ag deposits may exhibit the effect only as
electron traps, thus leading to the slight enhancement in the
AgTiO2 photocatalytic activity.
Acknowledgements
This research was partially supported by the Brain Korea
21 Project in 2001. Authors also thank Korea Energy Management Corporation (KEMCO) for its financial support.
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