Effect of multi walled carbon nanotubes loaded with ag nanoparticles on the

Applied Surface Science 257 (2011) 3620–3626

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Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc

Effect of multi-walled carbon nanotubes loaded with Ag nanoparticles on the
photocatalytic degradation of rhodamine B under visible light irradiation
Ya Yan, Huiping Sun, Pingping Yao, Shi-Zhao Kang, Jin Mu ∗
Key Laboratory for Advanced Materials, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China

a r t i c l e

i n f o

Article history:
Received 5 September 2010
Received in revised form
11 November 2010
Accepted 13 November 2010
Available online 19 November 2010
Keywords:
Multi-walled carbon nanotubes
Silver
Adsorption
Rhodamine B
Visible light photocatalysis

a b s t r a c t
Multi-walled carbon nanotubes loaded with Ag nanoparticles (Ag/MWNTs) were prepared by two methods (direct photoreduction and thermal decomposition). The photocatalytic activity of Ag/MWNTs for

the degradation of rhodamine B (RhB) under visible light irradiation was investigated in detail. The
adsorption and photocatalytic activity tests indicated that the MWNTs served as both an adsorbent and a
visible light photocatalyst. The photocatalytic activity of MWNTs was remarkably enhanced when the Ag
nanoparticles were loaded on the surface of MWNTs. Moreover, the visible light photocatalytic activity of
Ag/MWNTs depended on the synthetic route. On the basis of the experimental results, a possible visible
light photocatalytic degradation mechanism was discussed.
© 2010 Elsevier B.V. All rights reserved.

1. Introduction
Since discovered by Iijima in 1991 [1], carbon nanotubes (CNTs)
have captured the worldwide researchers’ interest because of their
small dimension, high surface area, unique structure, ultrastrong
mechanical property and high stability [2]. With the development
of CNTs chemistry in the past decade, the integration of onedimensional nanotubes with zero dimensional nanoparticles (NPs)
has received increased attention due to their interesting structural, electrochemical, electromagnetic and other properties which
are not available to the respective components alone [3–6]. Ag
NPs have been known for their unique properties of high catalytic activity [7,8], good antibacterial activity [9,10] and excellent
surface-enhanced Raman scattering (SERS) [11,12]. When Ag NPs
were loaded on CNTs, the Ag/CNTs nanocomposite exhibited not
only good electrocatalytic activity, remarkable antibacterial activity and excellent SERS properties but also high chemical stability,
excellent absorption capacity, high selectivity, etc. [13–22]. However, there is less literature concerning the photocatalytic activity
of Ag NPs/CNTs.
We had reported that the loading of Pt NPs on the surface
of MWNTs obviously enhanced the photodegradation of methyl
orange under visible light irradiation [23]. However, the high cost
of Pt is a great drawback for the full commercial application of
Pt/MWNTs. Therefore, it is necessary to search for an abundant,

∗ Corresponding author. Tel.: +86 21 64252214; fax: +86 21 64252485.
E-mail address: (J. Mu).

0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2010.11.089

inexpensive, stable and efficient visible light photocatalytic material as a substitute for Pt/MWNTs. Here, we found an interesting
phenomenon that Ag/CNTs could replace Pt/MWNTs for the visible light photodegradation of rhodamine B. To the best of our
knowledge, this study may be the first one about the visible light
photocatalytic activity of Ag/MWNTs. Therefore, our results not
only extend the applications of Ag/MWNTs, but also provide some
clew for developing visible light responsive photocatalyst for dye
degradation.
To date, strategies including physical method [24–26], electrochemical method [16,21,27] and chemical method [28], have been
developed to prepare the NPs/CNTs nanocomposites. The physical method is to deposit the metal particles onto the CNTs from
the metal vapor, which needs expensive apparatus. The electrochemical method is to apply a current through an aqueous metal
salt solution with the CNTs serving as one of the electrodes. Same
as the physical method, the electrochemical method also needs
expensive apparatus, and moreover, technique of producing gram
quantity of Ag/CNTs nanocomposites is very difficult [4]. In the general chemical method, metal salts or other precursors are usually
adsorbed onto the CNTs and then reduced to metal via using the
reducing agent such as hydrogen [29], sodium borohydride [14,30],
ethylene glycol [31] and hydrazine hydrate [32] etc. In these processes, impurities are easily to be involved since reducing agents
are needed.
In this study, we take two approaches, i.e. photoreduction and
thermal decomposition, to prepare Ag/MWNTs which are denoted
as Ag/MWNTs-P and Ag/MWNTs-T, respectively. No reducing agent
or electrical current is used in both of two methods. The photo-


Y. Yan et al. / Applied Surface Science 257 (2011) 3620–3626

catalytic activity of Ag/MWNTs is evaluated by the degradation of

RhB in aqueous solution under visible light irradiation. The effect
of different preparation methods on the visible light photocatalytic
activity of Ag/MWNTs is studied. To understand the photocatalytic
behavior of Ag/MWNTs, the adsorption properties of MWNTs for
RhB and the electrochemical impedance spectra of Ag/MWNTs are
measured. A possible photocatalytic mechanism is also discussed.
2. Experimental
2.1. Materials
MWNTs were obtained from Shenzhen Nanotechnology Co., Ltd.
(China) and purified according to the chemical oxidization method
reported previously [33]. Silver nitrate (AgNO3 ) and rhodamine B
(RhB) were purchased from Shanghai Chemical Reagent Co., Ltd.,
sulfuric acid (H2 SO4 ) and nitric acid (HNO3 ) from Shanghai Lingfeng
Chemical Reagent Co., Ltd., sodium sulfate (Na2 SO4 ) from Shanghai
No. 4 Reagent Factory. All the reagents were used as received. All
aqueous solutions were prepared using doubly distilled water.

The Ag/MWNTs were synthesized via photoreduction or thermal decomposition. The amount of Ag loaded on MWNTs was
calculated from an initial dosage. The samples were denoted as
x wt% Ag/MWNTs, where x indicated the mass percentage of starting Ag in theoretical products. In the first method, 200 mg MWNTs
were added into 50 mL double distilled water and dispersed ultrasonically for 10 min. Then 50 mL AgNO3 aqueous solution was
added into this suspension dropwise. The obtained mixture was
stirred magnetically for 24 h and illuminated for 4 h under UV light
from a 300 W high-pressure Hg lamp (365 nm). After filtrating,
washing with double distilled water and drying, the Ag/MWNTs-P
was obtained.
The second method is a straightforward “mix-and-heat” process
in the absence of any solvent, reducing agent or electric current.
The purified MWNTs were mixed with AgNO3 at a certain weight
ratio in an agate mortar and grounded for 30 min at room temperature. The obtained solid mixtures of AgNO3 and MWNTs were

transferred into the small alumina crucibles and heated in a nitrogen oven at 450 ◦ C for 6 h with a heating rate of 2 ◦ C/min. After
naturally cooled to the room temperature, the product denoted as
Ag/MWNTs-T was obtained.

ln Ce = − ln k +

H
RT

(2)

Equation derived from adsorption isotherm [35]:
G = −nRT

(3)

S=

H−
T

G

(4)

where k is a constant, Ce (mg L−1 ) is the equilibrium concentration
and n the Freundlich constant.
2.4. Photocatalytic experiment
The visible light photocatalytic activity of Ag/MWNTs was evaluated by the photocatalytic degradation of RhB under visible light
irradiation ( > 420 nm). The photocatalytic experiments were carried out in a reactor containing the 50 mL aqueous solution of RhB

(2.0 × 10−5 mol L−1 ) and 8 mg catalysts. The distance between the
lamp and the reactor was 15 cm. Before irradiation, the suspension
was magnetically stirred in dark for 4 h to establish an adsorption/desorption equilibrium under ambient conditions. Then, the
mixture was exposed to the visible light irradiation. At the given
irradiation time, the concentration of RhB was quantified by the
absorbance. The degradation efficiency was calculated according
to Eq. (5):
Degradation (%) =

A0 − A
× 100%
A0

(5)

where A0 and A represent the absorbances of the RhB solution
before and after visible light irradiation, respectively.

2.3. Adsorption experiment
To determine the adsorption equilibration time, 8 mg
MWNTs were added into the 50 mL aqueous solution of RhB
(2 × 10−5 mol L−1 ). After 10 min ultrasonic treatment, the suspension was magnetically stirred in dark at 25 ◦ C. At a given
time, the suspension was filtered through a 0.22 ␮m millipore
cellulose acetate membrane to remove the catalyst. According
to the standard curve, the concentration of RhB was monitored
by measuring the absorbance at the wavelength of 554 nm. The
amount of adsorption q (mg g−1 ) could be calculated by Eq. (1):
V (C0 − C) × 479.02

3.0 × 10−5 and 4.0 × 10−5 mol L−1 , respectively. After 10 min ultrasonic treatment, the suspensions were vibrated in dark at 25 ◦ C for

4 h to establish an adsorption/desorption equilibrium. The concentrations of RhB were quantified according to the method mentioned
above.
To explore the adsorption thermodynamics of MWNTs for the
RhB solution, 8.0 mg MWNTs were added into the 50 mL aqueous
solution of RhB (2.0 × 10−5 mol L−1 ). After 10 min ultrasonic treatment, the suspension was magnetically stirred in dark for 4 h at 25,
35, 40, 50, 55, 60 and 65 ◦ C, respectively, to establish an adsorption/desorption equilibrium. Then the concentrations of RhB were
quantified by the absorbance. Estimation of the heat of adsorption ( H), free energy change ( G) and entropy change ( S) was
calculated from the below expressions:
Clausius–Clapeyron equation [34]:

Gibbs–Helmholtz equation:

2.2. Synthesis of Ag/MWNTs

q=

3621

(1)

where C0 and C (mol L−1 ) are the concentration of the RhB solution
at initial and at the given time, respectively. V (mL) is the volume
of the solution, and ω (g) the mass of the dry adsorbent.
To determine the adsorption isotherm, 8 mg MWNTs were
added into the 50 mL aqueous solution of RhB with initial concentrations of 8 × 10−6 , 1.0 × 10−5 , 1.5 × 10−5 , 2.0 × 10−5 , 2.5 × 10−5 ,

2.5. Characterization
The morphological characterization of Ag/MWNTs was performed on a JEOL JEM 2010 field-emission transmission electron
microscope (FETEM) (Japan). The X-ray photoelectron spectra (XPS)
were measured using a Kratos AXIS Ultra DLD X-ray photoelectron

spectrometer (Japan). The UV–vis absorption spectra of the solutions were obtained on a UV-2102 PCS spectrophotometer (China).
The photoelectrochemical measurements were performed with
the PCI4/300 electrochemistry station (Gamry, USA) in a conventional three-electrode cell with a quartz window, using a saturated
calomel electrode (SCE) as the reference electrode, a platinum circle
as the counter electrode, and an Ag/MWNTs modified ITO glass as
the working electrode. The working electrode was prepared according to the following procedure. The ITO electrode with an area
of 10 mm × 15 mm was ultrasonicated successively in anhydrous
ethanol, acetone, and doubly distilled water for 30 min, respectively, then, dried in air. The Ag/MWNTs was dispersed in distilled


3622

Y. Yan et al. / Applied Surface Science 257 (2011) 3620–3626

Fig. 1. FETEM images of 3.0 wt% Ag/MWNTs-P (a) and 3.0 wt% Ag/MWNTs-T (b).

water ultrasonically to form a 0.1 mg mL−1 aqueous solution. 0.1 mL
of the dispersed solution was dropped onto the surface of the ITO
electrode and dried under an IR lamp. After the surface was dried
thoroughly, dropping 0.1 mL of the dispersed solution onto the surface again, repeating the process 3 times, the Ag/MWNTs modified
ITO electrode was obtained. A 1000 W halide lamp (FELCO, China)
with a cutoff filter (JB-420) was employed as the visible excitation
light source.
3. Results and discussion
3.1. Characterization of Ag/MWNTs
The FETEM images of 3.0 wt% Ag/MWNTs are shown in Fig. 1. It
can be observed that the inner radius of MWNTs is between 5 and
10 nm and the Ag NPs deposit on the MWNTs. For the Ag/MWNTs-

P (Fig. 1a), the size of Ag NPs is smaller (ca. 5 nm), some of them

could enter the cavities of MWNTs and deposit on the inner wall.
For the Ag/MWNTs-T, the Ag NPs are larger (ca. 22 nm) and can only
deposit on the surface of MWNTs, as shown in Fig. 1b.
The Ag/MWNTs is analysed by XPS, as shown in Fig. 2. It can
be observed from the survey XPS spectra (Fig. 2A) that the purified MWNTs are composed of C and O elements. Compared with
the MWNTs, there appears a peak at 368.5 eV in the XPS spectra
of Ag/MWNTs-P and Ag/MWNTs-T, which is ascribed to the Ag 3d
[36]. The existence of Ag 3d signal indicates that the Ag element
was successfully introduced onto the MWNTs with both methods.
In the high resolution XPS spectra of Ag 3d (Fig. 2B), there are
two peaks centered at 368.6, 374.6 eV for Ag/MWNTs-P and 368.5,
374.5 eV for Ag/MWNTs-T, which are ascribed to Ag 3d5/2 and Ag
3d3/2 , respectively. The binding energies of Ag 3d in Ag/MWNTs-P
are larger slightly than those in Ag/MWNTs-T, which may be caused

Fig. 2. Survey XPS spectra (A) and high resolution XPS spectra of Ag 3d (B) and C 1s (C). ((a) Purified MWNTs, (b) 3.0 wt% Ag/MWNTs-P and (c) 3.0 wt% Ag/MWNTs-T.).


Y. Yan et al. / Applied Surface Science 257 (2011) 3620–3626

3623

Fig. 5. Linear fitting of ln Ce vs. T−1 .
Fig. 3. Adsorption kinetic curve of RhB on the MWNTs at 25 ◦ C.

by the different sizes of Ag NPs. As we know, the binding energy
increases with decreasing the Ag particle size. The binding energies (BE) of Ag 3d5/2 for the Ag, Ag2 O and AgO are 368.2, 367.8
and 367.4 eV, respectively [37,38]. Here, no peak corresponding to
Ag2 O or AgO is observed in the XPS spectra of Ag/MWNTs. Therefore, the Ag species deposited on MWNTs is metal Ag. Compared
with the Ag/MWNTs-P, the peaks of Ag 3d in the XPS spectrum of

Ag/MWNTs-T are stronger, which means that the content of Ag in
Ag/MWNTs-T is higher than that in Ag/MWNTs-P. The quantification results of XPS show that the mass concentration of Ag element
is 1.42% and 2.11% for the Ag/MWNTs-P and Ag/MWNTs-T, respectively. The results indicate that the loss of Ag by the photoreduction
method is larger than that by the thermal decomposition method. In
the process of the photoreduction, Ag may lose during the washing
procedure. The full wave half maximum (FWHM) of Ag 3d5/2 in the
Ag/MWNTs-P and Ag/MWNTs-T is 0.964 and 0.860 eV, respectively,
indicating that the Ag NPs size in the Ag/MWNTs-P is smaller than
that in the Ag/MWNTs-T, which is in agreement with the FETEM
images shown in Fig. 1. From Fig. 2C, it can be observed that the
asymmetric BE peaks of C 1s in the spectra of MWNTs, Ag/MWNTs-P
and Ag/MWNTs-T are 284.5, 284.7 and 284.8 eV, respectively. Since
the binding energy correlates with the electron density around the
nucleus, the higher binding energy indicates the stronger interaction between Ag NPs and MWNTs, and the trapping electron
capability of the Ag NPs in the Ag/MWNTs-T is stronger than that
in the Ag/MWNTs-P.
3.2. Adsorption properties of MWNTs for RhB in aqueous solution
Fig. 3 shows the adsorption kinetic curve of RhB on the MWNTs
in aqueous solution. The adsorption/desorption equilibrium can
be achieved in 4 h. Therefore, 4 h is selected as the adsorption/desorption equilibrium time in the following experiments.

Fig. 4a shows the adsorption isotherm curve of RhB on the
MWNTs. The amount of adsorbed RhB increases acutely with
increasing initial concentration of RhB. The result fits the Freundlich
adsorption isotherm model:
ln qe = ln KF +

1
ln Ce
n

(6)

where qe (mg g−1 ) represents the amount of adsorbed RhB at the
equilibrium, Ce (mg L−1 ) is the equilibrium concentration of RhB.
KF and n are Freundlich constants which relate to the adsorption
capacity and intensity, respectively [39]. The linear equation is
ln qe = 4.37 + 0.42 ln Ce with the correlation coefficient of 0.98, as
shown in Fig. 4b. So KF and n are calculated to be 78.86 and 2.37,
respectively. The large KF value means strong adsorption capability
of MWNTs for RhB. The n value with a range of 2–10 means that the
adsorption is a preferential adsorption and takes place easily [40].
The effect of temperature on the adsorption of MWNTs for RhB
was also studied. Fig. 5 shows that the natural logarithm of the
equilibrium concentration of RhB depends linearly on the reciprocal value of temperature and the linear correlation coefficient of
the curve is −0.997. The Clausius–Clapeyron equation (Eq. (2)) can
be used to fit the experimental data. According to Eqs. (2)–(4), the
values of H, G and S can be calculated as follows:
H = R × (−6164.78) = −51.25 kJ mol−1
G = −nRT = −2.37 × 8.314 × 298.15 = −5.87 kJ mol−1
S=

H−
T

G

=

−51250 + 5870

= −152.21 J K−1 mol−1
298.15

The large negative value of H implies the adsorption is an
exothermic process and the interaction between MWNTs and RhB
is strong. The G is negative, indicating that the adsorption is a

Fig. 4. Adsorption isotherm curve of RhB on the MWNTs (a) and the linear fitting of the Freundlich isotherm equation (b).


3624

Y. Yan et al. / Applied Surface Science 257 (2011) 3620–3626

Fig. 6. Degradation efficiency of RhB using Ag/MWNTs as photocatalysts with various Ag contents under 6 h irradiation.

spontaneous process. The negative value of S reveals that the
randomness at the solid–solution interface decreases during the
adsorption process of RhB on the MWNTs.
3.3. Photocatalytic activity of Ag/MWNTs for the degradation of
RhB under visible light irradiation
To explore the effect of synthetic approach on the visible light
photocatalytic activity of Ag/MWNTs, the photocatalytic activities between Ag/MWNTs-P and Ag/MWNTs-T are compared, as
shown in Fig. 6. It can be observed that the photocatalytic activity of Ag/MWNTs-T is higher than that of Ag/MWNTs-P. In general,
the smaller particles possess higher catalytic activity due to the
more active sites. However, as we know from the FETEM images
(Fig. 1), the size of Ag NPs in the Ag/MWNTs-P is smaller than that
in the Ag/MWNTs-T. There are three possible reasons leading to
this abnormal result. Firstly, the XPS analysis results indicate that
the trapping electron capability of Ag NPs in the Ag/MWNTs-T is

stronger than that in the Ag/MWNTs-P. As a result, the charge separation is promoted and the photocatalytic activity is improved.
Secondly, it is confirmed by XPS that the amount of Ag NPs
deposited on the MWNTs by the thermal decomposition method
is higher than that by the photo reduction method. Thirdly, the
removal of impurities on the surface of Ag/MWNTs-T in the calcination process makes the excited electron move more smoothly on
the surface of MWNTs, which leads to better separation of photogenerated charges. Thus, higher visible light photocatalytic activity
is achieved.
As shown in Fig. 6, the photocatalytic activity of Ag/MWNTs
varies with the Ag NPs content and the optimum Ag content is
3.0 wt% for both synthetic methods. In order to further understand
the role of Ag NPs on the visible light activity of Ag/MWNTs, EIS
is used to characterize MWNTs and Ag/MWNTs electrodes under
visible light irradiation. The measurement results of EIS are shown
in Fig. 7.
As shown in Fig. 7, all the Nyquist plots (Zim vs. Zre ) include
a semicircle region lying on the Zre -axis observed at higher frequencies corresponding to the electron-transfer-limited process,
followed by a linear part at lower frequencies representing the
diffusion-limited process [41]. The semicircle diameter represents
the electron-transfer resistance which is controlled by the surface
modification of the electrode [42]. The size of the arc radius in the
Nyquist plot of the MWNTs electrode under visible light irradiation
reduces when the Ag NPs was loaded on the MWNTs, indicating that
the loading of Ag NPs induces the decrease of the electron-transfer
resistance and the enhancement of the interfacial electron-transfer

Fig. 7. Nyquist plots of impedance spectra for various electrodes under visible
light irradiation. Electrolyte: 0.1 mol L−1 Na2 SO4 . Scan rate: 2 mV s−1 (Zim : imaginary
impedance, Zre : real impedance).

rate. Thereby, the quenching of photogenerated electrons is effectively restrained and the visible light photocatalytic activity is

remarkably enhanced. The electron-transfer resistance decreases
with increasing the Ag NPs amount, and loading 5.0 wt% Ag on the
MWNTs induces the smallest electron-transfer resistance among
the four electrodes. It is notable that the Ag NPs can also act as
quenching centers, which is caused by the electrostatic attraction
of negatively charged silver and positively charged cationic radical
of RhB. So the presence of optimal content of Ag NPs, here is 3.0 wt%,
can reduce the possibility of excitons quenching and improve the
visible light photocatalytic activity.
The kinetic curves of RhB degradation using various photocatalysts under visible light irradiation are shown in Fig. 8. The
self-degradation of RhB is only 28% after 8 h visible light irradiation. The degradation percentage doubles when 8.0 mg MWNTs
are added into the RhB solution. The improvement of the degradation efficiency is probably due to the fast charge transfer ability
of MWNTs. The photocatalytic activity of MWNTs can be further enhanced when Ag NPs are loaded on them. When 3.0 wt%
Ag/MWNTs-T are added into the solution, the degradation efficiency of 72% can be achieved after 8 h irradiation. The results
indicate that the Ag/MWNTs-T are of high visible light photocatalytic activity for RhB degradation.
Our previous studies show that loading Pt NPs on the surface
of MWNTs can strongly enhance the visible light photocatalytic
activity of MWNTs for methyl orange degradation [23]. Therefore,
we make a comparison of visible light photocatalytic activities
between 3.0 wt% Ag/MWNTs-T and 3.0 wt% Pt/MWNTs for RhB
degradation. In the first 5 h, the visible light photocatalytic effi-

Fig. 8. Kinetic curves of RhB degradation using 8.0 mg photocatalysts under visible
light irradiation ( > 420 nm).


Y. Yan et al. / Applied Surface Science 257 (2011) 3620–3626

3625

e-

+

RhB eAg

RhB
Adsorption RhB
Ag

Ag

Ag

Ag
RhB

RhB*

RhB
Ag

RhB

Visible light
Photocatalysis

e-

ee- e O2

hv

eee-

O2- RhB Degradation
•

Ag/MWNTs

e- e
e-

OH

RhB

Degradation

RhB Degradation

RhB/Ag/MWNTs

Scheme 1. Photodegradation pathway of Ag/MWNTs.

ciency of Ag/MWNTs-T is lower than that of Pt/MWNTs. After being
irradiated for 6 h, the photocatalytic efficiency of Ag/MWNTs-T is
almost as high as that of Pt/MWNTs. Since the Ag/MWNTs-T possess
high photocatalytic activity and low cost, it is an ideal alternative
to the Pt/MWNTs.
3.4. Mechanism of visible light degradation of Ag/MWNTs for RhB

On the basis of the experimental results, we infer that the degradation of RhB on the Ag/MWNTs under visible light irradiation is
a self-sensitized photocatalytic process, as illustrated in Scheme 1.
The oxidation of MWNTs introduces many functional groups such
as hydroxyl (–OH), carboxyl (–COOH) and carbonyl (>C O) on the
surface of MWNTs. These functional groups act as the active sites
which help the MWNTs to adsorb Ag NPs [43]. Then the Ag–MWNTs
junctions are built up and the silver islands act as electron acceptors, which are similar to the semiconductor–metal junctions [44].
Since the MWNTs are a strong adsorbent for RhB in aqueous solution, a large amount of RhB molecules are adsorbed on the surface
of MWNTs because of the strong interactions between MWNTs and
RhB. Under the visible light irradiation, RhB molecules can be activated to the excited state (RhB* ). Due to the strong electron affinity
of MWNTs, the electrons transfer from RhB* to the MWNTs. The 1D
carbon-based nano-cylinder structure of MWNTs makes the electrons move freely without any scattering from atoms or defects
[45]. The moving electrons will be trapped when they encounter
the Ag NPs islands. The electrons accumulated on the Ag NPs reduce
the adsorbed oxygen species to superoxide anion radical (O2 •− )
and hydroxyl radical (OH• ). Subsequently, RhB is degraded by these
active oxygen species.
4. Conclusion
The photocatalytic activity of MWNTs can be promoted by loading Ag NPs on them via photoreduction or thermal decomposition
method. Moreover, the sample prepared by the thermal decomposition method shows higher visible light photocatalytic activity
than that by the photoreduction method. In the photodegradation
process, the MWNTs serve as absorbents and electron transmission
channels. The loaded Ag NPs can trap and accumulate electrons.
The photodegradation of RhB on the Ag/MWNTs is a self-sensitized
process. The results suggest that the Ag/MWNTs-T are an ideal
alternative to Pt/MWNTs for the RhB degradation due to the low
cost and high visible light photocatalytic activity.
Acknowledgement
This work was financially supported by the National High
Technology Research and Development Program of China (No.

2009AA05Z101).

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