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

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 tes

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Contents lists available atScienceDirect Applied Surface Science

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / a p s u s c

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 meth-ods (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

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

one-dimensional nanotubes with zero one-dimensional nanoparticles (NPs)

has received increased attention due to their interesting

struc-tural, 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

cat-alytic 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

activ-ity and excellent SERS properties but also high chemical stabilactiv-ity,

excellent absorption capacity, high selectivity, etc.[13–22]

How-ever, 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: jinmu@ecust.edu.cn (J Mu).

inexpensive, stable and efﬁcient visible light photocatalytic mate-rial as a substitute for Pt/MWNTs Here, we found an interesting phenomenon that Ag/CNTs could replace Pt/MWNTs for the vis-ible 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], electro-chemical method[16,21,27]and chemical method[28], have been developed to prepare the NPs/CNTs nanocomposites The physi-cal method is to deposit the metal particles onto the CNTs from the metal vapor, which needs expensive apparatus The electro-chemical 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 gen-eral 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 pro-cesses, 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

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reported previously[33] Silver nitrate (AgNO3) and rhodamine B

(RhB) were purchased from Shanghai Chemical Reagent Co., Ltd.,

sulfuric acid (H2SO4) and nitric acid (HNO3) from Shanghai Lingfeng

Chemical Reagent Co., Ltd., sodium sulfate (Na2SO4) from Shanghai

No 4 Reagent Factory All the reagents were used as received All

aqueous solutions were prepared using doubly distilled water

2.2 Synthesis of Ag/MWNTs

The Ag/MWNTs were synthesized via photoreduction or

ther-mal 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

start-ing Ag in theoretical products In the ﬁrst method, 200 mg MWNTs

were added into 50 mL double distilled water and dispersed

ultra-sonically 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 AgNO3at a certain weight

ratio in an agate mortar and grounded for 30 min at room

tem-perature The obtained solid mixtures of AgNO3and MWNTs were

transferred into the small alumina crucibles and heated in a

nitro-gen 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

2.3 Adsorption experiment

To determine the adsorption equilibration time, 8 mg

MWNTs were added into the 50 mL aqueous solution of RhB

(2× 10−5mol L−1) After 10 min ultrasonic treatment, the

sus-pension 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):

q =V(C0− C) × 479.02

where C0and 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, andω (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

concen-trations of 8× 10−6, 1.0× 10−5, 1.5× 10−5, 2.0× 10−5, 2.5× 10−5,

Clausius–Clapeyron equation[34]:

Equation derived from adsorption isotherm[35]:

Gibbs–Helmholtz equation:

S =H − G

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 eval-uated by the photocatalytic degradation of RhB under visible light irradiation ( > 420 nm) The photocatalytic experiments were car-ried out in a reactor containing the 50 mL aqueous solution of RhB (2.0× 10−5mol 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 adsorp-tion/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

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

2.5 Characterization The morphological characterization of Ag/MWNTs was per-formed 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 solu-tions were obtained on a UV-2102 PCS spectrophotometer (China) The photoelectrochemical measurements were performed with the PCI4/300 electrochemistry station (Gamry, USA) in a conven-tional 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 accord-ing to the followaccord-ing 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, respec-tively, then, dried in air The Ag/MWNTs was dispersed in distilled

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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−1aqueous 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

sur-face 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 inFig 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 inFig 1b

The Ag/MWNTs is analysed by XPS, as shown inFig 2 It can

be observed from the survey XPS spectra (Fig 2A) that the puri-ﬁed 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 3d5/2and 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

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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

ener-gies (BE) of Ag 3d5/2 for the Ag, Ag2O and AgO are 368.2, 367.8

and 367.4 eV, respectively[37,38] Here, no peak corresponding to

Ag2O or AgO is observed in the XPS spectra of Ag/MWNTs

There-fore, 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

quantiﬁca-tion results of XPS show that the mass concentraquantiﬁca-tion of Ag element

is 1.42% and 2.11% for the Ag/MWNTs-P and Ag/MWNTs-T,

respec-tively 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 3d5/2in 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 inFig 1 FromFig 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

inter-action 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 3shows 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

adsorp-tion/desorption equilibrium time in the following experiments

Fig 5 Linear ﬁtting of ln Ce vs T−1.

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:

lnqe= ln KF+1

where qe(mg g−1) represents the amount of adsorbed RhB at the equilibrium, Ce(mg L−1) is the equilibrium concentration of RhB

KFand 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 Cewith the correlation coefﬁcient of 0.98, as shown inFig 4b So KFand n are calculated to be 78.86 and 2.37, respectively The large KFvalue 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 5shows that the natural logarithm of the equilibrium concentration of RhB depends linearly on the recipro-cal 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 ofH, 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 − G

−51250 + 5870

298.15 = −152.21 J K−1mol−1 The large negative value of H implies the adsorption is an exothermic process and the interaction between MWNTs and RhB

is strong TheG is negative, indicating that the adsorption is a

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Fig 6 Degradation efﬁciency of RhB using Ag/MWNTs as photocatalysts with

var-ious Ag contents under 6 h irradiation.

spontaneous process The negative value ofS 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

activi-ties between Ag/MWNTs-P and Ag/MWNTs-T are compared, as

shown inFig 6 It can be observed that the photocatalytic

activ-ity 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

sep-aration 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

calci-nation process makes the excited electron move more smoothly on

the surface of MWNTs, which leads to better separation of

photo-generated charges Thus, higher visible light photocatalytic activity

is achieved

As shown inFig 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

inFig 7

As shown inFig 7, all the Nyquist plots (Zim vs Zre) include

a semicircle region lying on the Zre-axis observed at higher

fre-quencies 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 Na 2 SO 4 Scan rate: 2 mV s −1 (Z im : imaginary impedance, Z re : real impedance).

rate Thereby, the quenching of photogenerated electrons is effec-tively 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 photo-catalysts under visible light irradiation are shown in Fig 8 The self-degradation of RhB is only 28% after 8 h visible light irradi-ation The degradation percentage doubles when 8.0 mg MWNTs are added into the RhB solution The improvement of the degrada-tion efﬁciency is probably due to the fast charge transfer ability

of MWNTs The photocatalytic activity of MWNTs can be fur-ther enhanced when Ag NPs are loaded on them When 3.0 wt% Ag/MWNTs-T are added into the solution, the degradation efﬁ-ciency of 72% can be achieved after 8 h irradiation The results indicate that the Ag/MWNTs-T are of high visible light photocat-alytic 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).

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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

degra-dation of RhB on the Ag/MWNTs under visible light irradiation is

a self-sensitized photocatalytic process, as illustrated inScheme 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

accep-tors, which are similar to the semiconductor–metal junctions[44]

Since the MWNTs are a strong adsorbent for RhB in aqueous

solu-tion, 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

acti-vated 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

elec-trons 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

load-ing Ag NPs on them via photoreduction or thermal decomposition

method Moreover, the sample prepared by the thermal

decom-position 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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