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

Applied Surface Science 257 (2011) 3620–3626 Contents lists available at ScienceDirect 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 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 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: jinmu@ecust.edu.cn (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 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 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 Then 50 mL AgNO3 aqueous solution was added into this suspension dropwise The obtained mixture was stirred magnetically for 24 h and illuminated for 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 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 h with a heating rate of ◦ 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 mg catalysts The distance between the lamp and the reactor was 15 cm Before irradiation, the suspension was magnetically stirred in dark for 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, mg MWNTs were added into the 50 mL aqueous solution of RhB (2 × 10−5 mol L−1 ) After 10 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 ultrasonic treatment, the suspensions were vibrated in dark at 25 ◦ C for 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 ultrasonic treatment, the suspension was magnetically stirred in dark for 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, mg MWNTs were added into the 50 mL aqueous solution of RhB with initial concentrations of × 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 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 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 Results and discussion 3.1 Characterization of Ag/MWNTs The FETEM images of 3.0 wt% Ag/MWNTs are shown in Fig It can be observed that the inner radius of MWNTs is between 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 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 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 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 Linear fitting of ln Ce vs T−1 Fig 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 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 shows the adsorption kinetic curve of RhB on the MWNTs in aqueous solution The adsorption/desorption equilibrium can be achieved in h Therefore, 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 + 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 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 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 Degradation efficiency of RhB using Ag/MWNTs as photocatalysts with various Ag contents under 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 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 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 Nyquist plots of impedance spectra for various electrodes under visible light irradiation Electrolyte: 0.1 mol L−1 Na2 SO4 Scan rate: 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 The self-degradation of RhB is only 28% after 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 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 h, the visible light photocatalytic effi- Fig 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 Photodegradation pathway of Ag/MWNTs ciency of Ag/MWNTs-T is lower than that of Pt/MWNTs After being irradiated for 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 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• 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