MWCNT-Based Ag2S-TiO2 Nanocomposites Photocatalyst: Ultrasound-Assisted Synthesis, Characterization, and Enhanced Catalytic Efficiency

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2012, Article ID 586526, 10 pages doi:10.1155/2012/586526 Research Article MWCNT-Based Ag2S-TiO2 Nanocomposites Photocatalyst: Ultrasound-Assisted Synthesis, Characterization, and Enhanced Catalytic Efficiency Lei Zhu, Ze-Da Meng, and Won-Chun Oh Department of Advanced Materials Science & Engineering, Hanseo University, Chungnam 356-706, Republic of Korea Correspondence should be addressed to Won-Chun Oh, wc Received 8 March 2012; Revised 13 May 2012; Accepted 16 May 2012 Academic Editor: Jiaguo Yu Copyright © 2012 Lei Zhu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Multiwalled carbon nanotube based nanoscale Ag2 S and TiO2 composites have successfully synthesized via a facile ultrasoundassisted method. The nanocomposites were characterized by Fourier transform infrared (FT-IR) spectroscopy, UV-vis absorption spectra, BET surface area measurements, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The Ag2 STiO2 /CNT nanocomposites exhibited much higher photocatalytic activity than pure TiO2 for the degradation of Rhodamine B (Rh.B) under UV and visible light. The improved photocatalytic activities may be attributed to increased adsorbability of Rh.B molecules and increased charge transfer rate in the presence of a one-dimensional MWCNT network. 1. Introduction Industrial dyestuffs including textile dyes are recognized as being a major environmental threat. Physical, chemical, and biological methods are available for the treatment of such waste. However, they are not sufficient and have some disadvantages. Therefore, advanced oxidation processes (AOPs), including peroxone, nonthermal plasma, photo-Fenton, UV/O3 , UV/H2 O2 , and semiconductor photocatalysis processes, have been developed by many researchers as processes for the degradation of organic compounds that are suitable for achieving the complete elimination and mineralization of various pollutants [1–4]. The present work focuses on coupled nanoscale semiconductors. It has been reported that the development of coupled semiconductors with nanometer sizes is an effective approach to prepare photocatalytic materials that can utilize sunlight effectively [5]. In addition, these coupled semiconductors, including MS/TiO2 (where M = Pb, Zn, Cd), also exhibit fine optical properties (absorption and photoluminescence) compared with those of their corresponding bulk materials due to quantum confinement effects [6]. However, it has been revealed that MS/TiO2 photocatalysts have some disadvantages, such as difficult recovery, easy cohesion, and low utilization rate in practical applications, due to the weak surface forces, when the particle size is small, thus it can easily agglomerate. So by the accumulation of carriers the adsorption mass transfer rate and efficiency of photocatalytic degradation are reduced to a certain extent [7, 8]. Therefore, in order to solve this problem, an alternative method was employed in this study, which involved immobilizing nanoparticles onto an inert and porous supporting matrix. Multiwalled carbon nanotubes (MWCNTs) have been widely applied as a promising raw material in many areas of science and technology due to their outstanding physical and electrical properties, such as high tensile strength and elastic modulus and excellent thermal and electrical conductivity [9, 10]. So far, MWCNTs have been used to synthesize not only various composite materials, including polymer/MWCNTs [11] and metal/MWCNTs [12] composites, but also various MWCNTs/metal sulphide nanoparticle hybrid catalysts, where the MWCNTs can function as supporting materials [13–16]. Among the metal sulphide materials, Ag2 S is an important semiconductor that has been used in photovoltaic cells, electrochemical storage cells, IR detectors, photoconductors, 2 and so on [17, 18]. Its conduction band (−0.3 eV) is less anodic than the corresponding TiO2 band (−0.1 eV), and its valence band (+0.7 eV) is more cathodic than the TiO2 valence band (+3.1 eV) [19, 20]. Nanocrystalline Ag2 S will be a good candidate for the photosensitization of TiO2 catalysts. In this work, we focused on the fabrication and characterization of the nanoscale Ag2 S-TiO2 /CNT composite photocatalyst. The microstructure, surface state, and elemental compositions of the prepared Ag2 S-TiO2 /CNT composites were investigated. The new products were characterized by the Fourier transform infrared (FT-IR) spectroscopy, energy dispersive X-ray (EDX) analysis, nitrogen adsorption Brunauer-Emmett-Teller (BET) specific surface area analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV-vis diffuse reflectance spectra (DRS). The photocatalytic activity of the Ag2 S-TiO2 /CNT nanocomposites was assessed by examining the degradation of Rh.B in model aqueous solutions as a probe reaction under UV-vis light irradiation. The kinetics and mechanism of the photocatalysis were also studied. 2. Experimental 2.1. Materials and Reagents. MWCNT (95.5%) powder, containing nanotubes with diameters of 20 nm and lengths up to 5 μm, was purchased from Carbon Nano-material Technology Co., Ltd., Korea. To oxidize the surface of the MWCNTs, we used m-chloroperbenzoic acid (MCPBA) purchased from Acros Organics, New Jersey, USA as an oxidized reagent. Ethyl alcohol was purchased from Duksan Pure Chemical Co. (Korea) and used as received. Titanium (IV) n-butoxide (TNB, C16 H36 O4 Ti) was used as a titanium source. Silver nitrate (AgNO3 ) and sodium sulfide·5-hydrate (Na2 S·5H2 O) were used for the preparation of Ag2 S and were supplied by Duksan Pure Chemical Co., Ltd, Korea, and Yakuri Pure Chemicals Co., Ltd., Japan, respectively. Analytical-grade Rhodamine B (C28 H31 ClN2 O3 ) was purchased from Samchun Pure Chemical Co., Ltd., Korea. All chemicals were used without further purification, and all experiments were carried out using distilled water. 2.2. Preparation of Ag2 S-TiO2 /CNT Composite Photocatalysts. Although MWCNT present remarkable intrinsic properties, it is necessary to functionalize their surfaces to obtain higher performances and to produce well-dispersed supported catalysts [21]. The purposes of these oxidative treatments are (i) to improve the interaction of MWCNTs with solvents and their dispersion; (ii) to allow the grafting of nanoparticles; (iii) to modify the adsorption properties of MWCNTs; (iv) to perform chemical treatments on MWCNTs. Here, the original MWCNTs were treated with a particular acid (MCPBA) and not the nitric-sulfuric acid (HNO3 -H2 SO4 , volumetric ratio: v/v = 1/3) used elsewhere in the literature [22]. A total of 2.0 g of MCPBA was dissolved in 60 mL of benzene to obtain the oxidizing agent. Then 0.6 g of MWCNTs was put into the oxidizing (...truncated)