A simple and straightforward approach to prepare TiO2-coated carbon nanotubes (CNTs) is presented. Anatase TiO2 nanoparticles (NPs) with the average size ~8 nm were coated on CNTs from peroxo titanic acid (PTA) precursor even at low temperature of 100 °C. We demonstrate the effects of CNTs/TiO2 molar ratio on the adsorption capability and photocatalytic efficiency under UV–visible irradiation. The samples showed not only good optical absorption in visible range, but also great adsorption capacity for methyl orange (MO) dye molecules. These properties facilitated the great enhancement of photocatalytic activity of TiO2 NPs-coated CNTs photocatalysts. The TiO2 NPs-coated CNTs exhibited 2.45 times higher photocatalytic activity for MO degradation than that of pure TiO2.
KeywordsTiO2 Carbon nanotubes Nanoparticles Photocatalyst Methyl orange
TiO2 has been one of the most widely investigated and used materials over the past decades [1–3], because it is nontoxic, easy to be made, inexpensive and chemically stable. In recent years, TiO2-based nanomaterials have attracted significant research attention due to their broad applications in the fields of water and air purification [4, 5], H2 production [6, 7], and photovoltaic and photoelectrochemical cells [8, 9].
However, one of the major factors that limit the efficiency of TiO2 photocatalysis is its fast recombination of photo-generated electron/hole pairs, which releases energy in the form of unproductive heat or photons. To solve this problem, many efforts have been made to reduce charge recombination and enhance photocatalytic activity of TiO2. Recently, synthesis of TiO2-CNTs composites has also attracted significant attention to increase the photocatalytic efficiency of TiO2[11–21]. Synthetic methods of TiO2-CNTs composites include sol–gel , chemical vapor deposition (CVD) [18, 19], hydrothermal deposition , electrospinning technique , etc. Most of the composite materials exhibited better photocatalytic performances than pure TiO2 because they provide a large surface area and prevent charge recombination by trapping photo-excited electrons generated from TiO2. Furthermore, due to the conducting properties of CNTs, electrons that are transferred from TiO2 to CNTs could move freely along CNTs and thus the oxidative reactivity is enhanced. For example, CNTs coated with TiO2 exhibit higher photodegradation efficiency for phenol  and better photocatalytic inactivation of bacterial endospores  than CNTs or TiO2 alone.
However, most of the synthetic approaches of CNTs-TiO2 composites require special or expensive devices [18, 21], or high temperature process [19, 24]. Herein, we present a very simple and straightforward wet chemical process to synthesize composites comprising anatase TiO2 NPs-coated CNTs, which can be prepared at low temperature of 100 °C. By varying the ratio of TiO2/CNTs, a series of composites were prepared in order to optimize the synthetic condition of the TiO2 NPs-coated CNTs composites that exhibit the best photocatalytic activity. The photocatalytic property of the products was evaluated by degradation of MO under UV–visible irradiation.
Synthesis of TiO2 NPs-coated CNTs
The crystal structure of the samples was investigated by using powder X-ray diffraction (XRD) and Raman spectra. XRD was performed with Cu Kα radiation on a D/MAX-RB X-ray diffractometer (Rigaku, Japan), and Raman analysis was performed on a LabRAM HR instrument (Horiba Jobin–Yvon, France) in air environment. The morphologies and size of the samples were investigated using a Tecnai G2 F30 field emission transmission electron microscope (FETEM, FEI). UV–visible diffuse reflectance spectra (UV–Vis DRS) were performed on a S4100 UV–Vis spectrophotometer.
Photocatalytic Activity Measurements
Photocatalytic decomposition of MO was examined by optical absorption spectroscopy. Typically, 10 mg of photocatalyst was put into 40.0 mL of MO aqueous solution (10 mg/L) in a 50-mL beaker. Prior to irradiation, the mixture was ultrasonicated in a water bath for 2 min to ensure good dispersion of the photocatalysts. Subsequently, the mixture was stirred in darkness for 20 min in order to establish adsorption/desorption equilibrium between MO molecules and the surface of the photocatalysts. The mixture was then illuminated with a 500-W Xe lamp under a magnetic stirring. The solution of 2.0 mL was drawn out, and the MO solution was separated from the photocatalysts by centrifugation to determine the concentration of MO using a S4100 UV–Vis spectrophotometer.
Results and Discussion
Photocatalytic Activity of the TiO2 NPs-coated CNTs
We have demonstrated that composites of anatase TiO2 NPs-coated CNTs can be straightforwardly prepared by a simple wet chemical method. The molar ratio of CNTs/TiO2 affects the adsorption capability of the final products, and consequently influences the photocatalytic activity. By optimization, TiO2 NPs-coated CNTs sample with CNTs/TiO2 molar ratio of 1:1 exhibits 2.45 times higher efficiency than that of pure TiO2 NPs. The present chemical method is much simpler than previous-reported methods including CVD and electrospinning technique, and is suitable for large-scale production. Because of the efficient adsorption capability and prominent photodegradation efficiency, the anatase TiO2 NPs-coated CNTs might be used for other desired applications, such as photovoltaic and photochemical cells, sensor, and hydrogen production.
This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Ministry of Education, Science and Technology (MEST) (No. 2009-0081819).
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