Advanced Photocatalysis with Anatase Nano-coated Multi-walled Carbon Nanotubes
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Advanced Photocatalysis with Anatase Nano-coated Multi-walled Carbon Nanotubes Georgios Pyrgiotakis, Sung-Hwan Lee and Wolfgang Sigmund Materials Science and Engineering, University of Florida RHN 225,Gainesville, FL, 326011, USA ABSTRACT A novel approach is presented to synergistically enhance photocatalytic activity in a nanocomposite using the high aspect ratio of carbon nanotube (CNT) and their unique electrical properties. Composite nanoparticles were synthesized with sol-gel nano-coating on multi-walled carbon nanotubes (MWNTs). The nanostructure was characterized using SEM, TEM, XRD, Raman, FTIR and UV-VIS spectroscopies. Photocatalytic efficiencies of commercial photocatalysts (Degussa P25) and TiO2 nano-coated MWNTs were evaluated by Azo dye degradation tests. Superior photocatalytic activity was observed for the nanocomposite with UVA and with visible light only irradiation. INTRODUCTION Recently, TiO2 has attracted a great deal of attention due to its unique photoelectrochemical properties.1 Titania (TiO2) exists in nine different crystal structures, where only anatase and rutile demonstrate those photoelectrochemical processes. 2 It is a direct wide band gap semiconductor with Eg = 3.0 eV for rutile phase and Eg = 3.2 eV for anatase.3 Furthermore, titania is an oxygen deficient material; so there is an excess amount of electrons and consequently it is considered n-type semiconductor.4 The unique photoelectrochemical properties of TiO2 are apparent when it is irradiated with UV light (wavelength of 90 %, Alfa Aesar, 15-24 nm outer diameter, 1-10 m length) were used and the functionalization of the carbon surfaces was performed by chemical oxidation. The amount of 30 mg of MWNTs was weighed out and combined with 200 mL 70% nitric acid solution. The solution was then sonicated for 30 minutes and immediately refluxed and continuously stirred at 140 °C for 10 hours. This results in having nanotubes with an outer diameter less than 20 nm and surfaces partially covered with functional groups (COOH, >C=O, OH). These functional groups were used as anchoring points for chemical reactions in the sol-gel process. Additionally, these functional groups aided in dispersion of the carbon nanotubes in water, as compared to the as received MWCNT’s inability to stay dispersed. This is a critical parameter because under different conditions MWCNTs bundles would have been coated instead of individual CNTs. Once the CNT surfaces were modified, they were centrifuged and redispersed in 200 mL of DI water. An amount of 20 µL of titanium (III) sulfate (>45 %, Alfa Aesar) solution was added to the MWNT dispersion and the mixture was stirred for 1 hour. After washing with H2O, the anatase nano-coated MWNTs were centrifuged and dried at 60 °C for 2 days and heat treated at 500°C to crystallize the TiO2 coating layer. HRTEM and XRD were utilized to confirm crystal structure. Irradiation of UV
Electron-hole pair Electron-hole migration Oxidation by holes 10 s Reduction by electrons 10 s Electron-hole recombination 1
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