Paramagnetic properties of carbon-doped titanium dioxide
© Minnekhanov et al.; licensee Springer. 2012
Received: 15 May 2012
Accepted: 21 June 2012
Published: 21 June 2012
This paper reports the experimental results on paramagnetic properties of carbon-doped titanium dioxide. The electron paramagnetic resonance study of the samples has been carried out both in dark and under illumination. The nature of defects and their dynamics under illumination of carbon-doped TiO2 samples is discussed.
Titanium dioxide, because of its non-toxicity and high catalytic activity in various photo-oxidation reactions, represents the most important semiconductor photocatalyst. However, its large band gap (approximately 3.2 eV) requires the use of UV light and, therefore, does not allow utilizing also the much larger visible part of solar light . For this reason, during the last years, many attempts were made to obtain a modified titania which is photocatalytically active also with visible light. Typical examples are surface modification by transition metal ions [2, 3] and nonmetallic elements such as carbon , nitrogen , and sulfur . All these novel materials photocatalyze complete visible light mineralization of various pollutants in water and air, and some nitrogen- or carbon-doped titania powder are active even in diffuse indoor daylight of very weak light intensity [4, 5]. Experimental and theoretical results indicated that these dopants generate localized energy levels (surface states) just above the valence band from which visible light excitation becomes feasible [4, 5]. Due to these intra-bandgap states, the carbon-doped titania exhibits a weak sub-bandgap light absorption starting already at about 700 nm. These materials contained 0.4% to 4.0% carbon in the form of carbonate and elemental carbon as indicated by X-ray photoelectron spectra . To characterize these C-doped materials in more detail and to obtain basic information on the nature of the carbon dopant, we investigated the electronic properties by electron paramagnetic resonance ( EPR) spectroscopy. This very sensitive method allows detection and characterization of paramagnetic defects which may be of significant importance for the photocatalytic properties [7–12]. For example, Li et al. ascribed the visible light activity of C-doped TiO2 to the presence of oxygen vacancies as suggested by the EPR detection of Ti3+ species . However, it is noticed that the calculation of g-values is incorrectly performed, and details on the wavelength of exciting light is missing. In the following, we compare EPR data for a series of C-doped titania powders to clarify the nature of the paramagnetic centers and their change upon illumination under well-defined conditions.
Bulk-modified material, C-TiO2-1, containing 0.42 wt.% carbon was prepared through hydrolysis of titanium tetrachloride with tetrabutylammonium hydroxide followed by calcination at 400°C for 1 h and at 350°C for 2 h, respectively . The surface-modified sample C-TiO2-2 containing 1.05 wt.% carbon was prepared by suspending 3 g of titanium dioxide (Kerr-McGee Pigments GmbH, Krefeld, Germany) and 4 ml of glycerol in 50 ml of distilled water. After sonicating for 30 min, the suspension was stirred magnetically overnight, and the solvent was removed. Thereafter, the residue was crushed to a fine powder and calcined in air for 30 min at 300°C. The sample C-TiO2-3 was a commercially available surface-modified material and contained 0.46 wt.% of carbon (Kronos Incorporated, Asse-Zellik, Belgium). All weight percentages of carbon reported in this paper were obtained by elemental analysis. According to X-ray diffraction, all samples consist of the anatase modification .
EPR spectra were detected by the standard Bruker EPR spectrometer ELEXSYS-500 (X-band, sensitivity is around 1010 spin/G; Bruker BioSpin, Moscow, Russia). Mn2+ in MgO was employed as reference for g-values. After filling the powder into a quartz tube, air was pumped off at 5⋅10−6 Torr during 30 min followed by filling with He gas up to a pressure of 10−1-10−2 Torr and sealing of the tubes. The samples were investigated at 300 and 5 K.
The samples were illuminated (in situ) at 5 K with a 100-W tungsten halogen lamp in the spectral range of 400 to 1,000 nm.
Results and discussion
In summary, we should like to conclude that carbon-doped TiO2 samples have CO2− radicals (C-TiO2-1) as well as with carbon defects (dangling bonds) on its surface (C-TiO2-2, C-TiO2-3). Additional energy levels of both interstitial carbon atoms or surface defects (carbon particles) should be located in the band gap of TiO2, as it was shown for titania doped with tiny metal nanoparticles of Cu, Pd, Pt and Ag [22–24], when doping of TiO2 led to formation of electronic surface states in semiconductor band gap. Such additional levels created by dopants can absorb visible light, increasing photosensitivity of the carbon-doped TiO2.
EAK and PKK are both professors and doctorate degree holders in the Chair of General Physics and Molecular Electronics, Department of Physics, Lomonosov Moscow State University. ASV is a research associate. DMD is a PhD student, and AAM is a student at the same university.
The experiments were performed using the facilities of the Collective Use Center at the Moscow State University. We are grateful Prof. X. Kisch for the sample preparation. This work was supported by Russian Federation Ministry of Education and Science (State Contract No. 16.513.11.3141).
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