Preparation of Visible-light Responsive Nanocrystalline N/TiO2 and Photocatalytic Degradation of Benzoic Acid

yu xiujuan
Department of Environmental Science and Engineering, Heilongjiang University

wang* yongqiang
department of environmental science and engineering, Harbin Institute of technology

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     Last modified: March 12, 2006

Abstract
In order to utilize visible light in photocatalytic degradation of organic polltants in aqueous media, nitrogen-doped titanium dioxide (N/TiO2) particles with nanometeric size were directly synthesized by wet hydrolyzation method with ammonia aqueous solution as a source of nitrogen. The products were characterized by Brauenner-Emmet-Teller (BET), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), surface photovoltage spectroscopy (SPS), electric-field-induced surface photovoltage spectroscopy (EFISPS) and UV-vis diffuse reflectrance spectra.
The diffuse reflectrance spectra of N/TiO2 exhibited distinct absorption bands in the visible light region (400~800 nm). Its particle size is about 9nm and BET surface area is 139.3 m2/g. According to XRD spectra, the crystal form of N/TiO2 sample was anatase entirely. An anatase-to rutile transition in N/TiO2 seems to be restrained by nitrogen-doped.
The N atomic composition of N/TiO2 was analyzed with XPS. There are two N1s XPS peaks at 396.625ev and 400.875ev which corresponds to atomic β-N (396ev, Ti-N bonds) and molecularly chemisorbed γ-N2 (400ev, N-N bonds). These findings suggest that N/TiO¬2 prepared in this paper was composed of the bulk containing β-N atoms. In N/TiO2, N atoms replace the O of the TiO2 and form the Ti-N bonding which affected the visible light response of TiO2. And the chemical composition of N/TiO2 was approximately calculated from the peak intensities of the XPS spectra were TiO2-xNx (x=0.025).
Compared with TiO2 powders, the absorption edge of the N/TiO2 powders shifts from 330nm to 459nm in SPS spectrum. Photo-voltage swich of N/TiO2 exhibited red shift obviously. The band gap of N/TiO2 is decreased to 2.7ev. The proposed band structure of nitrogen doped TiO2 was given. It indicated that the isolated narrow band of N2p orbitals, which is formed above the valence band, was responsible for the visible light activity.
The photocatalytic activity of the prepared N/TiO2 photocatalyst was estimated on the basis of decomposition of 25mg/L benzoic acid (BA) in 0.1g/L of photocatalyst suspended system. The photoreactor consisted of a cylindrical borosilicate glass reactor with an effective volume of 1250ml, a cooling water jacket, and a light source of a 125W Xe lamp. The removal efficiency of benzoic acid was determined by the change of total organic carbon (TOC). After 120min, the TOC removal efficiency with N/TiO2 is 53.4%, but that with TiO2 photocatalyst was only 21.6%.
This study demonstrates a simple route for the synthesis of nanocrystalline nitrogen-doped anatase titania by calcinations of acidified tetra-butyl titanate in presence of ammonia aqueous solution. The N doped anatase titania showed excellent photocatalytic activity under simulated sunlight. And the primary ultraviolet activity of the TiO2 photocatalyst didn’t decrease.