Preparation And Visible Light Photocatalytic Performance Of Mn-N-codoped TiO₂

Photocatalysis based on powdered semiconductors has received much attention for the effective utilization of solar energy. Among all semiconductors, TiO₂ is the most widely studied photocatalyst, due to its strong redox ability, chemical stability, non-toxicity, and cheapness. Recently, TiO₂ is extensively applied to the purification of air such as De-NOx, De-VOCs, the bactericidal action, and degradation of organic pollutant compounds in wastewater. It can decompose most organic pollutants into CO2 , H2O, and other small inorganic molecules under UV light irradiation. However, TiO₂ absorbs only the very small ultraviolet part (3⁵%) of solar light due to its wide band-gap (3.2 eV for anatase form). In addition, easy recombination of photo-induced electrons and holes makes the efficiency of photoquantum lower. These unfavorable factors make the large scale application of TiO₂ as mutual photocatalyst hampered. Considering the above factors, manganese and nitrogen was doped into TiO₂ to make its absorbing wavelength move to the visible light region and improve its photocatalytic performance.Mn-doped TiO₂ nanocrystal photocatalysts, named Mn-TiO₂-S and Mn-TiO₂-C samples, were synthesized by a simple hydrothermal synthesis method using MnSO4·H2O and MnC2 O4·4H2O as manganese source, respectively. The samples were characterized by X-ray diffraction and UV-vis diffuse reflectance spectroscopy. The photocatalytic activity for the degradation of rhodamine B was evaluated, and the effects of the manganese source and Mn doping amount were investigated. The results showed that all of the Mn-TiO₂ had an anatase crystallite structure and the Mn doping inhibited the growth of the TiO₂ particles. The particle size of Mn-TiO₂ -S was smaller than that of Mn-TiO₂-C. The absorption edge of Mn-TiO₂ shifted to the visible light region. The Mn-TiO₂ showed obvious photocatalytic activity for the degradation of rhodamine B under visible light irradiation, and Mn-TiO₂-S samples showed higher photocatalytic activity than undoped TiO₂ and Mn-TiO₂-C samples.Mn-doped and Mn-N-codeped TiO₂ nanocrystal photocatalysts were synthesized by sol-gel method. XRD, UV-vis, XPS, ESR were employed to characterize the structure of undoped TiO₂ and doped TiO₂ powders, and the effects of the Mn-N-codoping, Mn doping amount and heat treatment temperature were investigated. The results showed that all of the Mn-TiO₂ and Mn-N-TiO₂ had an anatase crystallite structure and Mn-N-codoping promote the phase transition from anatase phase to rutile phase. The absorption edge of Mn-TiO₂ and Mn-N-TiO₂ shifted to the visible light region. The XPS survey spectrum of Mn-N-TiO₂-400 sample calcined at 400℃suggsted that some N atoms substitute O atoms in TiO₂ lattice, and the introduction of a small amount of Mn facilitates further N uptake. N-TiO₂, Mn-TiO₂ and Mn-N-TiO₂ samples showed obvious photocatalytic activity for the degradation of rhodamine B under visible light irradiation, and Mn-N-TiO₂ samples showed higher photocatalytic activity than undoped TiO₂ , N-TiO₂ and Mn-TiO₂ samples..