N-TiO2: chemical synthesis and photocatalysis

MATIAS FACTOROVICH; LUCAS GUZ; CANDAL ROBERTO

San Diego, CA

Conferencia; 15th International Conference on TiO2 Photocatalysis: Fundamentals and Applications; 2010

Redox-technologies

Non metal doped TiO2 (in particular N-TiO2) have attracted the interest of researchers all around the world, as possible photocatalysts with activity under visible light (VL, l > 400 nm). Several methods, based in different technologies, have been proposed for the synthesis of these materials. The chemical route is a real challenge for chemists, and in addition the process by which the non-metal is incorporated into the TiO2 is not clear. There are also doubts about the location of the non metallic atoms: are they located into the structure of TiO2? Or are they located at the surface of the TiO2 particles? Recently, it was reported that the formation of colored nitrogenated compounds on the surface of TiO2 are the responsible of the photocatalytic activity under VL (Horst Kisch et al). A better understanding of the synthesis mechanism/s is necessary in order to improve the reproducibility of the protocols propose for the production of non metal doped TiO2. By the other hand, there are other issues to be considered in relation with the activity of these materials under UVA or VL. Recently, it was reported that the photocatalytic mechanisms on TiO2 or N-TiO2 are different. In the case of N-TiO2 under VL the mechanisms are not mediated by OH radicals but other species with less oxidative capacity (Górska et al). Besides, when working under VL, the possibility of self-sensitization should be considered. Several substances, like phenol, chlorophenol and salicylic acid, can be oxidized in the presence of TiO2 and VL (Wang et al). We have studied the synthesis of N-TiO2 via the urea route, in an attempt to determine the formation of intermediates that leads to the doping of TiO2 with N. The intermediates detected by FTIR were similar to those found during the thermal decomposition of urea. Biuret, cyanuric acid, ammelide, ammeline and melanine were detected by FTIR in the temperature range 175-375 C. The crystalline degree was very low in this temperature range (anatase could be detected at 375 C). In the range 375-500 C, the crystalinity increased and cyanates predominated over the other intermediates. The color of the samples became bright yellow at this point. The better conditions for synthesis were reached by firing at 250 C for 3 hr, followed by firing at 500 C for 1 minute. Biuret, one of the intermediates of urea thermal decomposition, was successfully used as dopant. N-TiO2 samples with different level of doping were analyzed to determine the location of N. XPS-analysis indicated that N was located into the crystalline network, substituting O2-, and as cyanate or cyanide on the surface of the particles. The band gap of the N-doped samples shifted from 3.03 eV to 2.38 eV as the amount of N increases. Localized states were also detected by diffuse reflectance as a band at 2.75 eV. The activity of the photocatalysts was determined by using salicylic acid as target. Experiments were run under UVA or VL. The higher degradation rate was obtained with TiO2 and the lower for N-TiO2, both under UVA. In the last case localized states may have enhanced the electron-hole recombination leading to lower photocatalytic activity. Under VL the degradation rate was between the previous cases. The initial degradation rate was higher with N-TiO2 than with TiO2 but, in both cases, decreased with the time. This phenomenon may be related with the generation of recalcitrant byproducts that cannot be degraded by the oxidant species produced under visible light illumination. The similarity in degradation rate with both photocatalysts may be related with the self-sensitization of salicylic acid.