Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 58

within the forbidden band and usually is not desirable for catalysis applications due to
the sever recombination rate expected.

3.3.3.2 Anion Doping

The investigation of anion doped TiO 2 only have been reported within less than a
decade. The anion doping is meant to substitute the oxygen atoms with other anions
such as C, N, F, P, S, etc. as to improve the photoreactivity in terms of visible light
response.

There are only three types of anion dopants reported to have improved visible
light sensitivity: S, N, and C. Based on the optical and photoemission measurements,
Umebayashi et al. (2002) reported that the doping of S into TiO 2 improves the visible
light response of TiO 2 . They synthesized the S-doped TiO 2 by oxidation of the TiS
particles in controlled environments. Results of their band calculation suggest that the
substitutionally S-doped TiO 2 contributes to the visible light sensitivity due to the
mixing of the S 3 p states with valance band.

The substitutional N doping of TiO 2 has received the greatest attention in the
development of visible-light sensitive photocatalysts in recent years. Theoretical band
structure calculations of C-, N-, F-, P-, and S-doped TiO 2 , one the basis of the FLAPW
method suggest that N is the most proper anion dopant (Asahi et al., 2001), due to the
energy position of the impurity states and the likeness of the N ionic radius to that of O
atoms (Figure 3.7). Asahi et al. (2001) also concluded that substitutional N doping is
more effective than interstitial position since the latter one creates higher density of mid
gap states which can give hindering effect on charge carrier transfer kinetics.

Most commonly reported methods for the synthesis of N-doped TiO 2 include
plasma enhanced chemical vapor deposition (PECVD), sol-gel process, direct oxidation
of TiO 2 under NH 3 flow and ion implantation. The majority of reported N-doped TiO 2
is in the particulate formation. Their optoelectronic property is mostly examined only
based on optical absorption spectrum, which does not reveal the energy position of the
inserted impurity states. In addition, in the particulate system, the thermodynamic
limitation for the redox reaction in the aqueous phase is solely confined by the energy
position of the valance and conduction band edges. Consequently, there is no room for
the optimization of catalytic reactivity.

3.4 Reaction Variables

In addition to the photocatalyst's intrinsic activity, which is determined by
crystalline structure, particle size, specific area, porosity, surface modification and the