Assessment of CdS as Sensitizer for TiO2 and ZnO in Water Purification with Solar Light
Publication Type
Conference Paper

TiO2 and ZnO nano-particles are known to behave as catalysts for photo-degradation of water contaminants [1-7]. Once excited, the electrons jump from the valence band to the conduction band of the particles. This process creates the so-called electron-hole pairs. The electrons may then reduce different species, such as aqueous oxygen. On the other hand, the holes oxidize organic contaminant molecules.  Having a relatively high band gap, ~3.2 eV, and a valence-band edge with very highly positive potential, TiO2 and ZnO are expected to have very strong oxidizing power towards almost all organic contaminants. Such features make them good candidates for water purification using the cost-free solar light. Moreover, they are non-hazardous low cost materials, with a very high thermal and chemical stability. However, the high band gaps, limit their use in solar energy, since they demand UV light for excitation. Solar light that reaches our earth is mostly in the visible and IR region, with very little UV. Therefore, their applications are limited to the UV region.

In order to use it in the visible region, researchers modified TiO2 particles with dyes in a so-called sensitizationstrategy [8-15]. In sensitization, the dye molecules are excited with visible light. Electron-hole pairs are thus created. The hole would then oxidize contaminants, and electrons immigrate toward the TiO2 valence band.  Such model is explained in Scheme 1.  

In our search for efficient and economic catalytic systems for water purification processes, we have modified the surfaces, of TiO2 nano-particles, with different types of dyes. Synthetic (2,4,6-triphenylpyrilium hydrogen sulfate-TPPHS-, metalloporphyrins, CdS and CdSe) and natural (Henna, Pecan, and others) dyes have been attached to TiO2 surfaces. Despite the fat that many natural dyes lack high stability, they are potentially valuable due to their non-toxic nature. The TiO2/dye systems have been investigated as catalysts for photo-degradation of different organic contaminants in water, such as pesticides, phenols, oxalic acid, medically active ingredients, and others. Both UV and Visible regions have been studied. In the UV region, the dyes affected the rates of degradation by behaving as charge transfer mediators between the semiconductor solid surface and the contaminant molecules. In the visible region, the dyes behaved as sensitizers for the TiO2 semiconductor [16].  Despite this, the CdS leached out hazardous Cd2+ ions into solution.  We tried to prevent such leaching by supporting the TiO2/CdS onto solid silica, but the problems were not solved out.  This puts limits on the potential use of CdS as sensitizer for water purification.

Contrary to TiO2, the ZnO particles did not function with dyes. Their efficiency was lowered by the dye due to screening effects. We have seen that ZnO works efficiently in direct solar light by the UV tail available. This is due to the high UV absortivity of ZnO particles.

The TiO2/Dye and ZnO systems were then supported onto surfaces of insoluble supports, such as activated carbon, clay, glass and sand [16]. In the support/TiO2/Dye systems and support/ZnO the support showed further enhancement in catalytic degradation of contaminants in the UV and visible regions. The support surface presumably adsorbs contaminant molecules and brings them into close proximity with the catalytically active sites, thus enhances the efficiency. Moreover, the hydrophobic nature of the solid surfaces makes the support/catalyst easier to separate and isolate after use.  The supported/catalyst systems showed promising recovery-reuse features for more than three times with not much loss of efficiency. The results observed in these laboratories, together with relevant discussions and models, regarding sensitization and charge transfer re-mediations, will be presented in this plenary.  Environmental impact, technical difficulties, economic considerations and future perspectives will also be discussed.

Conference Title
The 1st International Workshop on Advanced Material
Conference Country
Conference Date
Jan. 1, 1970 - Jan. 1, 1970
Conference Sponsor