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The rutile form of titanium dioxide (r-TiO 2 ) usually shows a photocatalytic activity lower than that of anatase-TiO 2 (a-TiO 2 ). Nevertheless, there are numerous examples of sometimes unexpectedly high activity of r-TiO 2 . This material may appear to be particularly useful when a noncomplete and selective photocatalytic oxidation of organic substrates is required. On the basis of literature examples and our own studies, we compare the photocatalytic activities of r-TiO 2 , a-TiO 2 , and r-TiO 2 /a-TiO 2 composites. Because of a significantly better oxygen adsorption at the surface of r-TiO 2 and a lower redox potential of the excited electron, more efficient O 2•− production takes place at the surface of rutile. As a consequence, generation of 1 O 2 (involving O 2 reduction and the subsequent oxidation of superoxide) and reduction of H 2 O 2 to OH − and HO • are also favored with this material. Therefore, r-TiO 2 can be considered to be a particularly good photocatalyst for activation of molecular oxygen. On the other hand, a-TiO 2 appears to be a stronger oxidant. In its presence, efficient HO • generation (the result of water or surface hydroxyl group oxidation) and oxidation of H 2 O 2 to O 2•− are observed.

The effect of Zn-doping in CoFe2O4 nanoparticles (NPs) through chemical co-precipitation route was investigated in term of structural, optical, and magnetic properties. Both XRD and FTIR analyses confirm the formation of cubic spinel phase, where the crystallite size changes with Zn content from 46 to 77 nm. The Scherrer method, Williamson-Hall (W-H) analysis, and size-strain plot method (SSPM) were used to study of crystallite sizes. The TEM results were in good agreement with the results of the SSP method. SEM observations reveal agglomeration of fine spherical-like particles. The optical band gap energy determined from diffuse reflectance spectroscopy (DRS) varies increases from 1.17 to 1.3 eV. Magnetization field loops reveal a ferromagnetic behavior with lower hysteresis loop for higher Zn content. The magnetic properties are remarkably influenced with Zn doping; saturation magnetization (Ms) increases then decreases while both coercivity (HC) and remanent magnetization (Mr) decrease continuously, which was associated with preferential site occupancy and the change in particle size.

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Highly active photocatalysts were obtained by impregnation of nanocrystalline rutile TiO 2 powders with small amounts of Cu(II) and Fe(III) ions, resulting in the enhancement of initial rates of photocatalytic degradation of 4-chlorophenol in water by the factor of 7 and 4, compared to pristine rutile, respectively. Detailed structural analysis by EPR and X-ray absorption spectroscopy (EXAFS) revealed that Cu(II) and Fe(III) are present as single species at the rutile surface. The mechanism of the photoactivity enhancement was elucidated by a combination of DFT calculations and detailed experimental mechanistic studies including photoluminescence measurements, photocatalytic experiments using scavengers, OH radical detection, and photopotential transient measurements. The results demonstrate that the single Cu(II) and Fe(III) ions act as effective cocatalytic sites, enhancing the charge separation, catalyzing "dark" redox reactions at the interface, improving thus the normally very low quantum yields of UV light-activated TiO 2 photocatalysts. The exact mechanism of the photoactivity enhancement differs depending on the nature of the cocatalyst. Cu(II)decorated samples exhibit fast transfer of photogenerated electrons to Cu(II/I) sites, followed by enhanced catalysis of dioxygen reduction, resulting in improved charge separation and higher photocatalytic degradation rates. At Fe(III)-modified rutile the rate of dioxygen reduction is not improved and the photocatalytic enhancement is attributed to higher production of highly oxidizing hydroxyl radicals produced by alternative oxygen reduction pathways opened by the presence of catalytic Fe(III/II) sites. Importantly, it was demonstrated that excessive heat treatment (at 450 °C) of photocatalysts leads to loss of activity due to migration of Cu(II) and Fe(III) ions from TiO 2 surface to the bulk, accompanied by formation of oxygen vacancies. The demonstrated variety of mechanisms of photoactivity enhancement at single site catalyst-modified photocatalysts holds promise for developing further tailored single-site-modified photocatalysts for various applications. 19calculations that have the surface exposed to vacuum. Furthermore, the oxygen atom of the cluster forms a short bond of 1.79 Å with a fivefold coordinated titanium atom of the surface, closing a TiO 6 octahedra. Figure 10. (a) DOS of the TiO 2 (R)-Cu system, spin-up channel only, aligned to the DOS of the ideal TiO 2 (R) surface. (b) The spin up channel DOS of the final relaxed electron polaron system for the TiO 2 (R)-Cu system. Cu contribution shown. The zero of the x-axis is fitted to the VBM for both plots.The electronic structure of TiO 2 (R)-Cu is shown in Figure 10a. The Cu atom has a significant presence on the CBE. This is primarily due to the Cu d-states. Further, we compare the position of the decorated rutile TiO 2 surface to the bare rutile surface, by comparison and alignment of the electrostatic potential in the vacuum region. 61 When the alignment is taken into action, the Cu-state is margi...