**2.6. Immobilized TiO<sup>2</sup>**

reported to be improved by Ir-C-N tridoped TiO<sup>2</sup>

Ir4+ by substituting Ti in the lattice of TiO<sup>2</sup>

best photocatalytic activity, than that of TiO<sup>2</sup>

monodisperse mesoporous anatase TiO2

catalytic reaction [87]. Anodic TiO2

**2.5. Nanocarbon modified TiO<sup>2</sup>**

attributed to the synergistic effect of W, Bi and S.

incorporated into the surface of TiO2

46 Photocatalysts - Applications and Attributes

by W–Bi–S-tridoped TiO2

**2.4. Nano-structured TiO<sup>2</sup>**

studied. TiO2

bare O2

of O2

–TiO2

mance of GO–OTiO<sup>2</sup>

intimate interface of GO–OTiO<sup>2</sup>

–TiO2

TiO2

effect between Ir, C and N on the electron structure of TiO<sup>2</sup>

rowed to ~3.0 eV, resulting in improved photocatalytic H<sup>2</sup>

Amongst the various strategies that have been used to enhance TiO2

under UV-visible light, due the synergistic

nanoparticles, whereas the C and N were also

and Bi–S–TiO2

nanospheres using a template material and found the

nanotubes have been reported to allow a high control over

nanoparticles in interstitial mode. The absorption of

nanoparticles was expanded into the visible light region and the band gap was nar-

nanoparticles. It was found that the absorption edge of TiO2

, W–S–TiO<sup>2</sup>

irradiation. Tan et al. [83] investigated the photocatalytic degradation of methylene blue

expanded into visible-light region after doping with W, Bi and S and the catalytst showed the

improvement of morphology, crystal structure and surface area have also been considered important and widely investigated approach to achieve better photocatalytic performance. The nanotitania crystallinity can simply be enhanced by optimizing the annealing temperature. However, the stability of the structure and geometries have to be considered when annealing [84]. For the nanotitania morphology and surface area, various ordered structures have been

resulting catalysts to show high photocatalytic degradation efficiency and selectivity towards different target dye molecules and could be readily separated from a slurry system after photo-

the separation of photogenerated charge carriers in photocatalytic reactions. The nanotube array has as key advantage the fact that nanotube modifications can be embedded site specifically into the tube wall or at defined locations along the tube wall. This allows for engineering of reaction sites giving rise to enhanced photocatalytic efficiencies and selectivities [88].

The design and preparation of graphene-based composites containing metal oxides and metal nanoparticles have attracted attention for photocatalytic performances. For example,

significantly improved by the addition of GO, at which the resulting hybrid composite retained a high reactivity. The photoactivity attained was about 1.6 and 14.0 folds higher than that of

new possibilities in the development of novel, next generation heterojunction photocatalysts

and the commercial Degussa P25, respectively. This high photocatalytic perfor-

and an enhanced separation and transfer of photogenerated charge carriers at the

was attributed to the synergistic effect of the visible-light-responsiveness

heterojunctions. This study is reported to have opened up

Tan et al. [89] prepared a novel graphene oxide-doped-oxygen-rich TiO2

low-power energy-saving daylight bulbs. It was found that the photostability of O2

heterostructure and evaluated its activity for photoreduction of CO<sup>2</sup>

nanotubes [85, 86], nanowires [79], nanospheres [87], etc. Tang et al. fabricated

, S-TiO<sup>2</sup>

. It was found that Ir existed as

evolution under UV-visible light

was

. This might be

photocatalytic activity,

(GO–OTiO<sup>2</sup>

under the irradiation of

) hybrid

was

–TiO2

Another drawback of TiO<sup>2</sup> nanoparticles mentioned above is the formation of uniform suspension in water which makes its recovery difficult, therefore hindering the application of photocatalytic in an industrial scale. As a result, many studies have attempted the modification of TiO2 nanoparticles on support materials such as clays [93, 94] quartz [95], stainless steel [96], etc. Clays have been reported to be a significant support material for TiO<sup>2</sup> nanoparticles because of their layered morphology, chemical as well as mechanical stability, cation exchange capacity, non-toxic nature, low cost and availability. Therefore, TiO<sup>2</sup> /clay nanocomposites have attracted much attention for application in both water and air purification and have been prepared by numerous researchers. Belver et al. [97] investigated the removal of atrazine under solar light using a novel W-TiO2 /clay photocatalysts. It was found that the photocatalytic activity of W-TiO2 /clay catalyst exhibited higher photocatalytic performance than that of an un-doped TiO2 /clay, which was explained by the presence of W ions in the TiO<sup>2</sup> nanostructure. The substitution of Ti ions with W resulted in the increase of its crystal size and the distortion of its lattice and moderately narrower band gap of photocatalysts. Mishra et al. [98] reported the preparation of TiO2 /clay nanocomposites for photocatalytic degradation of VOC and dye. They found that the photocatalytic performance of TiO<sup>2</sup> /clay nanocomposites is highly dependent on the clay texture (as 2:1 clays show highest activity than 1:1) apart from their surface area and porosity. Moreover, the reactions involving TiO<sup>2</sup> /Clay photocatalyst were fast with rate constant of 0.02886 and 0.04600 min−1 for dye and VOC respectively than the other nanocomposites.

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