2.1. Selective photocatalytic degradation by controlling titanate morphology and crystal structure

The adsorption stage is a very important stage in the photocatalytic process; many factors affect on the selectivity in this stage like changing the size, amount, surface, and morphology of the photocatalyst, as well as the size or type of the target compounds. Recent studies revealed that altering the exposed surfaces of TiO2 nanoparticles is by controlling the morphologies of these particles. Sofianou et al. [25] found that the calcined TiO2 nanoplates showed the highest photocatalytic activity toward oxidizing NO gas to NO2 and NO3 ; on the other hand, the washed TiO2 nanoplates, preserving the initial morphology, showed the best photocatalytic activity toward acetaldehyde decomposition. It was concluded that the dominant exposed {1 0 1} or {0 0 1} crystal facet of the TiO2 nanoplates is considered as the key factor in controlling the selectivity in adsorption of these air pollutants. Also, it was demonstrated by other researchers that modifying the surface of TiO2 microspheres by varying the degree of the etching of {0 0 1} facets exhibited tunable photocatalytic selectivity toward the decomposition of azo dyes in water [28].

Recently, Zaki et al. [22] used three different colors (color yellow sunset, color red allura, and color red carmoisine) to test the selectivity of degradation of three different morphologies of TiO2 (spherical, layered, and tubular); the TEM images of these morphologies are shown in Figure 1. It is clear from Figure 1a that the starting TiO2 powder consists of nanosized particles, while Figure 1b shows the TEM image of the synthesized TiO2 nanosheets, and Figure 1c shows the TEM images of the obtained TiO2 nanotubes. The tubes have a diameter of about 16 nm, and they are randomly oriented with nearly homogenous dimensions with some intercalated tubes.

mineralization of organic pollutants to carbon dioxide, water, or low molecular weight compounds under visible or ultraviolet (UV) light radiation [2–5]. Photocatalytic degradation of various families of organic pollutants had been studied using semiconductors such as TiO2, ZnO, Fe2O3, CdS, GaP, and ZnS [6–15]. Titanium dioxide (TiO2) is considered as one of the most important photocatalysts used in water treatment application where it is stable, inexpen-

As it is well known, the mechanism of degradation of an organic pollutant using TiO2 nanostructures depends mainly on producing hydroxyl radicals (˙OH). TiO2 absorbs in the UV region and

peroxide radicals (˙OOH), hydroxyl ions (OH), and H+ ions. All these species will finally form the desired hydroxyl radicals (˙OH). Free radicals are aggressive species and highly active in the chemical reactions; these radicals attack the organic pollutants forming different oxygenated

TiO2 with variable morphologies, rods, spheres, tubes, fibers, sheets, and interconnected architectures, can be prepared with many methods such as hydrothermal method, sol-gel method,

The photocatalytic activity of TiO2 depends on many factors such as specific surface area, size, pore structure, pore volume, exposed surface facet, and crystalline phase [20, 22, 23]; achieving selectivity in degradation using TiO2 in some cases depends on crystallinity and crystal facet [24–26], and also depends on introducing molecular sites on TiO2 surface which produce selective adsorption and degradation for targeted compounds are also introduced [27].

2.1. Selective photocatalytic degradation by controlling titanate morphology and crystal

The adsorption stage is a very important stage in the photocatalytic process; many factors affect on the selectivity in this stage like changing the size, amount, surface, and morphology of the photocatalyst, as well as the size or type of the target compounds. Recent studies revealed that altering the exposed surfaces of TiO2 nanoparticles is by controlling the morphologies of these particles. Sofianou et al. [25] found that the calcined TiO2 nanoplates showed the highest photocatalytic activity toward oxidizing NO gas to NO2 and NO3

the other hand, the washed TiO2 nanoplates, preserving the initial morphology, showed the best photocatalytic activity toward acetaldehyde decomposition. It was concluded that the dominant exposed {1 0 1} or {0 0 1} crystal facet of the TiO2 nanoplates is considered as the key factor in controlling the selectivity in adsorption of these air pollutants. Also, it was demonstrated by other researchers that modifying the surface of TiO2 microspheres by varying the degree of the etching of {0 0 1} facets exhibited tunable photocatalytic selectivity toward the

electrodeposition, chemical vapor deposition, and microwave method [20, 21].

) pairs generate superoxide ions (O2˙

),

; on

sive, nontoxic, insoluble, and potentially reusable in water [2, 16–19].

produces electron-hole pairs. The electron-hole (e-h+

22 Photocatalysts - Applications and Attributes

intermediates and finally converting them into CO2 and H2O.

2. Selective photocatalytic degradation of dyes

decomposition of azo dyes in water [28].

structure

Figure 1. TEM image of spherical TiO2 (a), TiO2 nanosheets (b), and TiO2 nanotubes (c).

Zaki et al. in this work reported that changing the morphology of TiO2 from spherical to layered and tubular shape made each morphology preferentially decompose one dye of the three dyes [22]. From Figure 3a–c, it is clear that the photocatalytic degradation strongly depends on the morphology of TiO2. To test photocatalytic performance of all morphologies, they started the test with yellow sunset as a model, and they found that as moving from the spherical to the tubular structure via the sheet structure, the time of degradation reduced from 400 min for spherical, 75 min for nanotubes, and 55 min for nanosheets, while in the case of the other two colors, red allura and red carmoisine, they found other trends, as shown in Figure 4a–c, where the best

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Figure 3. Photocatalytic activity of spherical (a) TiO2 nanosheets and (c) TiO2 nanotubes.

Figure 2. XRD patterns of spherical TiO2 (a), TiO2 nanotubes (b), and TiO2 nanosheets (c). A, anatase; B, TiO2 (B).

Figure 2 shows XRD patterns of the three morphologies; it is clear from these patterns that the crystallinity of TiO2 nanotubes and TiO2 nanosheets is generally poor; this may be attributed to the small sizes of the prepared samples, which are confirmed by the presence of broad peaks. All the detected peaks confirmed the presence of anatase phase for spherical TiO2 nanoparticles; in the case of nanosheets and nanotubes, there is a contribution of TiO2 (B) with different ratios depending on the morphology; accordingly, we have preferred three. The crystal sizes of the three morphologies were calculated using Scherrer's formula and are listed in Table 1.


Table 1. Structural and kinetic parameter TiO2 nanostructures.

Zaki et al. in this work reported that changing the morphology of TiO2 from spherical to layered and tubular shape made each morphology preferentially decompose one dye of the three dyes [22]. From Figure 3a–c, it is clear that the photocatalytic degradation strongly depends on the morphology of TiO2. To test photocatalytic performance of all morphologies, they started the test with yellow sunset as a model, and they found that as moving from the spherical to the tubular structure via the sheet structure, the time of degradation reduced from 400 min for spherical, 75 min for nanotubes, and 55 min for nanosheets, while in the case of the other two colors, red allura and red carmoisine, they found other trends, as shown in Figure 4a–c, where the best

Figure 3. Photocatalytic activity of spherical (a) TiO2 nanosheets and (c) TiO2 nanotubes.

Figure 2 shows XRD patterns of the three morphologies; it is clear from these patterns that the crystallinity of TiO2 nanotubes and TiO2 nanosheets is generally poor; this may be attributed to the small sizes of the prepared samples, which are confirmed by the presence of broad peaks. All the detected peaks confirmed the presence of anatase phase for spherical TiO2 nanoparticles; in the case of nanosheets and nanotubes, there is a contribution of TiO2 (B) with different ratios depending on the morphology; accordingly, we have preferred three. The crystal sizes of the

K (min<sup>1</sup>

allura

) for Red

K (min<sup>1</sup>

carmoisine

) for Red

Figure 2. XRD patterns of spherical TiO2 (a), TiO2 nanotubes (b), and TiO2 nanosheets (c). A, anatase; B, TiO2 (B).

three morphologies were calculated using Scherrer's formula and are listed in Table 1.

) for Yellow

67.9 nm 0.0513 0.0059 0.0103

Spherical TiO2 95 nm 0.0065 0.0098 0.0082

TiO2 nanotubes 27.1 nm 0.0324 0.0060 0.0189

)

K (min<sup>1</sup>

sunset

Table 1. Structural and kinetic parameter TiO2 nanostructures.

Catalyst K (min<sup>1</sup>

24 Photocatalysts - Applications and Attributes

Morphology Crystal size (nm)

TiO2 nanosheets

3. Selective adsorption and degradation over decorated titanate nanotubes

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There is a major concern on contamination of water by organic pollutants [29–31]. Nowadays, intensive efforts have been devoted for water treatment especially when the problem of water shortage has begun to loom in some cities and countries, particularly in Africa and the Middle East. This problem has made scientists think of reusing water again after treatment. Therefore, many methods of treatment have been used such as filtration [32], biodegradation [33], adsorption [34], and photocatalytic degradation [35] of contaminant especially the organic molecules. Among these methods, adsorption is the simplest, cheapest, and most versatile technique for treating water pollutants [30, 36]. Photocatalytic degradation is also considered as one of the promising technologies used for wastewater treatment, where a suitable catalyst is used for the degradation of toxic organic molecules under irradiation with light [35, 37, 38]. Titanium dioxide (TiO2) is considered as one of the most important photocatalysts used in water treatment application where it is stable, inexpensive, nontoxic, and potentially reusable in water; however, its adsorption ability is very low. Therefore, the key challenge of using TiO2 for the treatment of industrial dye-mediated wastewater is to provide adsorption characteristics through surface or structure modification. Another drawback of TiO2 is the low selectivity due to the formed reactive species (radicals) by means of light irradiation which is difficult to be controlled. In addition, TiO2 efficiently degrades organic pollutants but requires ultraviolet light for activation. Thus, the ideal solution to it is to acquire TiO2 selective adsorption property which will be a powerful technique for imparting selectivity to photocatalytic degradation also.

Transformation of TiO2 nanoparticles to the tubular titanate forms could give some unique photocatalytic properties because of the one-dimensional (1D) geometry, which will enable the electron transfer faster for long distance. Moreover, its nanotubular structure will provide a large specific surface area and pore volume which is very important for providing more active

The treatment of highly enduring toxic dyes and leaving the alterable pollutants to be treated by the low-cost biological treatment systems are considered as great aims for scientists interested in wastewater treatment. Thus, provision of selective materials may be of great benefit. Therefore, one of the important challenges in water treatment is to possess a highly selective

Titanate nanotubes (TNTs) produced from the hydrothermal treatment of TiO2 nanoparticles in the presence of high concentration of NaOH (10 M) at 160C have been used for selective adsorption of specific dyes from water [40, 41]. The obtained sodium titanate (Na2TiO3, NaTNT) nanotubes were characterized before using as an adsorbent. In the TEM micrograph (Figure 5a and b), a clear tubular structure of about 5 nm inner cavity with a mean length of 148 (35) nm and thickness of 8 (1) nm is seen. Due to NaTNTs having a large bandgap (more than 3 ev), it needs high-energy light (UV light) for initiating its photocatalytic activity. Thus, for improving the photocatalytic properties of NaTNTs, gold (Au) nanoparticles were

sites for adsorption and photocatalytic degradation.

method for adsorption and photodegradation of contaminants [39].

Figure 4. Linear transform ln (Co/C) = f(t) of kinetic curves.

degradation rate for yellow sunset was achieved by TiO2 nanosheets, by spherical TiO2 for allura and by TiO2 nanotubes for carmoisine. Finally, they concluded that the preferred orientation of each morphology made it more specific in action; each dye is adsorbed preferentially by one of the three morphologies and decomposed more rapidly.
