3. Selective adsorption and degradation over decorated titanate nanotubes

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 sites for adsorption and photocatalytic degradation.

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 method for adsorption and photodegradation of contaminants [39].

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

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.

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

26 Photocatalysts - Applications and Attributes

deposited on the surface where the Au nanoparticles have a plasmonic response in the visible light. The in situ photoreduction deposition method was used for forming a good contact between the gold nanoparticles and NaTNTs. From Figure 5c and d, a homogeneous distribution of highly crystalline spherical Au nanoparticles with a mean diameter of 7.7 (1) nm was formed at the surface of the NaTNTs, and there is no any aggregation of Au particles.

As mentioned before, the adsorption ability of TiO2 nanoparticles is extremely low. El Rouby et al. reported that the titanate form can adsorb up to 55% of the methylene blue (MB) dye initial concentration after 180 min compared to the TiO2 nanoparticles which show negligible adsorption for MB (Figure 6a) [40]. This high adsorption ability of NaTNTs is due to their high porosity, large specific surface area, and strong electrostatic interaction between the positively charged dye and the negatively charged surface of NaTNTs. In addition, the internal cavity of the tubular structure can contribute to further extend the area on which organic molecules can be adsorbed.

The photocatalytic activity of NaTNTs and Au-decorated NaTNTs has been assessed under simulated solar light (350–2400 nm). Twenty-seven percent degradation of MB was achieved

Figure 5. (a) Low and (b) high magnification TEM images of the NaTNTs, (c) TEM images of au-NaTNTs, and (d) STEM images of the Au-NaTNTs [12] (copyright 2017, IOP Publishing Ltd).

Figure 6. (a) Effect of contact time on MB dye adsorption in the presence of TiO2 nanoparticles (black) and NaTNTs (blue) under agitation in the dark. (b) Photodegradation of MB in the presence of NaTNTs (gray triangles) and Au-functionalized NaTNTs (blue circles). (c) Photodegradation of the three mixed organic dyes: Tz (green triangles), RhB (red triangles), and MB (blue circles) using Au-functionalized NaTNTs as catalysts. The inset shows the absorption spectrum of the mixture after adsorption/desorption equilibrium and prior to the photocatalytic degradation. The absorption maxima for each dye in such

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a solution are 421 nm (Tz), 555 nm (RhB), and 674 nm (MB) [12] (copyright 2017, IOP Publishing Ltd).

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deposited on the surface where the Au nanoparticles have a plasmonic response in the visible light. The in situ photoreduction deposition method was used for forming a good contact between the gold nanoparticles and NaTNTs. From Figure 5c and d, a homogeneous distribution of highly crystalline spherical Au nanoparticles with a mean diameter of 7.7 (1) nm was

As mentioned before, the adsorption ability of TiO2 nanoparticles is extremely low. El Rouby et al. reported that the titanate form can adsorb up to 55% of the methylene blue (MB) dye initial concentration after 180 min compared to the TiO2 nanoparticles which show negligible adsorption for MB (Figure 6a) [40]. This high adsorption ability of NaTNTs is due to their high porosity, large specific surface area, and strong electrostatic interaction between the positively charged dye and the negatively charged surface of NaTNTs. In addition, the internal cavity of the tubular structure can contribute to further extend the area on which organic molecules can

The photocatalytic activity of NaTNTs and Au-decorated NaTNTs has been assessed under simulated solar light (350–2400 nm). Twenty-seven percent degradation of MB was achieved

Figure 5. (a) Low and (b) high magnification TEM images of the NaTNTs, (c) TEM images of au-NaTNTs, and (d) STEM

images of the Au-NaTNTs [12] (copyright 2017, IOP Publishing Ltd).

formed at the surface of the NaTNTs, and there is no any aggregation of Au particles.

be adsorbed.

28 Photocatalysts - Applications and Attributes

Figure 6. (a) Effect of contact time on MB dye adsorption in the presence of TiO2 nanoparticles (black) and NaTNTs (blue) under agitation in the dark. (b) Photodegradation of MB in the presence of NaTNTs (gray triangles) and Au-functionalized NaTNTs (blue circles). (c) Photodegradation of the three mixed organic dyes: Tz (green triangles), RhB (red triangles), and MB (blue circles) using Au-functionalized NaTNTs as catalysts. The inset shows the absorption spectrum of the mixture after adsorption/desorption equilibrium and prior to the photocatalytic degradation. The absorption maxima for each dye in such a solution are 421 nm (Tz), 555 nm (RhB), and 674 nm (MB) [12] (copyright 2017, IOP Publishing Ltd).

by NaTNTs that may be ascribed to the small portion of UV light irradiated from the solar simulator (350–400 nm). When Au-decorated NaTNTs were used as a photocatalyst, a remarkable 72% degradation of MB is observed (Figure 6b). This is due to the plasmonic photosensitization of gold nanoparticle which is considered as an effective way for increasing the photocatalytic activity of semiconductors holding large bandgaps such as TNTs.

This increase of specific surface area is due to the formation of a large number of pores because of the removal of sodium ions (larger ions) and insertion of cobalt ions (smaller ions). This leads to the increase in the surface area of pore as the cobalt content was increased in the samples (Table 2). It was found that MB uptake on NaTNTs was about 89 mg/g after 35 min. This indicated its ferocity to remove MB from water in a very short time at room temperature. In replacing the Na+ by Co2+, the surface area and number of pores were increased as mentioned previously. Thus, the available adsorption active sites will be more than in the case of NaTNT. This was reflected on the adsorption capacity of Co-doped TNT samples, where the uptake was increased by increasing the dopant content. The adsorption capacity reached 92.5, 91.9, and 91.8 for Co-doped TNT 1, Co-doped TNT 2, and Co-doped TNT 3, respectively,

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The problem in Co-doped TNT catalysts is its wide bandgap as in the cases of TiO2 and NaTNTs. So, for adding a photocatalytic property, the prepared Co-doped TNTs were calcined at 500C for enhancing its crystallinity and thus the photocatalytic activity. But, after calcination, the adsorption property was decreased, and the degradation was low (Figure 8a) due to light shielding by the high concentration of MB. The decrease in adsorption is due to the decrease in surface area which is caused by means of wall collapse and loose of tubular structure after calcination. Therefore, the previous concept of attaching Au nanoparticles to the surface of Co-doped TNTs was used. It was noticed that MB concentration (1 <sup>10</sup><sup>4</sup> M) was decreased gradually with time up to (1.2 <sup>10</sup><sup>5</sup> M) after 300 min, and with continuous illumination, MB was completely degraded (Figure 8b). The conclusion from these results is that a highly concentrated organic dye and pollutant solution can be photocatalytically degraded efficiently [41]. The prepared Au-decorated Co-doped TNT catalysts were used for the selective adsorption and degradation of MB in the same dye mixture (MB, RhB, TZ). From Figure 9a, the selective

Figure 7. Methylene blue dye (1 <sup>10</sup><sup>4</sup> M) uptake on titanate nanotubes doped with different ratios of cobalt (0.4 g/L)

[13] (copyright 2018, Elsevier).

depending on the concentration of Co2+ in NaTNTs (Figure 7).

The well-established Au-decorated NaTNT photocatalyst was applied for selective degradation of MB from dye mixture. Three different dyes as model (MB, rhodamine B (RhB), and tartrazine (Tz)) have been used where the absorption spectra of the three dyes allow for interference-free monitoring of the photodegradation for each one of them, thus facilitating their tracking over time through the change in its absorbance. As shown in Figure 6c, 65% of MB was decomposed after 240 min, while Tz and RhB remain in solution showing no trace of reaction after that time. These results clearly evidence for the selectivity of Au-NaTNT composite to MB degradation. This selectivity toward MB is attributed to the net charge of the dyes under consideration [40]. Through electrotactic interaction, the surface of the NaTNTs governs the preferential adsorption of the positively charged dye (MB), thus facilitating a high degree of contact between MB and the photocatalyst. On the other hand, in the case of Tz, it remains intact in solution due to its negatively charged surface which makes repulsion with the negative charges on NaTNTs. In a similar way, the interaction of RhB with the NaTNT surface is significantly weakened, due to the deprotonation of the RhB carboxyl group when dissolved in water and thus the formation of a zwitterionic species [40].

For the photocatalytic degradation of organic dyes in a highly concentrated solution, it will be difficult for the catalyst to achieve degradation unless it has a good adsorption property. This is because during the photocatalytic process by any semiconductor, the light strikes the catalyst surface leading to excitation of valence band electrons forming holes in the valence band that can make degradation for the pollutants. But in the case of high-concentrated dye solution, the dyes can strongly absorb the incident light and prevent the light to reach the catalyst surface forming impermeable solution. Thus, the electrons cannot be exited, and the degradation process will not start. Thus, in that case, it is very useful to use a catalyst with high adsorption property especially in addition to its high photocatalytic degradation properties.

In another work, El Rouby [41] tried to increase the adsorption ability of NaTNTs through doping by transition metal (Co). It was found that replacement of Na+ by Co2+ leads to increase the specific surface area as shown in Table 2 which will directly affect the adsorption extent.


Table 2. Effect contents on surface area, pore size, and pore volume of Co-doped TNT samples [13] (copyright 2018, Elsevier).

This increase of specific surface area is due to the formation of a large number of pores because of the removal of sodium ions (larger ions) and insertion of cobalt ions (smaller ions). This leads to the increase in the surface area of pore as the cobalt content was increased in the samples (Table 2). It was found that MB uptake on NaTNTs was about 89 mg/g after 35 min. This indicated its ferocity to remove MB from water in a very short time at room temperature. In replacing the Na+ by Co2+, the surface area and number of pores were increased as mentioned previously. Thus, the available adsorption active sites will be more than in the case of NaTNT. This was reflected on the adsorption capacity of Co-doped TNT samples, where the uptake was increased by increasing the dopant content. The adsorption capacity reached 92.5, 91.9, and 91.8 for Co-doped TNT 1, Co-doped TNT 2, and Co-doped TNT 3, respectively, depending on the concentration of Co2+ in NaTNTs (Figure 7).

by NaTNTs that may be ascribed to the small portion of UV light irradiated from the solar simulator (350–400 nm). When Au-decorated NaTNTs were used as a photocatalyst, a remarkable 72% degradation of MB is observed (Figure 6b). This is due to the plasmonic photosensitization of gold nanoparticle which is considered as an effective way for increasing the

The well-established Au-decorated NaTNT photocatalyst was applied for selective degradation of MB from dye mixture. Three different dyes as model (MB, rhodamine B (RhB), and tartrazine (Tz)) have been used where the absorption spectra of the three dyes allow for interference-free monitoring of the photodegradation for each one of them, thus facilitating their tracking over time through the change in its absorbance. As shown in Figure 6c, 65% of MB was decomposed after 240 min, while Tz and RhB remain in solution showing no trace of reaction after that time. These results clearly evidence for the selectivity of Au-NaTNT composite to MB degradation. This selectivity toward MB is attributed to the net charge of the dyes under consideration [40]. Through electrotactic interaction, the surface of the NaTNTs governs the preferential adsorption of the positively charged dye (MB), thus facilitating a high degree of contact between MB and the photocatalyst. On the other hand, in the case of Tz, it remains intact in solution due to its negatively charged surface which makes repulsion with the negative charges on NaTNTs. In a similar way, the interaction of RhB with the NaTNT surface is significantly weakened, due to the deprotonation of the RhB carboxyl group when dissolved

For the photocatalytic degradation of organic dyes in a highly concentrated solution, it will be difficult for the catalyst to achieve degradation unless it has a good adsorption property. This is because during the photocatalytic process by any semiconductor, the light strikes the catalyst surface leading to excitation of valence band electrons forming holes in the valence band that can make degradation for the pollutants. But in the case of high-concentrated dye solution, the dyes can strongly absorb the incident light and prevent the light to reach the catalyst surface forming impermeable solution. Thus, the electrons cannot be exited, and the degradation process will not start. Thus, in that case, it is very useful to use a catalyst with high adsorption property especially in addition to its high photocatalytic degradation

In another work, El Rouby [41] tried to increase the adsorption ability of NaTNTs through doping by transition metal (Co). It was found that replacement of Na+ by Co2+ leads to increase the specific surface area as shown in Table 2 which will directly affect the adsorption extent.

Table 2. Effect contents on surface area, pore size, and pore volume of Co-doped TNT samples [13] (copyright 2018,

/g) Pore size (Å) Pore volume (cm<sup>3</sup>

/g)

/g) Surface area of pores (m<sup>2</sup>

NaTNT 132.15 144.37 51.09 0.900 Co-doped TNT 1 161.47 163.25 53.23 0.243 Co-doped TNT 2 162.62 177.94 49.33 0.226 Co-doped TNT 3 170.83 185.14 51.62 0.255

photocatalytic activity of semiconductors holding large bandgaps such as TNTs.

in water and thus the formation of a zwitterionic species [40].

properties.

Elsevier).

Sample BET (m<sup>2</sup>

30 Photocatalysts - Applications and Attributes

The problem in Co-doped TNT catalysts is its wide bandgap as in the cases of TiO2 and NaTNTs. So, for adding a photocatalytic property, the prepared Co-doped TNTs were calcined at 500C for enhancing its crystallinity and thus the photocatalytic activity. But, after calcination, the adsorption property was decreased, and the degradation was low (Figure 8a) due to light shielding by the high concentration of MB. The decrease in adsorption is due to the decrease in surface area which is caused by means of wall collapse and loose of tubular structure after calcination. Therefore, the previous concept of attaching Au nanoparticles to the surface of Co-doped TNTs was used. It was noticed that MB concentration (1 <sup>10</sup><sup>4</sup> M) was decreased gradually with time up to (1.2 <sup>10</sup><sup>5</sup> M) after 300 min, and with continuous illumination, MB was completely degraded (Figure 8b). The conclusion from these results is that a highly concentrated organic dye and pollutant solution can be photocatalytically degraded efficiently [41].

The prepared Au-decorated Co-doped TNT catalysts were used for the selective adsorption and degradation of MB in the same dye mixture (MB, RhB, TZ). From Figure 9a, the selective

Figure 7. Methylene blue dye (1 <sup>10</sup><sup>4</sup> M) uptake on titanate nanotubes doped with different ratios of cobalt (0.4 g/L) [13] (copyright 2018, Elsevier).

Figure 8. Absorption spectra of MB dye (1 <sup>10</sup><sup>4</sup> M) in presence of (a) Co-doped TNT 1 calcined at 500C for 2 h and (b) Co-doped TNT 1 at Au in dark and under illumination [13] (copyright 2018, Elsevier).

Author details

Ayman Hassan Zaki\* and Waleed Mohamed Ali. El Rouby \*Address all correspondence to: ayman\_h\_zaki@yahoo.com

TNT 1 as a catalyst (0.4 g/L) [13] (copyright 2018, Elsevier).

Advanced Sciences (PSAS), Beni-Suef University, Beni-Suef, Egypt

Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for

Figure 9. Selective adsorption (a) and selective degradation (b) of MB dye from dye mixture (methylene blue, rhodamine B, and tartrazine dye [5 <sup>10</sup><sup>5</sup> M]) in dark and under simulated solar light illumination using Au-decorated Co-doped

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adsorption of MB from the dye mixture is clear for the same reason of surface charge as discussed before. After adsorption and subjecting the dye solutions to illumination of simulated light, the degradation of MB was started (Figure 9b). According to postulates of any photocatalytic process, the catalyst will initially adsorb the reactant, catalyze the reaction, and finally leave the products. By the same concept, the adsorbed dye was degraded, while dyes in the solution remained without effect [13].

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Figure 9. Selective adsorption (a) and selective degradation (b) of MB dye from dye mixture (methylene blue, rhodamine B, and tartrazine dye [5 <sup>10</sup><sup>5</sup> M]) in dark and under simulated solar light illumination using Au-decorated Co-doped TNT 1 as a catalyst (0.4 g/L) [13] (copyright 2018, Elsevier).
