Selectivity of Sol-Gel and Hydrothermal TiO2 Nanoparticles towards Photocatalytic Degradation of Cationic and Anionic Dyes

Titanium dioxide (TiO2) nanoparticles have been extensively studied for catalyzing the photo-degradation of organic pollutants, the photocatalyst being nonselective to the substrate. We, however, found that TiO2 nanoparticles prepared via the sol-gel and hydrothermal synthetic routes each possess a definite specificity to the charge of the substrate for photodegradation. The nanoparticles were characterized by SEM, FTIR, XRD, TGA, and UV-visible spectra, and the photocatalytic degradation under UV-B (285 nm) irradiation of two model compounds, anionic methyl Orange (MO) and cationic methylene blue (MB) was monitored by a UV-visible spectrophotometer. Untreated sol-gel TiO2 nanoparticles (Tsg) preferentially degraded MO over MB (90% versus 40% in two hours), while after calcination at 400 °C for two hours (Tsgc) they showed reversed specificity (50% MO versus 90% MB in one hour). The as-prepared hydrothermal TiO2 nanoparticles (Tht) behaved in the opposite sense of Tsg (41% MO versus 91% MB degraded in one and a half hours); calcination at 400 °C (Thtc) did not reverse the trend but enhanced the efficiency of degradation. The study indicates that TiO2 nanoparticles can be made to degrade a specific class of organic pollutants from an effluent facilitating the recycling of a specific class of pollutants for cost-effective effluent management.


Introduction
Synthetic dyes are extensively used in dying textiles, leathers, papers, plastics, and fibers.However, most of the synthetic dyes are toxic, mutagenic, carcinogenic, and allergenic.Discharging the wastewater of these dyeing industries has instigated a major problem of water contamination as these dyes contain stable aromatic rings that resist degradation under normal conditions [1][2][3][4][5].Thus, the removal of these organic contaminants from the effluents by degrading them into environmentally benign fragments is energy-intensive and costly.Accordingly, the cost-effective removal of synthetic dyes from the effluents to guard against their adverse effect on the environment is a big challenge.Selective degradation of some of the components from the effluents can pave the way for recycling and cost-effective management of the effluents of these industries.
TiO 2 has three crystalline polymorphs such as anatase, rutile, and brookite, each of which may show different photocatalytic activity.Moreover, the chemical reactivity may strongly depend on the crystalline structure, morphology, size, and shape of the TiO 2 nanoparticles of a particular crystalline polymorph [25][26][27][28].It has been demonstrated that nanoscale TiO 2 such as nano-spheres, nano-prisms, nano-wires, nano-rods, and nanotubes having appreciably large specific surface areas lead to better physical and chemical properties [29], including catalyzing photo-degradation of organic pollutants.This makes it essential to compare and evaluate different synthetic methods for obtaining control over the size, shape, surface properties, and crystalline polymorph of the TiO 2 nanoparticles for making the best use of their photocatalytic ability.
In this study, we have demonstrated that the TiO 2 nanoparticles prepared by two different synthetic routes, such as sol-gel and hydrothermal are quite specific toward the photocatalytic degradation of cationic and anionic dyes under UV-B irradiation (285 nm).We have found that the photocatalytic degradation efficiency of untreated TiO 2 nanoparticles prepared by the sol-gel technique is remarkably higher for the anionic model dye methyl orange (MO), compared to methylene blue (MB), a cationic dye (90% for MO versus 40% for MB degradation within two hours).The said specificity was reversed for the same nanoparticles calcined at 400 • C, especially in the early stage of the degradation reaction (50% for MO versus 90% for MB in one hour).On the other hand, the as-synthesized hydrothermal nanoparticles displayed specificity opposite of the as-synthesized sol-gel nanoparticles (91% for MB versus 41% for MO in one and a half hours).

Morphological, Structural, Photophysical, and Thermal Analysis of TiO 2 Nanoparticles
The morphology of the TiO 2 nanoparticles prepared by the sol-gel method: asprepared (T sg ), and calcined at 400 • C (T sgc ) and by the hydrothermal method: as-prepared (T sg ), and calcined at 400 • C (T htc ) were observed by a scanning electron microscope (SEM) and the SEM micrographs are shown in Figure 1.From the SEM images, we observe that the nanoparticles synthesized by both methods were agglomerated and non-uniform in size.The size distribution curves of the T sg , T sgc , T ht, and T htc nanoparticles have been shown in the insets of Figure 1.The size range of the T sg , T sgc , T ht, and T htc nanoparticles were 17-26 nm, 68-97 nm, 12-32 nm, and 27-39 nm, respectively.The shape of T sg and T sgc were approximately spherical (Figure 1a,b) whereas that of T ht and T htc were approximately rectangular prism and cubic (Figure 1c,d), and the surfaces of all the nanoparticles were rough.
Fourier transform infrared (FTIR) spectroscopy being a surface-sensitive analytical technique provides valuable information on the surface region of the nanoparticles.The FTIR spectra of the nanoparticles are displayed in Figure 2.These are typical of the TiO 2 nanoparticles confirming their successful synthesis.The broad absorption band observed in the region 400-800 cm −1 originated from Ti-O and bridging Ti-O-Ti stretching vibrations, while the absorption peak at 1386 cm −1 is associated with the Ti-O bending vibrations.The absorption band at 3420 and 1615 cm −1 represented the -O-H stretching and bending vibrations, respectively, of water chemisorbed on the TiO 2 nanoparticle surface.The two small absorption peaks at ~2915 and ~2842 cm −1 have been assigned to be associated with the Ti-OH bond vibrations [11,30].The extra absorption peaks at ~1428 and ~1528 cm −1 found in as-prepared sol-gel TiO 2 (T sg ) can be attributed to the adsorbed organic materials on the surface of the nanoparticles during synthesis.
vibrations, while the absorption peak at 1386 cm −1 is associated with the Ti-O bending vibrations.The absorption band at 3420 and 1615 cm −1 represented the -O-H stretching and bending vibrations, respectively, of water chemisorbed on the TiO2 nanoparticle surface.The two small absorption peaks at ~2915 and ~2842 cm −1 have been assigned to be associated with the Ti-OH bond vibrations [11,30].The extra absorption peaks at ~1428 and ~1528 cm −1 found in as-prepared sol-gel TiO2 (Tsg) can be attributed to the adsorbed organic materials on the surface of the nanoparticles during synthesis.The adsorbed organic material on the surface of the nanoparticles was estimated by the thermogravimetric analysis (TGA) profiles of the nanoparticles (Figure 3).The TGA trace of Tsg exhibited three-stage weight loss typical for TiO2 nanoparticles [4][5][6][7][8].The first ca.6% weight loss below 200 °C mainly originated from the evaporation of physisorbed water and alcohols on the nanocomposite surface.The second weight loss of ca.7% in the region 200 °C-500 °C for Tsg correlated mainly with the degradation of the organic components, which were adsorbed from the reaction mixture during synthesis.The third minor weight loss occurring above 500 °C was attributed to the loss of water produced by the condensation of neighboring terminal Ti-OH groups on the nanoparticle surfaces [10- The adsorbed organic material on the surface of the nanoparticles was estimated by the thermogravimetric analysis (TGA) profiles of the nanoparticles (Figure 3).The TGA trace of T sg exhibited three-stage weight loss typical for TiO 2 nanoparticles [4][5][6][7][8].The first ca.6% weight loss below 200 • C mainly originated from the evaporation of physisorbed water and alcohols on the nanocomposite surface.The second weight loss of ca.7% in the region 200 • C-500 • C for T sg correlated mainly with the degradation of the organic components, which were adsorbed from the reaction mixture during synthesis.The third minor weight loss occurring above 500 • C was attributed to the loss of water produced by the condensation of neighboring terminal Ti-OH groups on the nanoparticle surfaces [10][11][12][13].For the nanoparticles obtained after calcination of T sg at 400 • C, the weight loss beyond 200 • C was minor, apparently due to the almost complete removal of the adsorbed organic materials during calcination.Only a slight weight loss (<1%) of hydrothermal T ht and T htc indicated that they were pure TiO 2 with cleaner surfaces.The heavy loss of weight (ca.16%) for the as-prepared sol-gel nanoparticles, T sg indicated a high concentration of organic material adsorbed on the surface of this nanoparticle.The crystalline phases of the synthesized Tsg, Tsgc, Tht, and Thtc nanoparticles were determined by X-ray diffraction (XRD), and the representative patterns are displayed in Figure 4.The precise Bragg diffraction peaks signify the crystalline nature of the nanoparticles.The Bragg diffraction peaks designated by (A) agreed with the anatase phase of TiO2 with tetragonal arrangement [31][32][33], especially, the two most intense diffraction peaks found in all of the samples, viz., at (2θ) 25.0° and 48.0° are typical of the anatase phase.This result indicates that all of the prepared TiO2 nanoparticles were predominantly of the anatase phase.As observed in SEM images (Figure 1), the sol-gel TiO2 nanoparticles were smaller in size, which resulted in relatively broad diffraction peaks and consequently alongside peaks were not resolved.The hydrothermal TiO2 nanoparticles Tht prepared at 240 °C being larger, the diffraction peaks were well resolved and confirmed with the anatase polymorph.However, after calcination at 400 °C the hydrothermal TiO2, Thtc, displayed two additional diffraction peaks (designated by B) at (2θ) ~31° and ~58°, the former of which is the (d 121 ) peak of the brookite phase [34,35], confirming that Thtc is a mixture of anatase and brookite crystalline polymorphs of TiO2.The crystalline phases of the synthesized T sg , T sgc , T ht, and T htc nanoparticles were determined by X-ray diffraction (XRD), and the representative patterns are displayed in Figure 4.The precise Bragg diffraction peaks signify the crystalline nature of the nanoparticles.The Bragg diffraction peaks designated by (A) agreed with the anatase phase of TiO 2 with tetragonal arrangement [31][32][33], especially, the two most intense diffraction peaks found in all of the samples, viz., at (2θ) 25.0 • and 48.0 • are typical of the anatase phase.This result indicates that all of the prepared TiO 2 nanoparticles were predominantly of the anatase phase.As observed in SEM images (Figure 1), the sol-gel TiO 2 nanoparticles were smaller in size, which resulted in relatively broad diffraction peaks and consequently alongside peaks were not resolved.The hydrothermal TiO 2 nanoparticles T ht prepared at 240 • C being larger, the diffraction peaks were well resolved and confirmed with the anatase polymorph.However, after calcination at 400 • C the hydrothermal TiO 2 , T htc , displayed two additional diffraction peaks (designated by B) at (2θ) ~31 • and ~58 • , the former of which is the (d 121 ) peak of the brookite phase [34,35], confirming that T htc is a mixture of anatase and brookite crystalline polymorphs of TiO 2 .
The degree of crystallinity (λ) of the synthesized TiO 2 nanoparticles calculated from the XRD data [32] according to Equation (1), where A c is the area covered by the crystallization peaks and (A c + A a ) is the total crys- talline and amorphous area calculated by OriginLab's Origin software (version 2019b 64bit), is shown in Table 1.The crystal size (l) of the nanoparticles calculated from the XRD data according to the method of Scherrer [36] is also presented in Table 1.The XRD data revealed that the size of the sol-gel nanocrystals was much smaller than the hydrothermal nanocrystals.The size increased by about 40% and 24% after the calcination of the sol-gel and the hydrothermal nanocrystals, respectively.The degree of crystallinity of all the nanoparticles was low compared to the commercial Degussa (Evonik) P25 TiO 2 photocatalyst (total crystallinity 92% in a typical sample) [37].However, the degree of crystallinity was increased by 108% for the sol-gel nanoparticles while that of the hydrothermal nanoparticles decreased by 46% upon calcination.The decrease in crystallinity can be attributed to the fact that for the T htc sample, which is a mix of anatase and brookite, the growth of the brookite phase at the expense of the anatase phase during calcination slowed down the crystallization process [38].
Molecules 2023, 28, x FOR PEER REVIEW 6 of 21 The degree of crystallinity (λ) of the synthesized TiO2 nanoparticles calculated from the XRD data [32] according to Equation (1), where  is the area covered by the crystallization peaks and (  is the total crystalline and amorphous area calculated by OriginLab's Origin software (version 2019b 64bit), is shown in Table 1.The crystal size (l) of the nanoparticles calculated from the XRD data according to the method of Scherrer [36] is also presented in Table 1.The XRD data revealed that the size of the sol-gel nanocrystals was much smaller than the hydrothermal nanocrystals.The size increased by about 40% and 24% after the calcination of the sol-gel and the hydrothermal nanocrystals, respectively.The degree of crystallinity of all the nanoparticles was low compared to the commercial Degussa (Evonik) P25 TiO2 photocatalyst (total crystallinity 92% in a typical sample) [37].However, the degree of crystallinity was increased by 108% for the sol-gel nanoparticles while that of the hydrothermal nanoparticles decreased by 46% upon calcination.The decrease in crystallinity can be attributed to the fact that for the Thtc sample, which is a mix of anatase and brookite, the growth of the brookite phase at the expense of the anatase phase during calcination slowed down the crystallization process [38].
Table 1.The degree of crystallinity (λ) and crystal size (l) of the synthesized TiO2 nanoparticles.  Figure 5 displays the UV-visible spectra of stable sols of the photocatalysts in ethanol.The λ max for T sg , T sgc , T ht , and T htc were 300 nm, 313 nm, 335 nm, and 358 nm, respectively.Thus, there is a red shift of 13 nm and 23 nm of the λ max for the sol-gel and the hydrothermal nanoparticles, respectively, upon calcination.As the λ max is related to the absorption onset, it can be inferred that the optical bandgap of the materials significantly reduced upon calcination [39].
hydrothermal nanoparticles, respectively, upon calcination.As the   is related to the absorption onset, it can be inferred that the optical bandgap of the materials significantly reduced upon calcination [39].

Photocatalytic Degradation Study
The catalytic efficiency of the nanoparticles for photodegradation of two model organic dyes, MB and MO was conducted at 10 °C under UV-B irradiation; a detailed procedure is given in Section 3.4.MB is cationic whereas MO is anionic.The choice of these model compounds was made to exploit their contrasting chemical nature for studying the selectivity of the nanoparticles for catalyzing the photodegradation of a particular class of compounds, and also because they have previously been extensively used as model compounds for photodegradation study [4,5,32].

Control Experiment
A control experiment was conducted under identical experimental conditions without the presence of any photocatalyst, the data of which are presented in Figure 6.The intensity of absorption maximum of the dyes decreased only slightly during three hours of UV-B irradiation, displaying negligible degradation of the dyes without a photocatalyst (Figure 6a,b).

Photocatalytic Degradation Study
The catalytic efficiency of the nanoparticles for photodegradation of two model organic dyes, MB and MO was conducted at 10 • C under UV-B irradiation; a detailed procedure is given in Section 3.4.MB is cationic whereas MO is anionic.The choice of these model compounds was made to exploit their contrasting chemical nature for studying the selectivity of the nanoparticles for catalyzing the photodegradation of a particular class of compounds, and also because they have previously been extensively used as model compounds for photodegradation study [4,5,32].

Control Experiment
A control experiment was conducted under identical experimental conditions without the presence of any photocatalyst, the data of which are presented in Figure 6.The intensity of absorption maximum of the dyes decreased only slightly during three hours of UV-B irradiation, displaying negligible degradation of the dyes without a photocatalyst (Figure 6a,b).
Molecules 2023, 28, x FOR PEER REVIEW 7 of 21 hydrothermal nanoparticles, respectively, upon calcination.As the  is related to the absorption onset, it can be inferred that the optical bandgap of the materials significantly reduced upon calcination [39].

Photocatalytic Degradation Study
The catalytic efficiency of the nanoparticles for photodegradation of two model organic dyes, MB and MO was conducted at 10 °C under UV-B irradiation; a detailed procedure is given in Section 3.4.MB is cationic whereas MO is anionic.The choice of these model compounds was made to exploit their contrasting chemical nature for studying the selectivity of the nanoparticles for catalyzing the photodegradation of a particular class of compounds, and also because they have previously been extensively used as model compounds for photodegradation study [4,5,32].

Control Experiment
A control experiment was conducted under identical experimental conditions without the presence of any photocatalyst, the data of which are presented in Figure 6.The intensity of absorption maximum of the dyes decreased only slightly during three hours of UV-B irradiation, displaying negligible degradation of the dyes without a photocatalyst (Figure 6a,b).

Determination of the Optimum Dose of the Nanoparticles
Before starting a photocatalytic degradation experiment, we determined the minimum amount of the nanoparticle (adsorbent) that can degrade the maximum amount of the substrate.The experiment for dose effect was carried out in the presence of the nanoparticles under UV-B irradiation at 10 • C for three hours.Figure 7 displays the % degradation of the model dye MB as a function of the amount of the nanoparticles T sgc as representative.The maximum degradation efficiency of MB (98%) was observed at the optimum dose of 8 mg/10 mL of the catalyst.Subsequent degradation experiments were conducted by using this optimum dose.
Molecules 2023, 28, x FOR PEER REVIEW 8 of 21 Before starting a photocatalytic degradation experiment, we determined the minimum amount of the nanoparticle (adsorbent) that can degrade the maximum amount of the substrate.The experiment for dose effect was carried out in the presence of the nanoparticles under UV-B irradiation at 10 °C for three hours.Figure 7 displays the % degradation of the model dye MB as a function of the amount of the nanoparticles Tsgc as representative.The maximum degradation efficiency of MB (98%) was observed at the optimum dose of 8 mg/10 mL of the catalyst.Subsequent degradation experiments were conducted by using this optimum dose.

Adsorption Study
We conducted an adsorption study using the optimum dose of 8 mg/10 mL of the nanoparticles Tsg, Tsgc, Tht, and Thtc on 10 ppm solutions of MB and MO under dark conditions at ambient temperature (25 °C) for 30 min and the data are accumulated in Figure 8 and Figure 9, respectively.As we can observe from Figure 8 that the adsorption capacity of the sol-gel Tsg, and Tsgc nanoparticles for cationic MB was negligible, while that of the hydrothermal Tht and Thtc was quite high at 26% and 13%, respectively.The corresponding data for the adsorption of anionic MO are shown in Figure 9 where it is clear that the adsorption capacity of all the nanoparticles studied was negligible for MO excepting Tsg, which showed an appreciable adsorption of 16%.

Adsorption Study
We conducted an adsorption study using the optimum dose of 8 mg/10 mL of the nanoparticles T sg , T sgc , T ht , and T htc on 10 ppm solutions of MB and MO under dark conditions at ambient temperature (25 • C) for 30 min and the data are accumulated in Figures 8 and 9, respectively.As we can observe from Figure 8 that the adsorption capacity of the sol-gel T sg , and T sgc nanoparticles for cationic MB was negligible, while that of the hydrothermal T ht and T htc was quite high at 26% and 13%, respectively.The corresponding data for the adsorption of anionic MO are shown in Figure 9 where it is clear that the adsorption capacity of all the nanoparticles studied was negligible for MO excepting T sg , which showed an appreciable adsorption of 16%.

Photocatalytic Degradation of Cationic MB Dye under UV-B Irradiation
The photodegradation study of cationic MB was conducted in the presence of Tsg, Tsgc, Tht, and Thtc under UV-B irradiation.The photocatalytic efficiency of the nanoparticles was determined by measuring the intensity of the optical absorption of MB in UV-visible spectra (λmax = 664 nm) of the reaction mixture as a function of time (Figure 10).The intensity slowly decreased with time indicating slow degradation of the dye, and it was

Photocatalytic Degradation of Cationic MB Dye under UV-B Irradiation
The photodegradation study of cationic MB was conducted in the presence of T sg, T sgc , T ht , and T htc under UV-B irradiation.The photocatalytic efficiency of the nanoparticles was determined by measuring the intensity of the optical absorption of MB in UV-visible spectra (λ max = 664 nm) of the reaction mixture as a function of time (Figure 10).The intensity slowly decreased with time indicating slow degradation of the dye, and it was not completely degraded even after 4 h, as displayed by the large absorption maxima of the undegraded dye in Figure 10a.Now the photocatalytic degradation of an organic compound on the surface of a nanoparticle consists of two steps; in the first step, the compound is adsorbed on the surface of the nanoparticles, followed by a photocatalytic reaction in the second step [4].The highly active surface of the T sg nanoparticles was already occupied by adsorbed organic materials from the reaction mixture during synthesis, as revealed in the TGA experiment, which hindered the approach of MB to the nanoparticle surface.The adsorption study displayed in Figure 8 revealed that the sol-gel nanoparticles have zero or negligible adsorption ability for MB.Both factors might have contributed to the slow and incomplete degradation of the dye during that time.
The as-synthesized nanoparticles (T sg ) were calcined for 2 h at 400 • C to clean the surface.In the presence of the calcined nanoparticles (T sgc ), the intensity of UV-visible absorption maxima of the reaction mixture quickly declined revealing that the degradation was very fast at the early stage and it took about 3 h for complete degradation (the λ max became indistinguishable from the baseline) as displayed in Figure 10b.The increased crystallinity (108% increase) of the calcined particles might have contributed to the effectiveness of the degradation [38].It is interesting to note that the photodegradation of MB catalyzed by the hydrothermal nanoparticles (T ht ) was almost identical to that in the presence of calcined sol-gel nanoparticles (T sgc ) as revealed in the UV-visible absorption profile of the reaction mixtures shown in Figures 10b and 10c, respectively.This is because the surface of both the nanoparticles was almost clean (cf. Figure 3), and also, they both were an anatase polymorph of TiO 2 having the same level of crystallinity (cf.Table 1).The degradation efficiency of the hydrothermal nanoparticles increased after calcination at 400 • C (T htc ), as observed in the absorbance profile of the reaction mixture shown in Figure 10d, although its degree of crystallinity decreased upon calcination.This is because T htc is a mix of the anatase and brookite phases of TiO 2 , which is well known to have much better photocatalytic efficiency than pure anatase or brookite due to a synergistic effect that facilitates charge separation and reduces electron-hole recombination [40][41][42].This fact is also reflected in the large red shift of the absorption maxima of the nanoparticle dispersion in UV-visible spectra of T htc in our study (cf. Figure 5).
Molecules 2023, 28, x FOR PEER REVIEW 10 of 21 not completely degraded even after 4 h, as displayed by the large absorption maxima of the undegraded dye in Figure 10a.Now the photocatalytic degradation of an organic compound on the surface of a nanoparticle consists of two steps; in the first step, the compound is adsorbed on the surface of the nanoparticles, followed by a photocatalytic reaction in the second step [4].The highly active surface of the Tsg nanoparticles was already occupied by adsorbed organic materials from the reaction mixture during synthesis, as revealed in the TGA experiment, which hindered the approach of MB to the nanoparticle surface.The adsorption study displayed in Figure 8 revealed that the sol-gel nanoparticles have zero or negligible adsorption ability for MB.Both factors might have contributed to the slow and incomplete degradation of the dye during that time.The as-synthesized nanoparticles (Tsg) were calcined for 2 h at 400 °C to clean the surface.In the presence of the calcined nanoparticles (Tsgc), the intensity of UV-visible absorption maxima of the reaction mixture quickly declined revealing that the degradation was very fast at the early stage and it took about 3 h for complete degradation (the λmax became indistinguishable from the baseline) as displayed in Figure 10b.The increased crystallinity (108% increase) of the calcined particles might have contributed to the effectiveness of the degradation [38].It is interesting to note that the photodegradation of MB catalyzed by the hydrothermal nanoparticles (Tht) was almost identical to that in the presence of calcined sol-gel nanoparticles (Tsgc) as revealed in the UV-visible absorption profile of the reaction mixtures shown in Figure 10b and Figure 10c, respectively.This is because the surface of both the nanoparticles was almost clean (cf. Figure 3), and also, they both were an anatase polymorph of TiO2 having the same level of crystallinity (cf.Table 1).The degradation efficiency of the hydrothermal nanoparticles increased after calcination at 400 °C (Thtc), as observed in the absorbance profile of the reaction mixture shown in Figure 10d, although its degree of crystallinity decreased upon calcination.This is because Thtc is a mix of the anatase and brookite phases of TiO2, which

Photocatalytic Degradation of Anionic MO Dye under UV-B Irradiation
The photodegradation of anionic MO was conducted in the same set-up under identical conditions and the results are shown in Figure 11.The photocatalytic efficiency of the nanoparticles was determined from the intensity of the λ max (464 nm) of MO.The UV-visible absorption profiles of the reaction mixtures revealed that the photodegradation of MO was fast at the initial stage and it was almost complete within one and a half hours in the presence of the T sg nanoparticles as displayed in Figure 11a, reflecting some attractive interaction of the dye with the nanoparticle surface due to the presence of the adsorbed organic materials on it.After calcination at 400 • C (T sgc ), the nanoparticle surface became clean and the surface gained a negative character due to the lone pair electrons on the oxygen atoms of the TiO 2 surface, creating a repulsive interaction with the negatively charged MO, as a result, the photocatalytic efficiency decreased [30] and it took almost three hours for complete degradation, as observed in Figure 11b.The adsorption study presented in Figures 9a and 9b displaying a 16% and 0% adsorption of MO on T sg and T sgc , respectively may be taken as a support of the photocatalytic behavior of the two nanoparticles.In the case of the as-prepared hydrothermal nanoparticles (T ht ), the degradation efficiency was much slower relative to the as-prepared sol-gel (T sg ) nanoparticles as revealed in Figure 11c.Even though the crystallinity decreased upon calcination, it made the hydrothermal nanoparticles (T htc ) a somewhat better photocatalyst for degradation of MO, as the same level of degradation was achieved within two hours that took more than 3.5 h by T ht .This is because T htc is an anatase-brookite mixed phase as revealed by XRD analysis, which is well known to have a much better photocatalytic efficiency than pure anatase or brookite [40][41][42], as explained in the previous section.However, near 100% degradation was not observed in the presence of the hydrothermal nanoparticles before or after calcination, which is in clear contrast with the efficiency of the as-prepared sol-gel nanoparticles T sg that catalyzed the photodegradation of MB to near completion within one and a half hours.

Degradation Study in the Absence of Light
The photocatalytic nature of the degradation of the model dyes by the synthesized nanoparticles was confirmed by carrying out the reaction between the dye and the nanoparticles under identical conditions but in the absence of any light (dark conditions) and the results are presented in Figure 12 for MB and in Figure 13 for MO.In the case of MB, the removal of the dye was 0% by the sol-gel nanoparticles Tsg and Tsgc whereas the maximum removal was 35% and 41% by the hydrothermal nanoparticles Tht and Thtc, respectively.The data are consistent with the adsorption study presented in Figure 8, revealing that the removal of the dye by the hydrothermal nanoparticles was solely due to adsorption.Also, the UV-visible spectra in Figure 12c,d were not consistent with time, presumably due to the uncertainty arising from the desorption of the dye from the nanoparticle surface during centrifugation and handling, revealing the physical nature of the adsorption.In the case of MO, a maximum removal of 10% was observed from corresponding UV-visible absorption spectra (Figure 13) for both the as-prepared sol-gel and hydrothermal nanoparticles Tsg and Tht, which was also inconsistent with reaction time; whereas the calcined nanoparticles Tsgc and Thtc did not show any removal efficiency.The results confirm that the removal of the dye by the nanoparticles under UV-B irradiation discussed in Sections 2.2.4 and 2.2.5 was photocatalytic in nature.

Degradation Study in the Absence of Light
The photocatalytic nature of the degradation of the model dyes by the synthesized nanoparticles was confirmed by carrying out the reaction between the dye and the nanoparticles under identical conditions but in the absence of any light (dark conditions) and the results are presented in Figure 12 for MB and in Figure 13 for MO.In the case of MB, the removal of the dye was 0% by the sol-gel nanoparticles T sg and T sgc whereas the maximum removal was 35% and 41% by the hydrothermal nanoparticles T ht and T htc , respectively.The data are consistent with the adsorption study presented in Figure 8, revealing that the removal of the dye by the hydrothermal nanoparticles was solely due to adsorption.Also, the UV-visible spectra in Figure 12c,d were not consistent with time, presumably due to the uncertainty arising from the desorption of the dye from the nanoparticle surface during centrifugation and handling, revealing the physical nature of the adsorption.In the case of MO, a maximum removal of 10% was observed from corresponding UV-visible absorption spectra (Figure 13) for both the as-prepared sol-gel and hydrothermal nanoparticles T sg and T ht , which was also inconsistent with reaction time; whereas the calcined nanoparticles T sgc and T htc did not show any removal efficiency.The results confirm that the removal of the dye by the nanoparticles under UV-B irradiation discussed in Sections 2.2.4 and 2.2.5 was photocatalytic in nature.

XRD Analysis of the Recovered Nanoparticles
The nanoparticles were recovered after each photocatalysis experiment by centrifugation, repeatedly washed with ultrapure water and methanol, and dried in a vacuum oven at 50 • C. The stability of the photocatalysts was assessed by taking XRD of the recovered nanoparticles.The XRD patterns are displayed in Figure 14 and the crystallinity and crystallite size data obtained from XRD analysis are presented in Table 2.

XRD Analysis of the Recovered Nanoparticles
The nanoparticles were recovered after each photocatalysis experiment by centrifugation, repeatedly washed with ultrapure water and methanol, and dried in a vacuum oven at 50 °C.The stability of the photocatalysts was assessed by taking XRD of the recovered nanoparticles.The XRD patterns are displayed in Figure 14 and the crystallinity and crystallite size data obtained from XRD analysis are presented in Table 2. Table 2.The degree of crystallinity (λ) and crystal size (l) of the TiO2 nanoparticles recovered after the photocatalytic degradation reactions, calculated from the XRD data.The letters in parenthesis after the name of a nanoparticle represent the name of the dye it degraded by photocatalysis.It is revealed from Table 2 that the crystallite size of the nano-catalysts remained practically unchanged after the photocatalytic reactions.However, the effect of the harsh reaction condition on the crystallinity of the nanoparticles was mixed (cf.Table 1).In the case of the sol-gel nanoparticles, the trend in the change of crystallinity was the same irrespective of the dye photodegraded: crystallinity of the uncalcined nanoparticle dramatically increased while it decreased to an appreciable extent for the calcined species.In the case of hydrothermal nanoparticles, the crystallinity of the uncalcined species increased after the photodegradation reaction of MB while it slightly decreased after photodegradation MO.For the calcined hydrothermal nanoparticles, a huge increase in crystallinity was observed after the degradation of both the model dyes: 159% after MB degradation and 120% after MO degradation.From the results, it is inferred that the moderate energy UV-B radiation played a major role in altering the crystallinity of the photocatalyst species rather than the dye substrate it degraded.It is to be noted that the increase in the crystallinity in the photocatalyst was more pronounced for the species that had relatively lower levels of crystallinity at the outset.The huge (159%) increase in Table 2.The degree of crystallinity (λ) and crystal size (l) of the TiO 2 nanoparticles recovered after the photocatalytic degradation reactions, calculated from the XRD data.The letters in parenthesis after the name of a nanoparticle represent the name of the dye it degraded by photocatalysis.It is revealed from Table 2 that the crystallite size of the nano-catalysts remained practically unchanged after the photocatalytic reactions.However, the effect of the harsh reaction condition on the crystallinity of the nanoparticles was mixed (cf.Table 1).In the case of the sol-gel nanoparticles, the trend in the change of crystallinity was the same irrespective of the dye photodegraded: crystallinity of the uncalcined nanoparticle dramatically increased while it decreased to an appreciable extent for the calcined species.In the case of hydrothermal nanoparticles, the crystallinity of the uncalcined species increased after the photodegradation reaction of MB while it slightly decreased after photodegradation MO.For the calcined hydrothermal nanoparticles, a huge increase in crystallinity was observed after the degradation of both the model dyes: 159% after MB degradation and 120% after MO degradation.From the results, it is inferred that the moderate energy UV-B radiation played a major role in altering the crystallinity of the photocatalyst species rather than the dye substrate it degraded.It is to be noted that the increase in the crystallinity in the photocatalyst was more pronounced for the species that had relatively lower levels of crystallinity at the outset.The huge (159%) increase in crystallinity obtained in T htc , a mix of the brookite and the anatase TiO 2 nanomaterial, is reflected in the XRD pattern of material by the appearance of several additional diffraction peaks characteristic of the brookite phase TiO 2 , which may indicate that the reaction condition favored the formation of the brookite phase of TiO 2 .Further study on the matter is in progress.

Name of
2.2.8.Selectivity of the Prepared Nanoparticles towards Catalyzing the Photodegradation of MB and MO under UV-B Irradiation Figure 15 compares the results of the photocatalytic degradation efficiency of the TiO 2 nanoparticles synthesized by the sol-gel and the hydrothermal synthetic routes, and also the effect of calcination of these nanoparticles on the degradation efficiency.The untreated TiO 2 nanoparticles prepared by the sol-gel technique were remarkably selective in catalyzing the photodegradation of the model anionic dye MO over the cationic MB (90% for MO versus 40% for MB degradation in two hours) (Figure 15a).The said selectivity was reversed for the nanoparticles calcined at 400 • C, especially in the early stage of the degradation reaction (50% for MO versus 90% for MB in one hour) (Figure 15b).The as-prepared hydrothermal TiO 2 nanoparticles displayed selectivity in the opposite sense of the as-prepared sol-gel nanoparticles (T sg ) towards the degradation of the said model dyes: 41% degradation for MO versus 91% degradation for MB in one and a half hours (Figure 15c).the sol-gel particles, the selectivity towards degrading the model dyes did not alter for the hydrothermal nanoparticles after calcination (Figure 15d); however, the rate of conversion increased significantly due to improved crystallinity and formation of a mix of anatase and brookite phases in the photocatalyst upon calcination [38] that presumably decreased optical bandgap of the calcined materials, as discussed in Section 2.1 under Figure 5 15 compares the results of the photocatalytic degradation efficiency of the TiO2 nanoparticles synthesized by the sol-gel and the hydrothermal synthetic routes, and also the effect of calcination of these nanoparticles on the degradation efficiency.The untreated TiO2 nanoparticles prepared by the sol-gel technique were remarkably selective in catalyzing the photodegradation of the model anionic dye MO over the cationic MB (90% for MO versus 40% for MB degradation in two hours) (Figure 15a).The said selectivity was reversed for the nanoparticles calcined at 400 °C, especially in the early stage of the degradation reaction (50% for MO versus 90% for MB in one hour) (Figure 15b).The asprepared hydrothermal TiO2 nanoparticles displayed selectivity in the opposite sense of the as-prepared sol-gel nanoparticles (Tsg) towards the degradation of the said model dyes: 41% degradation for MO versus 91% degradation for MB in one and a half hours (Figure 15c).Unlike the sol-gel particles, the selectivity towards degrading the model dyes did not alter for the hydrothermal nanoparticles after calcination (Figure 15d); however, the rate of conversion increased significantly due to improved crystallinity and formation of a mix of anatase and brookite phases in the photocatalyst upon calcination [38] that presumably decreased optical bandgap of the calcined materials, as discussed in Section 2.1 under Figure 5 and in Sections 2.2.4 and 2.2.5 above.For a better quantitative description of the photocatalytic specificity of the synthesized nanoparticles, the UV-visible spectroscopic data were utilized for analyzing the kinetics of the photocatalytic degradation of the dyes.initial degradation reaction reasonably followed the apparent first-order kinetic path [30,43,44] according to Equation (2), where C 0 and C t are the initial concentration and the concentration of the substrate at time t, during the photocatalytic reaction under UV-B irradiation at 10 • C in the presence of the nanoparticles; k is the apparent first-order rate constant.The plots of ln(C 0 /C t ) versus time (t) of UV-B irradiation are presented in Figure 16 and the values of the apparent first-order rate constant k, along with the half-life for the degradation of the model dyes are presented in Table 3.The selectivity of the untreated sol-gel TiO 2 nanoparticle T sg for catalyzing the photodegradation of MO over MB is also clear from the apparent t 1/2 values of 36.5 min for MO versus 210 min for MB.For the calcined sol-gel nano-catalyst T sgc, the t 1/2 values are 57.8 min and 16.0 min for MO and MB, respectively, clearly revealing the reversal of the specificity.Both untreated (T ht ) and calcined (T htc ) hydrothermal nanoparticles were more selective for degrading the cationic dye MB than the anionic dye MO which is reflected in the t 1/2 data, with T htc having higher rate constants because it is a mix of the anatase and brookite phases [40][41][42].

ln(C0/Ct) = kt
where C0 and Ct are the initial concentration and the concentration of the substrate at t, during the photocatalytic reaction under UV-B irradiation at 10 °C in the presence o nanoparticles; k is the apparent first-order rate constant.
The plots of ln(C0/Ct) versus time (t) of UV-B irradiation are presented in Figur and the values of the apparent first-order rate constant k, along with the half-life fo degradation of the model dyes are presented in Table 3.The selectivity of the untre sol-gel TiO2 nanoparticle Tsg for catalyzing the photodegradation of MO over MB is clear from the apparent t1/2 values of 36.5 min for MO versus 210 min for MB.Fo calcined sol-gel nano-catalyst Tsgc, the t1/2 values are 57.8 min and 16.0 min for MO and respectively, clearly revealing the reversal of the specificity.Both untreated (Tht) calcined (Thtc) hydrothermal nanoparticles were more selective for degrading the cat dye MB than the anionic dye MO which is reflected in the t1/2 data, with Thtc having hi rate constants because it is a mix of the anatase and brookite phases [40][41][42].

Materials
Titanium tetraisopropoxide (TTIP) was obtained from Tokyo Chemical Industry Ltd. (Tokyo, Japan).Analytical-grade chemicals such as aqueous ammonia solution
3.2. of the TiO 2 Nanoparticles 3.2.1.Preparation of TiO 2 Nanoparticles by Sol-Gel Technique TiO 2 nanoparticles were synthesized by the sol-gel method according to the scheme shown in Figure 17a.In a typical procedure, 9.68 g of TTIP were dissolved in 20 mL of isopropanol by stirring magnetically at 450 rpm for 20 min in a three-necked 500 mL round-bottom flask.Then acetic acid (1.2 mL) and ultrapure water (250 mL) were added into the TTIP solution, with the stirring continued.The reaction was carried out for 24 h at 60 • C. Following the reaction, the product was centrifuged and washed with EtOH.The centrifugation-redispersion washing procedure was repeated five times.Finally, the TiO 2 nanoparticles were dried in a vacuum oven at 60 • C for 24 h.The product obtained was 1.978 g (73% yield).A portion of the product was calcined at 400 • C for 2 h in a muffle furnace.
at 60 °C.Following the reaction, the product was centrifuged and washed with EtOH.The centrifugation-redispersion washing procedure was repeated five times.Finally, the TiO2 nanoparticles were dried in a vacuum oven at 60 °C for 24 h.The product obtained was 1.978 g (73% yield).A portion of the product was calcined at 400 °C for 2 h in a muffle furnace.

Preparation of TiO2 Nanoparticles by Hydrothermal Technique
TiO2 nanoparticles were also synthesized from TTIP following the hydrothermal technique, as described schematically in Figure 17b.Sodium hydroxide was used as the mineralizer and TTIP as the source material.In a typical procedure, 14.50 g TTIP was added to 50.0 mL of ultrapure water with vigorous magnetic stirring.A milky white sol was produced.The pH of the sol was adjusted to 10 by dropwise addition of an aqueous 1.0 M NaOH solution over several minutes.Finally, the sol was diluted to 90 mL by adding ultrapure water and was transferred to a 200 mL Teflon-lined autoclave jar.The jar was sealed in a stainless steel housing and was heated to 240 °C for 12 h in a thermostatic oven.Following the reaction, the product was purified from a 3:2 ultrapure water/EtOH medium by five consecutive centrifugation-redispersion cycles.Finally, TiO2 nanoparticles were dried at 60 °C for 24 h in a vacuum oven (3.1030 g, 76% yield).A portion of the product was calcined at 400 °C for 2 h in a muffle furnace.

Preparation of TiO 2 Nanoparticles by Hydrothermal Technique
TiO 2 nanoparticles were also synthesized from TTIP following the hydrothermal technique, as described schematically in Figure 17b.Sodium hydroxide was used as the mineralizer and TTIP as the source material.In a typical procedure, 14.50 g TTIP was added to 50.0 mL of ultrapure water with vigorous magnetic stirring.A milky white sol was produced.The pH of the sol was adjusted to 10 by dropwise addition of an aqueous 1.0 M NaOH solution over several minutes.Finally, the sol was diluted to 90 mL by adding ultrapure water and was transferred to a 200 mL Teflon-lined autoclave jar.The jar was sealed in a stainless steel housing and was heated to 240 • C for 12 h in a thermostatic oven.Following the reaction, the product was purified from a 3:2 ultrapure water/EtOH medium by five consecutive centrifugation-redispersion cycles.Finally, TiO 2 nanoparticles were dried at 60 • C for 24 h in a vacuum oven (3.1030 g, 76% yield).A portion of the product was calcined at 400 • C for 2 h in a muffle furnace.

Instruments
The surface Morphology of the nanoparticles was studied by a Scanning Electron Microscope (Model no.JSM-7610F from JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 15 kV.Fourier-transform infrared (FTIR) spectra of the materials were taken on a FTIR spectrophotometer, model number FTIR-100.Thermogravimetric analysis (TGA) of the materials was conducted on a PerkinElmer STA-6000 instrument at a heating rate of 10 • C/min under an N 2 gas environment.The XRD of the samples was taken on a Rigaku, SmartLab-SE X-Ray Diffraction Analyzer.A Shimadzu (Kyoto, Japan) DR 1800 UV-visible spectrometer was used for optical absorbance measurement as a function of the wavelength in the range of 200-800 nm.A centrifuge machine (Model no.BKC-TH1611, Biobase Corporation, Jinan, China) operated at 10,000 rpm was used for the purification of the nanoparticles by repeated washing.

Adsorption and Degradation Experiments
An aqueous solution of MB or MO (10 mg/L) was mixed with either T sg or T sgc or T ht or T htc (8 mg/10 mL) and placed in an ultrasonic bath for 5 min and then kept in the dark for 30 min at ambient temperature (25 • C) under unstirred condition after which a portion of the mixture was centrifuged at 6000 rpm and UV-visible spectra of the supernatant was taken.The extent of adsorption was calculated from the difference in the UV-visible absorption of the supernatant from that of the initial dye solution.
For the degradation experiment, the sonicated dye/nanoparticle dispersion was taken in a double-walled Pyrex glass reaction vessel (100 mL) connected to a thermostatic water bath set at 10 • C. Before applying UV-B radiation, the reaction mixture was magnetically stirred in the dark for 30 min.Then, a UV light source (Quartz UV-B bulb, 3 w, 285 nm) was applied to the dye/nanoparticle dispersion for a desired period.The degradation reaction was followed by withdrawing 5.0 mL aliquots of the reaction mixture at regular intervals, centrifuging for 5 min at 6000 rpm, and measuring the absorbance of the supernatant with a UV-visible spectrophotometer.A degradation study of the model dye compounds was also carried out in an identical manner in the absence of light for comparison.
The extent of removal of the dye by adsorption or degradation in the presence of the TiO 2 -nanoparticles in a certain time (r rem in %) was calculated from the following Equation (3): where a 0 and a t are the absorbances at times 0 and t, respectively.

Conclusions
TiO 2 nanoparticles synthesized by sol-gel and hydrothermal routes and annealed (calcined) at 400 • C were characterized by standard methods.The catalytic efficiency of the nanoparticles for the photodegradation of two organic model compounds, cationic methylene blue (MB) and anionic methyl orange (MO), under UV-B (285 nm) irradiation was investigated.The untreated sol-gel nanoparticle (T sg ) showed a higher selectivity for catalyzing photodegradation of MO than MB (90% versus 40% in two hours).This was attributed to an attractive interaction between anionic MO and the nanoparticle surface that adsorbed organic matter during synthesis acquiring a positive charge, facilitating photodegradation of the former.For the annealed nanoparticles (T sgc ) the selectivity for photocatalysis reversed (50% for MO versus 90% for MB in one hour) as the surface of T sgc was clean and negative due to the lone-pair electrons of oxygen of TiO 2 ; it preferentially adsorbed and more efficiently degraded the cationic MB relative to MO.The surface of the hydrothermal nanoparticle (T ht ) was clean at the outset because it was synthesized at an elevated temperature (240 • C) for a longer period, and also had a negative character.Consequently, it showed a photocatalytic selectivity for the cationic MB over MO, a behavior similar to the calcined sol-gel nanoparticle T sgc .The annealing of these particles (T htc ) did not alter the nature of the surface and hence the photocatalytic selectivity for MB was sustained.However, T htc being an anatase-brookite mixed phase showed better photocatalytic efficiency than the as-prepared T ht for the photodegradation of both the model substrates.

Figure 1 .
Figure 1.SEM images of Tsg (a), Tsgc (b), Tht (c), and Thtc (d).Inserts represent the size-distribution curves (blue lines) of the respective nanoparticles.Figure 1. SEM images of T sg (a), T sgc (b), T ht (c) , and T htc (d).Inserts represent the size-distribution curves (blue lines) of the respective nanoparticles.

Figure 2 .
Figure 2. FTIR spectra of the synthesized photocatalysts as indicated in the legend.

Figure 4 .
Figure 4. XRD patterns of the synthesized photocatalysts as indicated in the legend.Diffraction peaks corresponding to the anatase phase (A) and brookite phase (B) are also indicated.

Figure 4 .
Figure 4. XRD patterns of the synthesized photocatalysts as indicated in the legend.Diffraction peaks corresponding to the anatase phase (A) and brookite phase (B) are also indicated.

Figure 5 .
Figure 5. UV-visible spectra of stable sols (1 mg/10 mL) of the photocatalysts in ethanol, as indicated in the legends.

Figure 6 .
Figure 6.UV-visible absorbance data of the model dye solutions (10 mg/L) at various reaction times during photodegradation at 10 °C in a control experiment (without any photocatalyst) under UV-B irradiation: (a) MB, (b) MO.

Figure 5 .
Figure 5. UV-visible spectra of stable sols (1 mg/10 mL) of the photocatalysts in ethanol, as indicated in the legends.

Figure 5 .
Figure 5. UV-visible spectra of stable sols (1 mg/10 mL) of the photocatalysts in ethanol, as indicated in the legends.

Figure 6 .
Figure 6.UV-visible absorbance data of the model dye solutions (10 mg/L) at various reaction times during photodegradation at 10 °C in a control experiment (without any photocatalyst) under UV-B irradiation: (a) MB, (b) MO.

Figure 6 .
Figure 6.UV-visible absorbance data of the model dye solutions (10 mg/L) at various reaction times during photodegradation at 10 • C in a control experiment (without any photocatalyst) under UV-B irradiation: (a) MB, (b) MO.

Figure 7 .
Figure 7. Effect of the dose of the photocatalyst (Tsgc) on the degradation of MB under UV-B irradiation.10 mL of a 10 ppm MB solution was irradiated for three hours in the presence of the photocatalyst.

Figure 7 .
Figure 7. Effect of the dose of the photocatalyst (T sgc ) on the degradation of MB under UV-B irradiation.10 mL of a 10 ppm MB solution was irradiated for three hours in the presence of the photocatalyst.

Figure 8 .
Figure 8. UV-visible absorption spectra of methylene blue before and after adsorption by T sg (a), T sgc (b), T ht (c), and T htc (d) at ambient temperature (25 • C).

Figure 9 .
Figure 9. UV-visible absorption spectra of methyl orange before and after adsorption by T sg (a), T sgc (b), T ht (c), and T htc (d) at ambient temperature (25 • C).

Figure 13 .
Figure 13.UV-visible absorption spectra of an aqueous solution of MO during degradation under dark conditions in the presence of T sg (a), T sgc (b), T ht (c), and T htc (d).Reaction parameters: dye = 10 mg/L, nanoparticle = 8 mg/10 mL, temperature = 10 • C.

Figure 14 .
Figure 14.XRD patterns of the nanoparticles (indicated by the legends) recovered after the photocatalytic degradation reaction of MB (a) and MO (b) carried out under UV-B irradiation.Diffraction peaks corresponding to the anatase phase (A) and brookite phase (B) are also indicated.The brookite diffraction peaks indicated by the green color were absent in the fresh photocatalyst.

Figure 14 .
Figure 14.XRD patterns of the nanoparticles (indicated by the legends) recovered after the photocatalytic degradation reaction of MB (a) and MO (b) carried out under UV-B irradiation.Diffraction peaks corresponding to the anatase phase (A) and brookite phase (B) are also indicated.The brookite diffraction peaks indicated by the green color were absent in the fresh photocatalyst.

Figure 15 .
Figure 15.Comparison of the extent of degradation (r deg ) of MB and MO dyes in the presence of T sg (a), T sgc (b), T ht (c), and T htc (d) under UV-B irradiation.Reaction parameters: dye = 10 mg/L and nanoparticles = 8 mg/10 mL, temperature = 10 • C.

Figure 16 .
Figure 16.Apparent first-order kinetic plots prepared from UV-visible absorption data of the ninety minutes of photocatalytic degradation of MB and MO under UV-B irradiation in the pres of nanoparticles Tsg (open circles), Tsgc (closed circles), Tht (open triangles), and Thtc (closed trian at 10 °C.

Figure 16 .
Figure 16.Apparent first-order kinetic plots prepared from UV-visible absorption data of the first ninety minutes of photocatalytic degradation of MB and MO under UV-B irradiation in the presence of nanoparticles T sg (open circles), T sgc (closed circles), T ht (open triangles), and T htc (closed triangles) at 10 • C.

Figure 17 .
Figure 17.The schematic diagrams for the preparation of TiO 2 nanoparticles from titanium tetraisopropoxide (TTIP) by: (a) sol-gel and (b) hydrothermal methods.

Table 1 .
The degree of crystallinity (λ) and crystal size (l) of the synthesized TiO 2 nanoparticles.

Name of Samples λ/% l (± Error)/nm
and in Sections 2.2.4 and 2.2.5 above.Thtc, a mix of the brookite and the anatase TiO2 nanomaterial, is reflected in the XRD pattern of the material by the appearance of several additional diffraction peaks characteristic of the brookite phase TiO2, which may indicate that the reaction condition favored the formation of the brookite phase of TiO2.Further study on the matter is in progress.2.2.8.Selectivity of the Prepared Nanoparticles towards Catalyzing the Photodegradation of MB and MO under UV-B IrradiationFigure

Table 3 .
The values of the apparent first-order rate constant (k) and the half-life (t1/2) fo degradation of the model dyes MB and MO in the presence of nanoparticles Tsg, Tsgc, Tht, an under UV-B irradiation.

Table 3 .
The values of the apparent first-order rate constant (k) and the half-life (t 1/2 ) for the degradation of the model dyes MB and MO in the presence of nanoparticles T sg , T sgc , T ht , and T htc under UV-B irradiation.