3.1. FTIR analysis
Fig. 1 and Fig. 2 illustrate the Fourier transform infrared (FTIR) spectrums of MCT#1 and MCT#2, respectively. As can be observed in Fig. 1, two detectable transmission bands around 1625 cm-1 and 3419 cm-1 are assigned to the stretching vibration of O-H [16] bonds that is present in the generated carboxylic groups (HO-C=O) during the oxidation of MWCNTs in NH3. The presence of these kinds of oxygen containing groups on the surface of MWCNTs can enhance the hydrophilic properties. Therefore, the stability of MWCNTs in the organic solutions is improved. Meanwhile, these functional groups can as active sites for nucleation of different kinds of nanoparticles. Besides these two bands, there is a major peak in the range of 440 cm-1 to 520 cm-1 that can be attributed to the O-Ti bending of TiO2 nanoparticles [16,22]. The results of Fig. 2 reveal that all of three mentioned transmission bands that are observed in MCT#1 can be detected for synthesized MCT#2. Therefore, it can be confirmed that the hydrolysis of TiCl4 in in the solution containing functionalized MWCNTs can lead to the synthesis of TiO2 nanoparticles and covalent attachment on the sidewalls of MWCNTs. The comparison between intensity of Ti-O groups in MCT#1 and MCT#2 reveals that the content of TiO2 nanoparticles in the prepared MCT#2 is higher than that of MCT#1. It can be attributed to the applied amount of TiCl4 as precursor of TiO2 nanoparticles in the hydrolysis process.
3.2. TEM study
Fig. 3 and Fig. 4 show the TEM images of the synthesize MCT#1 and MCT#2, respectively. According to these Figures, the presence of TiO2 nanostructures with spherical shape can be confirmed on the outer surface of MWCNTs. Comparison between TEM images of MCT#1 and MCT#2 confirms that the amount of introduced TiO2 nanostructures on the sidewalls of MCT#1 is lower than that of MCT#2. It can be due to the amount of soluble Ti+4 ions in the suspension of MWCNTs. As the amount of applied TiCl4 as precursor of TiO2 nanostructures in the synthesis of MCT#2 is higher than that of MCT#1, the produced soluble ions can be enhanced due to the hydrolysis. Therefore, most of ions bind to the negative charges on the surface of MWCNTs (–COOH and –OH) and cause nucleation [23]. The particle size distributions of decorated TiO2 nanoparticles on the sidewalls of MCT#1 and MCT#2 are represented if Fig. 5 and Fig. 6, respectively. The particle size distributions reveal that the particle size of TiO2 nanostructures in the synthesized MCT#1 and MCT#2 are ranging from 5 nm to 25 nm and 10 nm to 20 nm, respectively. However, the average particle size of the most decorated nanoparticles in the synthesized MCT#1 and MCT#2 are about 13 nm and 15 nm, respectively.
3.3. Degradation rate study
Fig. 7 and Fig. 8 show the variation of the ratio of MO concentration at each interval to the initial concentration with respect to the irradiation time and weight fraction of the synthesized MCT#1 and MCT#2, respectively. According to these Figures, it is clear that the ratio of MO concentration at each irradiation time to the initial concentration decreases by enhancement of time and weight fraction. It means that the enhancement of time and weight fraction of applied catalysts leads to the decreasing of organic pollutant concentration. The influence of irradiation time on the decomposition of organic pollutants can be due to the excited and transmitted electrons from valence band to the conduction band [24,25]. In fact, UV-irradiation of the photocatalysts surface stimulates the capacitance layer electrons. Thus, the excited electrons transfer to the conduction layer. The transfer of electron from the valence band to the conduction band leads to the creation of cavity (h+) and electron (e-) in the conduction band and valence band, respectively. The number of created e- - h+ pairs is equal to the number of transferred electrons. Thus, as the UV irradiation time increases, the number of transferred electrons and consequently the number of created e- - h+ pairs increases. The created e- - h+ pairs can react with the dissolved oxygen in the suspension to form the active oxidizing radicals such as hydroxyl (OH.). Therefore, the number of formed oxidizing radicals can be enhanced with increasing the irradiation time [5,24]. The reduction of MO concentration with enhancement of the applied photocatalysts concentration (MCT#1 and MCT#2) is due to the augmentation of the contact surface of the photocatalysts with UV irradiation and organic pollutants. The enhancement of the contact surface leads to the increasing of the exited electrons and the created e- - h+ pairs [7,9,11]. Therefore, the enhancement of the photocatalysts weight fraction has a positive effect on the decomposition rate of MO.
Fig. 9 shows the comparison between variation rates of MO concentration with respect to the irradiation time using synthesized MCT#1 and MCT#2 at different weight fraction. It can be observed that at three studied weigh fractions (0.1 %wt, 0.2 %wt and 0.3 %wt) the decreasing rate of MO concentration using MCT#2 is higher than that of MCT#1. Therefore, at the same irradiation time the final concentration of MO in the suspension containing MCT#2 is lower than that of MCT#1. It may be due to the amount of decorated TiO2 nanoparticles on the sidewalls of MWCNTs. Increasing the TiO2 nanoparticles content leads to the enhancement of active contact surface of photocatalyst that is exposed with UV irradiation. Therefore, it increases the excitation of electrons in the valence band. As the excited electrons are able to move to the conduction layer, the amount of produced e- - h+ pairs and active oxidant radicals such as hydroxyl can be increased [12,26,27]. Thus, it can be confirmed that the created oxidizing radicals in the suspension containing MCT#2 is greater than MCT#1.
3.4. The statistical analysis based on Duncan’s multiple range test
Fig. 10 and Fig. 11 show the analysis results of MO concentration based on Duncan’s multiple range test. The influence of different levels of each main factor such as irradiation time and weight fraction of prepared photocatalysts on the MO concentration can be evaluated according to the Duncan’s multiple range test. Fig. 10 represents the effect of irradiation time on the variation of MO concentration. According to the Fig. 10, it can be observed that the MO concentration decreases by increasing the irradiation time from 5 min to 35 min. it may be due to the effect of irradiation time on the amount of generated electrons and holes [5,25]. Meanwhile, the results of Fig. 10 depict that at each irradiation time the MO concentration in the suspension containing synthesized MCT#2 is lower than that of MCT#1. It can be attributed to the TiO2 content in the synthesized MCT#1 and MCT#2. As mentioned in the before section, the amount of TiO2 nanoparticles as main photocatalyst in the sample of MCT#2 is higher than that of MCT#1. Therefore, active surface of MCT#2 that is exposure with light source is higher than that of MCT#1. Thus, the excited electrons from valence band to the conduction band and formed oxidizing radicals can be increased in the suspension involving MCT#2. In addition, the results of Fig. 10 confirm that all studied levels of irradiation time have a significant effect on the variation of MO concentration at significance level equal to 0.05.
The influence of weight fractions of MCT#1 and MCT#2 on the variation of MO concentration can be observed in Fig. 11. As can be seen, different levels of the weight fractions of prepared MCT #1 and MCT #2 (0.1, 0.2 and 0.3 %wt) are significant (at 5% level of probability) on the variation of MO concentration. Meanwhile, it is clear that the degradation rate of MO enhances by increasing the weight fraction of the synthesized MCT #1 and MCT #2. The active surface area of the prepared photocatalysts can be enhanced by increasing the weight fraction of them[8,15]. Therefore, more electrons can migrate from the valence band to the conduction band. It can be eventuate to the enhancement of the produced active radicals such as hydroxyl (OH.) [12,27] . These kinds of radical can act as decomposer of different dye organic pollutants such as MO.
3.5. Response surface study
Fig. 12 and Fig. 13 illustrate the response surface of changes in the MO concentration using synthesized MCT#1 and MCT#2, respectively. The response surface method (RSM) is a common graphical approach for investigation the simultaneous influence of the studied parameters (such as irradiation time and weight fraction) on the variation of response (MO concentration). The results of the Fig. 12 and Fig. 13 confirm that the MO concentration in the presence of synthesized MCT#1 and MCT#2 as photocatalysts decreases by increasing the irradiation time and weight fraction. Although, it can be observed that the influence of irradiation time on the decomposition of MO is more than that of weight fraction. It can be attributed to the effect of irradiation time on the excitation of electrons that are located in the valence band. Thus, the excited electrons can be transferred from the valence band to the conduction band. It can be eventuated to the formation of electrons and holes in the valence band and conduction band, respectively [1,3]. The produced charges can act as a decomposer of the organic pollutants such as MO. Therefore, the enhancement of generated charges can decrease the MO concentration in the suspension.
Fig. 14 and Fig. 15 illustrate the contour lines of the variation of MO concentration with respect to the irradiation time and weight fraction of synthesized MCT#1 and MCT#2, respectively. It is clear that the enhancement of irradiation time leads to the decreasing of desired weight fraction of MCT#1 and MCT#2 as photocatalysts. It means that for decreasing the MO concentration to the desired value, the required weight fraction of the both photocatalysts decreases by enhancement of irradiation time.