The Dependence of Photocatalytic Activity on Tio2 to MWCNT Ratio in the Composite


 Here, we prepared two kinds of nanocomposites (MCT#1 and MCT#2) involving MWCNTs and TiO2 nanopaerticles. The characterization of the samples is carried out based on FTIR spectroscopy and TEM. The Ti-O groups that is attibuted to the TiO2 nanopaerticles can be confirmed according to the FTIR analysis. TEM images show that the average particle size of TiO2 nanopaerticles in prepare MCT#1 and MCT#2 is equal to 13 nm and 15 nm, respectively. The influence of nanocomposites weight fraction and illumination time are investigated on the decomposition rate of methyl orange (MO) as pollutant. The photocatalytic results exhibit that the decomposition rate of MO is increased with respect to the weight fraction and illumination time. Meanwhile, higher decomposition rate can be observed using MCT#2 compared to MCT#1. Statistical analysis of the results based on Duncan’s multiple range test at α = 0.05 reveals that all of the applied levels of the factors have a significant effect on the decomposition rate. The response surface results confirm that the effect of illumination time is high that that of weight fraction of MCT#1 and MCT#2.


Introduction
Organic dyes are commonly applied in different industries such as textile and dyestuff. Some of these organic dyes, which are often toxic, can be entered in the sewage of the industries that is discharged into water systems. It can be leads to the signi cant environmental pollutions. Colored pollutants reduce the amount of sunlight penetrating deep into the water and disrupt the photosynthetic process of aquatic organisms [1,2]. Therefore, reducing or eliminating dye contaminants from industrial wastewater is critical to protecting the environment. In recent years, various methods including reverse osmosis, ozonization, photocatalytic degradation and adsorption have been studied for the decomposition of dye contaminants [3,4]. Among the mentioned techniques, advanced oxidation process (AOP) using semiconductor photocatalysts is an e cient technique that can completely decompose the dye organic pollutants [5,6]. TiO 2 due to the wide band gap (3.2 eV) is applied as a dominant photocatalyst for the photocatalytic oxidation of environmental organic pollutants. The AOP is based on the irradiation of suspension containing the pollutant and semiconductor using UV light source. The UV irradiation leads to the excitation of electrons that are located in the valence band of the semiconductors. The excited electrons can transmit from the valence band to the conduction band [7,8]. Therefore, due to the migration of an electron from the valence layer to the conduction layer, a hole and an electron are produced on the valence band and conduction band, respectively. The generated electron and hole pairs have a signi cant effect on the degradation of pollutants [9,10]. The recombination of the produced charges can reduce the degradation rate of pollutants. Therefore, it leads to the enhancement of operating cost and decreasing the removal e ciency. The coupling of TiO 2 nanoparticles with materials that have a high aspect ratio is expected to preparation of a novel nanocomposite, which can reduce the recombination rate of generated electron and hole pairs. In the recent years, there are several reports that studied the effect of different supports such as carbon nanotubes (CNTs), graphene oxide (GO) and reduced graphene oxide (rGO) on the recombination rate of produced electron and hole pairs [11,12,13].
The reported results show that the coupled systems containing applied support and semiconductor have higher photocatalytic performance rather than the semiconductor alone. It can be related to the positive in uence of support on the decrement on recombination rate [14,15]. Multi-walled carbon nanotubes (MWCNTs) are widely applied for preparation of nanocomposites, sensors and nanoelectronics [16,17]. It can be due to the excellent physical, mechanical, electrical, thermal and chemical properties of MWCNTs [18,19,20]. Recently, the application of MWCNTs for synthesis of catalytic template materials has great deal of attention due to the unique high surface area and excellent aspect ratio. Meanwhile, the coupled MWCNTs@TiO 2 have been applied as photocatalyst for degradation of different kinds of organic pollutants such as methyl orange (MO) and methylene blue (MB). However, the effect of TiO 2 content in the prepared coupled system has not yet been investigated. In addition, there is no report on the statistical analysis of the photocatalytic performance of MWCNTs@TiO 2 .
In this study, we prepare a new nanocomposite containing MWCNTs and TiO 2 nanoparticles. The

Preparation of MCT nanocomposites
The applied multiwalled carbon nanotube (MWCNTs) in this study have the average diameter aruond 40-60 nm and length 5-15μm. The typical method for preparation of the oxidized MWCNTs is based on the acid-treatment of pristin MWCNTs in HNO 3 that is discribed in our previous works [16,21]. The prepared oxidized MWCNTs contain the functional groups that can be applied for synthesized the TiO 2 @MWCNTs composites. For this purpose, 0.08 g of oxidized MWCNTs is dispersed in 100 ml of distilled water and proccessd in ultrasonic bath for about 30 min. Subsequently, the desired amount of tetra chloride titanium (TiCl 4 , 99%, Merck) is added to the oxidized MWCNTs suspention as precursor of TiO 2 nanoparticles. The obtained mixture is agitated at ambient temperature for about 5 h. Then the temperature is increased until 65°C for about 12 h. Finally, the suspention is ltered, washed and calcined at 350°C for 3h. It should be mentioned that by variation of TiCl 4 content, we can synthesis different kinds of nanocomposites. Therefore, in this study, the TiCl 4 content is varied from 0.4 ml to 0.8 ml and the synthesized sapmles are indexed as MCT#1 and MCT#2, respectively.

Characterization
The surface functional groups of the prepared nanocomposites are determined using a Tensor 70 Fourier transform infrared (FTIR) spectrometer. The morphology, decoration quality and particle size of TiO 2 nanoparticles are determined by transmission electron microscopy (TEM, LEO 912AB). The decomposition rate of MO as an organic dye pollutant model is evaluated using UV-Vis absorption spectroscopy (Lambda EZ 201spectrophotometer, Perkin Elmer Company). For evaluatuion of the MO decomposition, the speci c amount of MCT#1 and MCT#2 (0.1, 0.2 and 0.3 %wt) as photocatalyst is added to 80 mL of MO solution (10 ppm). The prepared suspensions are stirred for 60 min in a dark chamber. It leads to the adsorption and desorption equiblirium. The adsorbance of the MO in the suspension is recorded at 464 nm. The recorded adsorbance is assigned as initial adsorbance (A 0 ) that can be attributed to the initial concentration (C 0 ). Then, the suspension is irradiated using an Hg vapor lamp (150W). The adsorbance of the MO at each 5 min interval is evaluated and assigned as A t , which is coresponded to the remained concentration (C t ). The decomposition rate of MO can be calculated based on the Equation 1.  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 TiO 2 nanostructures with spherical shape can be con rmed on the outer surface of MWCNTs. Comparison between TEM images of MCT#1 and MCT#2 con rms that the amount of introduced TiO 2 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 TiCl 4 as precursor of TiO 2 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 TiO 2 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 TiO 2 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. 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 nal 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 TiO 2 nanoparticles on the sidewalls of MWCNTs. Increasing the TiO 2 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 con rmed that the created oxidizing radicals in the suspension containing MCT#2 is greater than MCT#1. Fig. 10 and Fig. 11 show the analysis results of MO concentration based on Duncan's multiple range test. The in uence 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.

The statistical analysis based on Duncan's multiple range test
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 TiO 2 content in the synthesized MCT#1 and MCT#2. As mentioned in the before section, the amount of TiO 2 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 con rm that all studied levels of irradiation time have a signi cant effect on the variation of MO concentration at signi cance level equal to 0.05.
The in uence 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 signi cant (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 in uence 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 con rm 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 in uence 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.

Conclusions
MWCNTs act as substrates for synthesis of TiO 2 nanoparticles via hydrolysis method. TEM images show that TiO 2 nanopaerticles are successfully attached on the surface of MWCNTs in the both of MCT#1 and MCT#2. FTIR analysis and TEM images con rm that the TiO 2 nanoparticles content in MCT#2 is higher than that of MCT#1. The photocatalytic results show that the concentration of MO as pollutant is dramatically decreased by enhancemnt of irradiation time and weight fraction of MCT#1 and MCT#2. Meanwhile, the reults con rmed that the photocatalytic performance of MCT#2 is higher than that of MCT#1. The analysis of the results based on response surface depicts that the in uence of irradiation time on the degradation of MO is more than that of weight fraction of MCT#1 and MCT#2. 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.

Figure 9
Comparison between variation of the ratio of MO concentration at each interval to the initial concentration with respect to the irradiation time using synthesized MCT#1 and MCT#2 at different weight fraction, a) 0.1 %wt, b) 0.2 %wt, c) 0.3 %wt.

Figure 10
The effect of irradiation time on the MO concentration using synthesized MCT#1 and MCT#2, the analysis based on Duncan's multiple range test at α= 0.05.

Figure 11
The effect of weight fraction of synthesized MCT#1 and MCT#2 on the MO concentration, the analysis based on Duncan's multiple range test at α= 0.05.

Figure 12
Response surface of the variation of MO concentration using synthesized MCT#1.

Figure 13
Response surface of the variation of MO concentration using synthesized MCT#2.

Figure 14
Contour lines of the variation of MO concentration using synthesized MCT#1. Figure 15 Contour lines of the variation of MO concentration using synthesized MCT#2.