The nanocomposite of TiO2-MWCNT has been synthesized by simple hydrothermal route showing significant enhancement in the photocatalytic activity for the degradation of methyl orange dye (MO). Several characterizations employed were X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), Transmission electron microscopy (TEM), Raman spectroscopy. XRD pattern shows the formation of anatase phase in prepared TiO2 and which was retained in TiO2-MWCNT composite as well. The Raman spectrum of prepared TiO2-MWCNT shows the interface integration of TiO2 and MWCNTs which is further supported by TEM data. Complete decolonization and degradation of dye using TiO2-MWCNT nanocomposite has been observed only in 45 minutes of UV irradiation. 65 % reduction in Chemical Oxygen Demand (COD) value of treated dye shows substantial mineralization of dye by composite catalyst. Dye degradation reactions were found to follow first order kinetics.
Keywords. MWCNTs; TiO2; methyl orange; photocatalysis
*Corresponding Author : Dr. Pragati Thakur, Associate Professor,
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Department of Chemistry, University of Pune, Ganeshkind Road, Pune-411007,India
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Titanium dioxide is a benchmark photocatalyst for the successful decontamination of the organic pollutants in both liquid and gas phases. Although TiO2 has several advantages it has two critical limitations to be used for large scale technical applications. They are (i) Electron hole recombination which limits its efficiency.  (ii) It absorbs only 2-3 % of the solar light impinging on the Earth's surface as it can be excited only under UV irradiation with wavelengths shorter than 400 nm. To overcome the limitations innumerable methods have been applied to enhance the photocatalytic activity of TiO2 by increasing active sites of reaction, retardation of electron-hole recombination and visible light catalysis by modification of band-gap.[2-4] Recently, among carbon materials, after the discovery of Carbon nanotubes (by Ijmer 1991),TiO2-MWCNT composites have attracted much attention of researchers because of the remarkable electrical,[6-7] mechanical and thermal properties of MWCNTs and the promising applications of TiO2-MWCNT composites for the big problem of pollutions  due to their high capability to conduct electrons and adsorb hydrophobic organic pollutants, hardly adsorbed by TiO2 nanoparticles themselves. [11-12] The synergistic effect of carbon nanotubes on the activity of composite catalyst can be explained in terms of its action as adsorbent and dispersing agent. Further MWCNTs consisting of multiple layers of graphite superimposed and rolled in on them to form tubular shape conductive structure might facilitate the separation of the photo-generated electron/hole pairs at the TiO2-CNT interface leading to the faster rates of photocatalytic oxidation and enhancement in the efficiency of titanium dioxide.
Different techniques have been employed for the preparation of TiO2-CNT composites. Mostly TiO2 is coated on the surface of CNT. The Composites can be prepared by various methods [13-14] which includes sol-gel,[15-18] impregnation, Electro-spinning[20-21] electrophoretic deposition, [ 22] chemical vapor deposition[ 23] and hydrothermal method.  Among this sol-gel has been used extensively for mechanical mixing of CNT- TiO2 composites. [25-26] Composites prepared by hydrothermal methods are mostly found to give better results as it favors a decrease in agglomeration among particles, narrow particle size distribution, phase homogeneity and controlled particle morphology. In present study, we have prepared TiO2-CNT composites by in-situ deposition of CNT on the TiO2 sol followed by hydrothermal treatment.
Functionalization of MWCNTs is needed to improve the solubility of MWCNTs in water by introducing anionic groups on their surfaces. In the present investigation functionalization was done by giving acid treatment to Pristine MWCNTs followed by purification of MWCNTs by removal of amorphous carbon and metal catalyst. Pristine MWCNTs (5-20 nm in diameter, 1-10 μm in Length carbon purity is min. 95 %, purchased from Reinste Nano Ventures Pvt. Ltd.) were subjected to conventional acid treatment. For the purification of MWCNTs an amount of 0.05g of pristine MWCNTs were sonicated several times in the equal volume of concentrated HCl till the color of acid was unchanged followed by washing with double distilled water and drying at 110 oC in oven..  These purified MWCNTs were refluxed in conc. HNO3 for 6h at 80 oC followed by washings with double distilled water till neutral pH is obtained, The sample was then dried at 110 oC.
Synthesis of TiO2 and TiO2-MWCNT nanocomposite
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Titanium dioxide and TiO2-MWCNT nanocomposite were prepared by hydrothermal method, using titanium tetraisopropoxide (TTIP) as the precursor.  For synthesis all used chemicals were of analytical grade. In synthesis of TiO2-MWCNT nanocomposite, the functionalized MWCNTs (fMWCNTs) were added to provide a weight ratio of MWCNT over TiO2 was 10%. First fMWCNTs were dispersed into double distilled water and sonicated for 1h. A predetermined amount of TTIP was mixed with ethanol in 1:5 ratios. After complete dispersion of MWCNTs in water, TTIP: Ethanol solution was added drop wise under sonication and was kept overnight with vigorous stirring. On the next day, whole solution was transferred to the Teflon lined stainless steel autoclave and was placed in muffle furnace for hydrothermal treatment at 1400C for 24h. In autoclave fMWCNTs interact with TiO2 at high temperature and at elevated pressure. After cooling the furnace to the room temperature autoclave was removed from the furnace opened and dried the composite formed on hot plate. The composite was calcined at 400oC for 2h. The photocatalytic efficiency of the composite was compared with bare Titanium dioxide prepared by same method without adding fMWCNTs.
Characterization of Sample
The TiO2 and TiO2-MWCNT nanocomposite were characterized by a range of analytical techniques. The X-ray powder diffraction (Philips X' Pert PRO) patterns were recorded with Cu Kα radiation (α = 0.15406nm) in the range 10o to 80o 2θ at a scanning speed of 0.02osec-1 to determine the crystal structure. The Raman measurements were performed by Micro Raman Spectroscopy (Horiba Jobin Yvon Lab RAM HR 800 spectrometer) for the study of chemical bonding and nature of disorder in the materials. The morphology and structure of TiO2 nanoparticle and TiO2-MWCNT nanocomposite were examined by Transmission electron microscopy (TECNAI G2 20 TWIN FEI, Netherlands) and Scanning electron microscopy (JEOL JSM-6360 A) along with Energy Dispersive X-ray (EDX). The sample was subjected to thermal gravimetric analysis (DTG-60H simultaneous DTA-TG apparatus, Shimadzu) to get regarding thermal stability of the composite. The formation of functional groups on the surface of pristine MWCNTs after acid treatment was studied by the FTIR spectroscopy (Shimadzu FTIR-8400 spectrophotometer) with KBr-disc technique.
The photocatalytic activities of the TiO2 nanoparticles and TiO2-MWCNT nanocomposite were monitored from the results of photocatalytic degradation of Methyl Orange. The initial concentration of methyl Orange was 0.01mmols and catalyst dose was 0.02g/50mL. The reaction temperature was controlled at 30±1oC by an air cooling The photocatalytic activity was analysed using home made multilamp photoreactor consisting of quartz reaction vessel in the center surrounded by four 8w UV lamps at the edge of the square. The reaction solution was constantly aerated using aerator pump (Philips, TUV 8W/G8 T5). Before UV irradiation the suspension was stirred in dark for 1h to ensure the establishment of adsorption-desorption equilibrium. The un-decomposed Methyl Orange at various time intervals during UV irradiation was determined using Shimadzu UV-visible (UV-1800 PC) spectrophotometer. Conversion of Methyl Orange was defined as the following :
% conversion = (C0 - C)/C0 x 100
Where C0 is the initial concentration of Methyl Orange and C is the concentration of Methyl Orange after photocatalytic reaction. The photocatalytic degradation and mineralization of methyl orange was further confirmed by COD analysis. For COD, the digestion of sample was carried out by open reflux method using COD digester (Spectralab COD digestor 2015M) for 2h at 150 oC. After cooling, the solutions were titrated against ferrous ammonium sulfate by COD titrator (Spectralab COD titrator CT-15) using double distilled water as a blank solution.
Results and Discussion
The XRD of TiO2 (refer Figure 1 a) shows the formation of anatase and brookite mixture. The peaks for anatase are 25.33(101), 37.88(004), 47.98(200), 54.74(105), 62.80(204), 70.00(116), 75.16(301) [PCPDF WIN # 211 272] and one small peak for brookite 30.68(211) [PCPDF WIN # 761 937]. XRD of MWCNT (refer Figure 1 ) shows two characteristic peaks at 26.00(002) and 43.11(100) [PCPDF WIN # 411 487]. The composite shows all these above mentioned peaks confirming the formation of TiO2- MWCNT composite.
The Raman spectra for TiO2, MWCNTs and TiO2-MWCNT nanocomposite samples are as shown in Figure 2. The Raman spectrum for the pure TiO2, assign the 148.10cm-1 (very strong Eg), 397.82 cm-1 (B1g), 518.07 cm-1 (A1g) and 641.53 cm-1 (Eg) bands to anatase[30-31] and 246.89cm-1(A1g) and 326.17 cm-1 (B1g) bands to brookite It confirms the preparation of TiO2 as a mixture of anatase and brookite, anatase phase being dominant.. Which was earlier confirmed by XRD data. In the Raman spectrum of MWCNT the band at 1597.21 cm-1 indicates the G band, this G band shows the crystalline nature of the MWCNTs. The band at 1318 cm-1(D band) indicates the distortions on the MWCNT surface. It is worth noting that the synergistic effect occurs only if the TiO2 is chemically attached to the carbon nanotubes. The main three bands (397.82 cm-1, 518.07 cm-1, 641.53 cm-1) in the Raman spectrum representative of anatase TiO2 (refer Figure 2 : inset) are broadened and shifted in the case of TiO2-MWCNT nanocomposite sample as compared to the pure TiO2. Such broadening and shifting may occur due to strain gradients originating from interface integration of TiO2 and MWCNT.
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Figure 3a, 3b and 3c shows the SEM image of TiO2, MWCNTs and TiO2-MWCNT nanocomposite . . The TEM images of TiO2, and TiO2-MWCNT nanocomposite are shown in Figure 4a and 4b). SEM and TEM images of TiO2, shows the uniform average distribution. Figure 3c depicts the SEM of TiO2-MWCNT nanocomposite showing TiO2 nanoparticles agglomeration on MWCNT surface. The TEM image (Figure 4b) reveals nanocomposite formation made up of TiO2 nanoparticle agglomerates embedded with MWCNTs. Because of the more quantity of TiO2 over MWCNTs (10% MWCNTs content) most of the MWCNTs are covered and hidden under the TiO2 nanoparticles. The aggregation of TiO2 over MWCNTs indicates the supporting role of MWCNTs as center for deposition and growth of TiO2 nanoparticles. Also confirming the intimate contact between the MWCNTs and TiO2. The Energy dispersive X-ray (Figure 5) spectrum analysis of TiO2 - MWCNT samples shows the presence of C, O and Ti elements.
The TGA gives information about thermal stability of compound. As seen in Figure 6, the highest rate of mass loss is at 500 oC which is the combustion point MWCNTs. In case of TGA of TiO2, initial loss in weight can be seen due to evaporation of water molecules around 100 oC and further due to decomposition of organic residue around 200-350 oC. In the composite of TiO2-MWCNTs, the early weight loss around 100 oC was because of water evaporation followed by decomposition of organic residue and combustion of MWCNTs around 200-350 oC and 550-650 oC respectively. The thermal analysis suggests the stability of TiO2 - MWCNT nanocomposite at calcination temperature of 400 oC . Further the MWCNTs present in composite are thermally more stable than pristine MWCNTs.
To obtain the hydrophilic surface structure of oxygen containing surface groups, chemical oxidation of MWCNTs is carried out using concentrated nitric acid. The oxidation of MWCNTs with nitric acid introduces some functional groups like -OH (Hydroxyl), -COOH (carboxyl) and some more on the surface of MWCNTs. These surface groups are helpful to form interaction and chemical bonding between MWCNTs and TiO2. The FTIR spectrum of pristine(refer Figure 7a) and functionalized MWCNTs provides information of surface functional groups. As shown in Figure 7(b) functionalized MWCNTs exhibits characteristic strong and broad band between 3173-3600 cm-1 which can be attributed to O-H stretching vibrations in C-OH groups. The broad band between 1766-2017 cm-1 is attributed to C=O stretching vibrations in carboxyl, aldehyde and acid anhydride groups.
The photocatalytic activity of TiO2 and TiO2-MWCNT was evaluated by studying the oxidation of Methyl Orange dye solution under UV light irradiation. Figure 8a and Figure 8b shows UV-Visible absorbance spectral changes of Methyl Orange Dye solution during the photocatalytic degradation in the presence of prepared TiO2 and prepared TiO2-MWCNT nanocomposite. Obtained results show (refer Figure 9), two fold enhancement in the photocatalytic activity of TiO2-MWCNT nanocomposite as as compared to pure prepared TiO2 nanoparticles in only 45 minutes of UV irradiation. Degradation of Methyl Orange dye was found to follow the first order reaction kinetics (refer Figure 10). The obtained results show that the rate of degradation of Methyl Orange dye using TiO2-MWCNT nanocomposites as photocatalyst is 10 times higher as compared to prepared TiO2. The enhanced photocatalytic efficiency of as synthesized nanocomposite can be attributed to the presence of MWCNTs which act as an adsorbent, dispersing agent and electron reservoir and facilitating the separation of the photo-generated electron/hole pairs at the TiO2-MWCNT interface leading to the faster rates of photocatalytic oxidation. The COD data depicted in Table 1 shows substantial degradation and mineralization of methyl orange dye solution when TiO2-MWCNT nanocomposite was used.
Nanosized and interface integrated TiO2-MWCNT composite was successfully synthesized using hydrothermal method and characterized by XRD, Raman spectroscopy, SEM, TEM, EDX, FTIR and TGA. The enhanced photocatalytic efficiency of as synthesized TiO2-MWCNT nanocomposite suggests that the MWCNTs acts as an adsorbent, dispersing agent and electron reservoir and hence facilitating the separation of the photo-generated electron/hole pairs at the TiO2-MWCNT interface leading to the faster rates of photocatalytic oxidation. Degradation of Methyl Orange dye was found to follow the first order reaction kinetics. COD values of degraded methyl orange dye solution shows substantial mineralization of methyl orange dye solution when TiO2-MWCNT nanocomposite was used.