Preparation, Characterisation and Photocatalytic Applications of TiO2-MWCNTs Composite

The nanocomposite of TiO2-MWCNTs has been synthesised by simple hydrothermal route showing significant enhancement in the photocatalytic activity for the degradation of methyl orange dye (MO). Several characterisations 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 which was retained in TiO2-MWCNTs 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 decolorisation and degradation of dye using TiO2-MWCNTs nanocomposite has been observed only in 45 minutes of UV irradiation. 65 per cent reduction in chemical oxygen demand (COD) value of treated dye shows substantial mineralisation of dye by composite catalyst. Dye degradation reactions were found to follow first order kinetics.

Keywords:  MWCNTsTiO2methyl orangephotocatalysis 

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  for large scale technical applications. They are (i) electron hole recombination which limits its efficiency1. (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–gap2-4. Recently, among carbon materials, after the discovery of carbon nanotubes5, TiO2-MWCNTs composites have attracted much attention of researchers because of the remarkable electrical6-7, mechanical8 and thermal9 properties of MWCNTs and the promising applications of TiO2-MWCNTs composites for the big problem of pollutions10 due to their high capability to conduct electrons and adsorb hydrophobic organic pollutants, hardly adsorbed by TiO2 nanoparticles themselves11,12. 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-CNTs composites. Mostly TiO2 is coated on the surface of CNT. The composites can be prepared by various methods13,14 which includes sol-gel,15-18 impregnation19, electro-spinning,20,21 electrophoretic deposition22, chemical vapor deposition23 and hydrothermal method24. Among this sol-gel has been used extensively for mechanical mixing of CNT-TiO2 composites25,26. 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 the present study, we have prepared TiO2-MWCNTs nanocomposites by in-situ deposition of TiO2 on the MWCNTs by hydrothermal treatment.

2.1 MWCNTs Functionalization

The pristine MWCNTs (Multi Walled Carbon Nanotubes) were purchased from Plasma Chem. GmbH Berlin, having diameters of 5-20 nm, lengths of 1-10μm and carbon purity is minimum 95%. Purification and functionalisation of MWCNTs is needed to remove impurities and to improve the solubility of MWCNTs in water by introducing anionic groups on their surfaces27.

In a typical purification treatment, 1g pristine MWCNTs were sonicated in the equal volume of conc. HCl for 1 hr and allowed to settle down, after settling yellow coloured supernatant acid was observed. This colour indicates that there is a Fe impurity present in the pristine MWCNTs. This Fe impurity has been removed by repeating same procedure and removing the acid layer on the surface of MWCNTs till the supernatant acid became colorless. The clear supernatant acid confirms the complete removal of iron impurity. These purified MWCNTs were further  washed with doubled distilled water till  complete removal of acid was achieved, which was confirmed by litmus paper and then dried at 110 °C in hot air oven for about 6-7 hrs28. For functionalization, purified MWCNTs were refluxed in conc. HNO3 for 6 hrs at boiling temperature of acid (≈80oC). After refluxing the above solution, it was allowed to cool to room temperature and decanted the upper layer of acid. It was then washed several times with double distilled water till neutral pH was achieved, which was confirmed by performing litmus paper test and was then dried at 110 °C in vacuum oven for 6-7 hrs.  The dried MWCNTs were collected and subjected for FTIR analysis27.

2.2 Synthesis of TiO2 and TiO2-MWCNTs Nanocomposite

Titanium dioxide and TiO2-MWCNTs nanocomposite were prepared by hydrothermal method, using titanium tetraisopropoxide (TTIP) as the precursor29. For synthesis all used chemicals were of analytical grade. In synthesis of TiO2-MWCNTs nanocomposite, the functionalized MWCNTs (fMWCNTs) were added to provide a weight ratio of MWCNTs over TiO2 as 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 140 °C 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 400 °C for 2h. The photocatalytic efficiency of the composite was compared with bare Titanium dioxide prepared by same method without adding fMWCNTs.>

2.3 Characterisation of Sample

The TiO2 and TiO2-MWCNTs 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 = 0.15406 nm) in the range 10° to 80° at a scanning speed of 0.02° s-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 TiO2nanoparticle and TiO2-MWCNTs 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 information 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.      

2.4 Photocatalytic Activity

The photocatalytic activities of the TiO2 nanoparticles and TiO2-MWCNTs 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/50 mL. The reaction temperature was controlled at 30±1oC by an air cooling. The photocatalytic activity was analysed using homemade multilamp photoreactor consisting of quartz reaction vessel in the center surrounded by four 8W UV lamps at the edges 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 concentration of 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 reaction16. The photocatalytic degradation and mineralisation 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 °C. 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.

3.1 Crystal Structure

The XRD of TiO2 (refer Fig.1 c) 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) [JCPDS 21-1272] and one small peak for brookite 30.68(211) [JCPDS 76-1 937]. XRD of MWCNTs (Fig 1(a)) shows two characteristic peaks at 26.00(002)

Figure 1.XRD patterns of (a) MWCNTs (b) TiO2-MWCNTs (c) TiO2.

and 43.11(100) [JCPDS 41-1487]. The composite shows all these above mentioned peaks confirming the formation of  TiO2 - MWCNTs nanocomposite (Fig. 1(b)).

Figure 2.Raman spectrum of (a) MWCNTs (b) TiO2(c) TiO2-MWCNTs.

The Raman spectra for TiO2, MWCNTs and  TiO2 - MWCNTs nanocomposite samples are as shown in Fig. 2. The Raman spectrum for the pure  TiO2,  assign the 148.10 cm-1 (very strong Eg), 397.82 cm-1 (B1g), 518.07 cm-1 (A1g) and 641.53 cm-1 (Eg) bands to anatase30-31 and 246.89 cm-1 (A1g) and 326.17 cm-1 (B1g) bands to brookite32. 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 MWCNTs 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 MWCNTs surface. It is worth noting that the synergistic effect occurs only if the TiO2 is chemically attached to the carbon nanotubes33. 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 Fig. 2 : inset) are broadened and shifted in the case of TiO2-MWCNTs 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 MWCNT34.

Figure 3.SEM images of the (a) synthesised TiO2 (b) MWCNTs (c) TiO2-MWCNTs nanocomposite (d) EDX of the TiO2-MWCNTs nanocomposite and TEM images of the (e) synthesized TiO2 (f) TiO2-MWCNTs nanocomposite.

3.2 Morphological Characterisation

Figures 3(a), 3(b) and 3(c) shows the SEM images of TiO2, MWCNTs and TiO2-MWCNTs nanocomposite. The TEM images of TiO2, and TiO2-MWCNTs nanocomposite are shown in Fig. 3(e) and 3(f). SEM and TEM images of TiO2, shows the uniform average distribution. Figure 3(c) depicts the SEM of TiO2-MWCNTs nanocomposite showing TiO2 nanoparticles agglomeration on MWCNTs surface. The TEM image (Fig. 3(f)) reveals nanocomposite formation made up of TiO2 nanoparticle agglomerates embedded with MWCNTs35. 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 nanoparticles35. Also confirming the intimate contact between the MWCNTs and TiO2. The energy dispersive X-ray (Fig. 3(d)) spectrum analysis of TiO2- MWCNTs samples shows the presence of C, O and Ti elements.

3.3 Thermogravimetric Analysis

The TGA gives information about thermal stability of compound. As seen in Fig. 4, the highest rate of mass loss is at 500 °C which is the combustion point of MWCNTs. In case of TGA of TiO2, initial loss in weight can be seen due to evaporation of water molecules around 100 °C and further due to  decomposition of  organic residue around 200 °C - 350 °C. In the composite of TiO2-MWCNTs, the early weight loss around 100 °C was because of water evaporation followed by decomposition of organic residue and combustion of MWCNTs around 200-350 °C and 550-650 °C respectively36.The thermal analysis suggests the stability of TiO2 - MWCNTs nanocomposite at calcination temperature of 400 °C. Further the MWCNTs present in composite are thermally more stable than pristine MWCNTs.

3.4 FTIR Spectroscopy

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 MWCNTs16. These surface groups are helpful to form interaction and chemical bonding between MWCNTs and TiO2. The FTIR spectrum of pristine (refer Fig.5(a)) and functionalised MWCNTs provides information of surface functional groups. As shown in Fig. 5(b) functionalised 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  groups37.

3.5 Photocatalytic Application

The photocatalytic activity of TiO2 and TiO2-MWCNTs was evaluated by studying the oxidation of methyl orange dye solution under UV light irradiation. Figures 6(a) and 6(b) shows UV - Visible absorbance spectral changes of methyl orange dye solution during the photocatalytic degradation in the presence of prepared TiO2 and prepared TiO2-MWCNTs nanocomposite. Obtained results show (refer Fig. 6(c)), two fold enhancement in the photocatalytic activity of  TiO2-MWCNTs nanocomposite as compared to  pure prepared TiO2 nanoparticles in only 45 minutes of UV irradiation.

Figure 4. TGA graph of the (a) MWCNTs (b) TiO2-MWCNTs (c) TiO2.

Figure 5. FTIR spectrum of (a) Pristine MWCNTs (b) functionalized MWCNTs.

Figure 6. Absorption spectra for the photo catalytic degradation of MO using catalyst (a) TiO2 (b) TiO2 - MWCNTs nanocomposite (c) C/C0 vs time for photodegradation of methy1orange dye solution (d) kinetics of photodegradation of MO by prepared TiO2 and TiO2-MWCNTs nanocomposite concentration of Mo=0.01mmols, catalyst dose=0.02g/50ml.

Degradation of methyl orange dye was found to follow the first order reaction kinetics (refer Fig. 6d). The obtained results show that the rate of degradation of Methyl Orange dye using TiO2-MWCNTs 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 facilitating the separation of the photo-generated electron-hole pairs at the TiO2-MWCNTs interface leading to the faster rates of photocatalytic oxidation19.

The COD data depicted in Table 1 shows substantial degradation and mineralisation of methyl orange dye solution when TiO2 - MWCNTs nanocomposite was used.

Table 1. The percentage reduction in COD.

Nanosized and interface integrated TiO2-MWCNTs composite was successfully synthesised using hydrothermal method and was further characterized by XRD, Raman spectroscopy, SEM, TEM, EDX, FTIR and TGA techniques. The enhanced photocatalytic efficiency of as synthesised TiO2 - MWCNTs 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 mineralisation when TiO2 -MWCNTs nanocomposite was used.

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Mr Kirti D. Shitole received her MSc in Analytical Chemistry in 2009 from Department of Chemistry, Pune University, India and currently pursuing her MPhil in Material Chemistry from the same University. Her research work involves synthesis, characterisation and photocatalytic applications of nanocomposites of metal oxides.

Mr Roshan K. Nainani has done MSc (Physical Chemistry) from Mumbai University and MPhil in Physical Chemistry from Department of Chemistry, Pune University, India. Presently, he is pursuing PhD in Physical Chemistry at Department of Chemistry, Pune University. His research interests are synthesis of metal oxide nanoparticles and their nanocomposite with graphene oxide and zeolites for applications in photocatalytic degradation of hazardous pollutants, photocatalytic hydrogen evolution etc.

Dr Pragati Thakur received her PhD from Laxminarayan Institute of Technology, Nagpur University in 2003. After that, she joined as a Lecturer in Physical Chemistry at Department of Chemistry, Pune University, Pune. Presently, she is working as an Associate Professor of Physical Chemistry in Pune University. Her areas of research include: Synthesis of semiconductor nanoparticles and their composites with carbon nanotubes, graphene oxide etc. for applications in energy and environment, heterogeneous photocatalysis, photocatalytic water splitting, industrial wastewater treatment, solar cells etc.