Abstract
Multiwalled carbon nanotubes (MWCNTs) were synthesized using arc discharge method at a magnetic field of 430 G and purified using HNO3/H2O2. Transmission electron micrographs revealed that MWCNTs had inner and outer diameter of ~2 nm and ~4 nm, respectively. Raman spectroscopy confirmed formation of MWCNTs showing G-band at 1577 cm−1. ZnFe2O4 and ZnFe2O4/MWCNT were produced using one step hydrothermal method. Powder X-ray diffraction (XRD) confirmed the formation of cubic spinel ZnFe2O4 as well as incorporation of MWCNT into ZnFe2O4. Visible light photocatalytic degradation of methylene blue (MB) was studied using pure ZnFe2O4 and ZnFe2O4/MWCNT. The results showed that ZnFe2O4/MWCNT composite had higher photocatalytic activity as compared to pure ZnFe2O4. After irradiation for 5 hours in the visible light, MB was almost 84% degraded in the presence of ZnFe2O4 photocatalyst, while 99% degradation was observed in case of ZnFe2O4/MWCNT composite. This enhancement in the photocatalytic activity of composite may be attributed to the inhibition of recombination of photogenerated charge carriers.
1. Introduction
Solar energy active photocatalysts have been a promising material for an environmental purification process. Efforts have been made to synthesize materials capable of utilizing solar spectrum for the photo degradation of industrial waste pollutants and dyes [1]. A wide use of dyes as redox indicators, biological stains, and pharmaceutical industries had adverse effects on gastrointestinal, genitourinary, and cardiovascular system of human body [2, 3]. Degradation of organic pollutants and dyes such as methylene blue, bromophenol blue, and Chicago sky blue from industrial waste water remains as a challenge because of low visible light photo catalytic activity of metal oxides and sulphides [4–6]. Large band gap of ZnS, TiO2, SrTiO3, and Ag3VO4 [7–9] prevents their functioning as effective catalyst for visible light photo degradation [10].
However, the advantage of ferrites as photo catalysts is their capability to absorb visible light solar irradiation because of low band gap and large availability of catalytic sites for adsorption of noxious waste and dyes [11]. Also, ferrites are magnetic [12–15] in nature giving them added advantage of easy recovery after photocatalytic reaction.
Albeit, the numerous literature has been reported on the photo degradation of different dyes using ZnFe2O4 as catalyst having spinel structure, where Zn2+ cations occupy tetrahedral site and Fe3+ enters octahedral site [16]. The band gap of ZnFe2O4 is small (1.9 eV) due to which it utilizes visible light to act as a photocatalyst [17]. Fan et al. [18] prepared nanocrystalline ZnFe2O4 by hydrothermal method and investigated photodegradation of acid orange II azo dye. The authors observed better photocatalytic activity as compared to bulk ZnFe2O4 sample prepared by conventional solid state method. The effect of sintering temperature on photodegradation of methyl orange was studied by Jadhav et al. [11] and observed maximum activity for samples sintered at 500°C. Su et al. [19] synthesized ZnFe2O4 by hydrothermal process and reported photodegradation of acid orange II dye within 2 hours. Li et al. [20] synthesized spinel zinc ferrite nanospheres of 212 nm and noticed enhanced photo catalytic activity of the ZnFe2O4 nanospheres as compared to ZnFe2O4 nanoparticles in the decomposition of rhodamine B. However, low quantum efficacy as a photo catalyst of the zinc ferrite has been improved by loading ZnFe2O4 to titanium dioxide [10]. Main drawback of ZnFe2O4/TiO2 composite is that it can absorb only UV light in predominance and only 50% of the dye degrades.
Therefore, the process of photo degradation requires absorption of visible light form solar or artificial sources. For the purpose of improving photo catalytic activity of the ferrites and to efficiently utilize visible light, a high aspect ratio and tubular nanometric dimensions of multiwalled carbon nanotubes (MWCNTs) make themselves a prospective candidate for photo catalytic activity [21, 22]. Decoration of CNTs surface by foreign materials especially metal oxides [23] extends their field of application and improves their chemical, electrical, mechanical, and thermal properties that can be used in advanced nanotechnology [24]. Magnetic CNT composites have been found to be a potential alternate in future for materials with low photo catalytic activity. Degradation of organic pollutants and toxic dyes has been an important aspect to study the photo catalytic efficiency of magnetic CNT composites [25].
Thus, our effort is to synthesize MWCNTs by arc discharge method. This method is advantageous over other previously reported methods because it is a low cost method which gives completely aligned multiwalled carbon nanotubes of high purity. Further, the surface of MWCNTs was decorated by ZnFe2O4. This was envisaged so as to efficiently design a green nanocomposite for the absorption of visible light from the spectrum. One step hydrothermal process is employed to produce ZnFe2O4 and ZnFe2O4/MWCNTs composite. Combination of two semiconductors, that is, ZnFe2O4 and MWCNT, would enhance the photo catalytic activity. Due to enhancement in photochemical activity of ZnFe2O4/MWCNTs composite, it is found to degrade almost 99% of the MB dye.
2. Experimental
2.1. Synthesis of MWCNTs
A metal-free arc discharge method was used to synthesize MWCNTs. An arc excitation between the graphite rods submerged in the deionized water was carried out by applying an AC voltage of 50 V to generate 100 A of current. The schematic diagram for the same is given in Figure 1. The electromagnets having soft iron core and copper wire winding around the core serve the purpose for the generation of 430 Gauss of magnetic field in a direction perpendicular to the arcing rods. The gap between the inter electrodes was maintained at a separation of ~0.5 mm for the generation of arc. A gap control screw unit was used for the movement of graphite electrodes. Carbon soot was produced by discharging graphite electrodes for 1 hr, and liquid turned black which was separated by decantation. This product formed was washed with de-ionized water 3-4 times followed by refluxing with 8 M aqueous HNO3 for 24 hrs. The sample was then refluxed again in 30% H2O2 as done by Feng et al. [26] for 24 hrs for further removal of impurities left. pH was then neutralized by washing the sample with de-ionized water. The sample was air dried and annealed at 400°C for 30 minutes.
2.2. Synthesis of ZnFe2O4/MWCNT Nanocomposite
A variety of methods are available to synthesize ferrites [27–31], but here ZnFe2O4 and ZnFe2O4/MWCNT were prepared by hydrothermal process [32]. The starting materials, that is, ferric nitrate, zinc nitrate, and sodium hydroxide, used for the preparation were of analytical grade. 10 mmol of ferric nitrate and 5 mmol of zinc nitrate were dissolved in minimum amount of water. 10 mg of prepared MWCNT was added into the solution and sonicated for 10 minutes. The pH of the solution was set to 12-13 with the help of 6 M sodium hydroxide. The mixture was vigorously stirred for 30 minutes and transferred into Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180°C for 12 hours. The product was allowed to cool to room temperature, filtered, and washed with distilled water followed by drying in vacuum oven for 12 hours. The same procedure was applied without adding MWCNT to get pure ZnFe2O4.
2.3. Photocatalytic Activity
The photocatalytic activity of ZnFe2O4 and ZnFe2O4/MWCNT was evaluated by degradation of aqueous solution of methylene blue (MB) under visible light irradiation. 50 mg of catalyst was added to 50 mL of 10 mg/L aqueous solution of MB. Before illumination, the mixture was stirred in dark for 30 min to achieve adsorption desorption equilibrium between catalyst and dye solution. The solution was exposed to visible light under stirring after addition of 2 mL of 30% H2O2. At given time intervals, 3 mL of aliquots was withdrawn and centrifuged to remove catalyst. The concentration of methylene blue in aqueous solution was determined with the help of UV-Vis spectrophotometer.
2.4. Physical Measurements
Transmission electron micrographs (TEM) were recorded using Hitachi (H7500) Transmission Electron microscope, operated at 120 kV. Raman data were recorded using Invia Raman Microscope (Renishaw) at room temperature with argon ion laser excitation 514 nm (5 mW power) over a range of 0–3200 cm−1. X-ray diffractographs of the prepared samples were recorded using an X-ray diffractometer (model PANalytical’s X’pert PRO spectrophotometer) with Cu-Kα radiation. Photoirradiation was carried out using 160 W Hg lamp. The concentration of methylene blue during the degradation was monitored by UV-visible spectrophotometer JASCO, V-530.
3. Results and Discussions
3.1. TEM Characterization
The morphology of the MWCNTs was studied using TEM analysis. The sample was agitated ultrasonically in ethanol for 20 minutes to avoid aggregation of the sample. Nanotubes thus obtained had inner and outer diameter of ~2 nm and ~4 nm. The corresponding wall thickness of ~2 nm can be attributed to the formation of approximately seven walled CNTs (interlayer distances between graphene sheets being 0.33 nm). The TEM micrographs of MWCNTs, after refluxing using HNO3/H2O2 are shown in Figure 3 which indicated the formation of pure nanotubes. CNTs obtained after purification has cleaner surface without any amorphous carbon impurities. TEM image also revealed that there is no effect on the shape and surface morphology of MWCNTs. Hence, the obtained nanotubes were further used for the formation of nanocomposite.
3.2. Raman Spectroscopy
Raman spectroscopy revealed invaluable insights into the purification of nanotubes. G-band (1577 cm−1) corresponded to the confirmation of MWCNTs. Defect induced D-band (1355 cm−1) was minimized after purifying CNTs with HNO3/H2O2 for 24 hrs as shown in Figure 4. D-band arises due presence of sp3 hybrid carbon, whereas G-band is due to existence sp2 hybrid carbon as in the case of MWCNTs. Intensity of D-Band is smaller when 430 Gauss of magnetic field was generated perpendicular to arcing rods. Occurrence of G-Band at 1581 cm−1 also confirmed that semiconducting MWCNTs were produced [33]. Large intensity of G′-Band at ~2700 cm−1 indicated the reduction of amorphous carbon [34]. These results were also supported by transmission electron microscopy (TEM). Decrease in the intensity of D-Band and increase in the intensity of G′-Band clearly confirmed the removal of amorphous carbon impurities and carbon lamellas from the MWCNT sample.
3.3. X-Ray Diffraction Studies
The phase composition of all the synthesized samples was investigated using powder X-ray diffraction (XRD) technique. XRD pattern of ZnFe2O4/MWCNT nanocomposite, pure ZnFe2O4, and MWCNT is presented in Figure 2. A strong diffraction peak at 2θ = 26° can be indexed to (002) plane of MWCNT (Figure 2(a)) corresponding to the P63/mmc space group. No peak corresponding to any other metal impurity was observed for purified sample. The lattice parameters were determined using the Powley and Le Bail refinement method, and the values of “a” and “c” were 2.45 Ǻ and 6.75 Ǻ, respectively, corresponding to the hexagonal structure of graphite [35]. The diffraction peaks of pure ZnFe2O4 (Figure 2(b)) confirmed formation of cubic spinel structure with Fd-3m space group. The peaks indexed to (220), (311), (400), (511), and (400) planes correspond to cubic spinel structure. The lattice parameter was calculated using the Powley refinement method and was found to be 8.46 Å. Based on the following Debye-Scherrer relation [36], crystallite size was found to be 10 nm. where is the diameter of the particle, is the full width of half maximum (FWHM), is the X-ray wavelength (1.542 Ǻ), and is the angle of diffraction. In case of ZnFe2O4/MWCNT composite (Figure 2(c)), besides the diffraction peaks of ZnFe2O4, a new peak at was also observed indicating coexistence of ZnFe2O4 and MWCNT in the composite. However, the intensity of the peak is low due to addition of very small amount of MWCNT.
3.4. Photocatalytic Activity
The photocatalytic activities of ZnFe2O4 and ZnFe2O4/MWCNT nanocomposite were evaluated by degradation of methylene blue (MB) solution. Figure 5 shows degradation of MB in the presence of ZnFe2O4/MWCNT nanocomposite under visible light irradiation. The UV-visible spectra of MB solution show four characteristic peaks at 246, 292, 617, and 663 nm. Addition of H2O2 masks peaks at 246 and 292 nm, while the peaks at 617 and 663 nm regularly decrease in intensity with increasing irradiation time. The peak at 664 nm disappeared completely in 5 hours indicating complete degradation (~99%) of MB dye. The photocatalytic degradation was calculated by applying following [37]: where is initial MB concentration and is the concentration of MB at time . For comparison, the degradation of MB was performed using pure ZnFe2O4. Photocatalytic degradation efficiency of MB in the presence of ZnFe2O4 and ZnFe2O4/MWCNT nanocomposite is shown in Figure 6. It was observed that after irradiation for 5 hours, about 84% of MB was degraded in the presence of ZnFe2O4 photocatalyst, while 99% degradation was observed in case of composite. Therefore, it can be concluded that ZnFe2O4/MWCNT nanocomposite exhibits stronger photocatalytic activity as compared to pure ferrite. This may be due to stronger interaction between ZnFe2O4 and MWCNT which inhibited recombination of photogenerated charge carriers.
4. Conclusion
An economical arc discharge method was used to synthesize multiwalled carbon nanotubes. Predominance of MWCNTs having diameter of around 2–4 nm in the prepared sample was clearly shown by TEM image. Oxidizing agent HNO3/H2O2 was used to purify the MWCNTs without affecting the surface of nanotubes. Raman spectra show the peak intensity of D- and G′-bands which clearly revealed the purity of as-synthesized carbon nanotubes. Decrease in the intensity of defect induced D-band at 1350 cm−1 and increase in the intensity of G′-band at 2700 cm−1 indicating the removal of amorphous carbon. Further synthesis of ZnFe2O4 and ZnFe2O4/MWCNT composite was successfully carried out using one step hydrothermal method. Formation of composite was confirmed using XRD. The peaks indexed to (220), (311), (400), (511), and (400) planes correspond to cubic spinel structure along with a very small peak of CNTs. A comparative photocatalytic study of ZnFe2O4 and ZnFe2O4/MWCNT nanocomposite clearly indicated the enhancement of photocatalytic activity in ZnFe2O4/MWCNT nanohybrid. MB was almost 84% degraded in the presence of ZnFe2O4 photocatalyst, while 99% degradation was observed in case of ZnFe2O4/MWCNT composite after irradiation for 5 hours in the visible light. MWCNTs significantly enhance photocatalytic activity of ZnFe2O4 as strong interaction between ZnFe2O4 and MWCNT inhibits recombination of photo generated carriers.
Acknowledgment
The authors are highly grateful to the Council of Scientific and Industrial Research (CSIR) for providing the necessary financial support.