Photocatalytic Activity of Sol-Gel Electrospun Co<sub>3</sub>O<sub>4</sub> Nanofibers in Degrading Methylene Blue and Methyl Orange

Research Article

Ann Materials Sci Eng. 2015; 2(2): 1025.

Photocatalytic Activity of Sol-Gel Electrospun Co3O4 Nanofibers in Degrading Methylene Blue and Methyl Orange

George G and Anandhan S*

Department of Metallurgical and Materials Engineering, National Institute of Technology-Karnataka, India

*Corresponding author: Anandhan Srinivasan, Department of Metallurgical and Materials Engineering, National Institute of Technology-Karnataka, Srinivas Nagar, Mangaluru-575025, India

Received: October 28, 2015; Accepted: December 20, 2015; Published: December 23, 2015


In this work, Co3O4 ceramic oxide nanofibers were fabricated from solgel electrospun poly(styrene-co-acrylonitrile)/cobalt acetate tetrahydrate precursor composite fibers by calcination. The photocatalytic properties of Co3O4 nanofibers obtained at different calcination temperatures were studied using methylene blue and methyl orange as the model organic pollutants. The degradation of dyes occurred in the presence of an oxidative medium and the maximum degradation was observed for the fibers obtained at the lowest calcination temperature, which had a narrow band gap and smaller grains. The morphological and compositional features of the Co3O4 nanofibers obtained after calcination were studied by scanning electron microscopy and Fourier transform infrared spectroscopy. The results of X-ray diffraction study and Raman spectra revealed that the average grain size composing the fibers increased with the calcination temperature. A clear evidence of defects in the fibers was observed in ultraviolet-visible-near infrared (UV-Vis-NIR) and energy dispersive spectroscopic measurements.

Keywords: Electrospinning; Co3O4; Nanofiber; Photocatalysis


Oxide nanomaterials have been widely used as photocatalysts for hydrogen evolution [1], methanol reduction, degradation of organic pollutants [2-6], antimicrobial activities [7], HCHO conversion [8] etc. ZnO, NiO, NbO, TiO2 etc. and oxide nanomaterials doped with noble metals such as Ag, Au and Pt are some examples for such photocatalysts. The inertness or stability towards hazardous chemicals and high surface area of the metal oxide nanomaterials make them ideal materials for catalytic reactions. Most of the metal oxides are semiconducting in nature; therefore, the catalytic activity of these materials is triggered by excitation energy, which can excite the electrons from the valance band to the conduction band. These excited electrons act as initiators for the whole degradation process. Different regions of the visible-sun light are used as excitation energy, which are namely UV-A, UV-B, UV-C, visible white light and infrared. It has been proven that the nanofibrous oxides are strong candidates for solid state [9-11] and electrochemical sensing [12], where the catalyzed reaction between the target analytes and the oxide, and the electron transport are important. The high aspect ratio nanofibers are well suited for catalysis as well. Electrospinning assisted sol-gel processing is a widely accepted technique for the production of oxide nanofibers.

Many industrial effluents are carcinogenic and synthetic dyes are the most common among them. A wide range of physical, chemical and biological methods have been developed [13-15] for the purification of wastewater containing dyes. Chemical oxidation methods using chlorine, hydrogen and ozone as oxidants were widely employed for the purification purpose. However, the chemical method does not result in the complete neutralization of the organic pollutants [16]. Catalytic wet air oxidation and several other methods have been developed for removal of dissolved toxic organic pollutants from wastewaters; however, most of them failed due to either their inefficiency or due to an increased cost of treatment [17].

Heterogeneous catalytic reactions are successful in the complete oxidation of a variety of hazardous pollutants in waste water effectively and economically. A suitable selection of catalyst and reaction conditions can lead to a high selectivity towards environmentally harmful products under mild conditions. The presence of a good chemical oxidant in the medium can bring out non-catalytic chemical oxidation of the substrate in the bulk solution in addition to the heterogeneous catalytic processes taking place on the catalyst surface. Heterogeneous catalytic processes start by the adsorption of the organic pollutants on to the surface of the catalyst. As a whole, three simultaneous processes take place in a photocatalytic reaction, i.e., adsorption of organic dye on to the catalyst surface, excitation of catalyst and chemical oxidation of dye in the presence of chemical oxidant.

In this study, Co3O4 nanofibers were fabricated in three steps. In the first step, a spinnable sol of poly(styrene-co-acrylonitrile) (SAN)/ cobalt acetate tetrahydrate (CATH) was prepared. In the second step, the precursor composite fibers of SAN/CATH were fabricated through electrospinning process. These fibers were calcined above the degradation temperatures of SAN and CATH in the final step. Fine structural features are expected for the fabricated Co3O4 nanofibers as the interaction between the nitrile group of SAN and the carbonyl group of cobalt acetate can ensure the uniform mixing of metal salt in the polymer solution [18]. The firm styrene side groups of SAN can act as a truss to transform the morphology of SAN/CATH composite fibers to Co3O4 nanofibers. The morphological and spectral characteristics of Co3O4 nanofibers were studied by different techniques. The photo-catalytic property of Co3O4 nanofibers were evaluated by investigating the degradation of two organic dyes, Methylene Blue (MB), a cationic dye, and Methyl Orange (MO), an anionic dye, in aqueous medium in the presence of hydrogen peroxide (H2O2) as a chemical oxidant under UV illumination.

Materials and Methods

SAN [Grade: Santron IMS 1000, acrylonitrile content: 30%, specific gravity: 1.07, Mv : 2.46×106 (viscometry), MFI: 35 g/10 min at 220°C under a load of 10 kg, ASTM D-1238] purchased from Bhansali Engineering Polymers, Rajasthan, India, cobalt (II) acetate tetrahydrate (Co(OCOCH3)2.4H2O, assay 98%) (CATH) procured from High Purity Laboratory Chemicals, Mumbai, India and N, N-dimethylformamide (DMF) purchased from Sisco Research Laboratories Pvt. Ltd, Mumbai, India, were used without further purification to fabricate the precursor composite fibers. Hydrogen peroxide (H2O2) (molarity 34.1 g/mol) procured from Merck, Bangalore, India, methylene blue (MB) procured from Nice chemicals, Cochin, India and methyl orange (MO) purchased from BDH chemicals, Bombay, India were used for the degradation studies.

The precursor composite fibers were fabricated using electrospinning technique (E-Spin Nano, Physics Equipments Co., Chennai, India) using the sol containing 20 wt % SAN and CATH, respectively in DMF solution. The proportion by weight of SAN to CATH in the spinnable sol was 1:1 in this study. The optimal electrospinning conditions were determined, which correspond to minimum fiber diameter and a maximum yield. To do so, the applied voltage was varied as 15, 17, 20 and 22 kV, the tip to collector distance was 17 cm and the solution flow rate was 1000 μL h-1.

The obtained SAN/CATH composite nanofibers were calcined in a programmable high temperature furnace (Indfur, Chennai, India) at a heating rate of 4 K.min-1 and the calcination temperature was varied as 773, 873 and 973 K, the dwell period was 2 hours in all cases. The minimum calcination temperature was based on the degradation temperature of the SAN/CATH composite fibers during their thermogravimetric analysis (TGA) (EXSTAR 6000 TG/DTA 6300, Japan) in a nitrogen atmosphere at a heating rate of 10 K min-1.

The morphological features of the fibers were examined by scanning electron microscope (SEM) (JEOL JSM-6380LA, Japan) and transmission electron microscope (TEM) (S-5500, Hitachi, Japan). The complete elimination of the organic phases were confirmed using Fourier transform infrared (FTIR) spectra (Jasco FTIR-4200, Japan), X-ray diffraction patterns (XRD) (JEOL X-ray diffractometer, DXGE- 2P, Japan) and Raman spectra (inVia, Renishaw, UK). The band gap energies were estimated using UV-Vis-NIR spectra (Varian, Cary 5000 UV-Vis-NIR, USA) of the ceramic fibers. The specific surface areas of the Co3O4 nanofibres were measured by the BET method (Smart sorb 92/93, Smart Instruments Co. Pvt. Ltd, Dombivli, India).

The photocatalytic activity of Co3O4 nanofibers on the degradation of MB and MO in the presence of UV light were carried out in a photocatalytic reactor chamber equipped with four 8 Watt UV lamps (Philips, 8W, TUV, UV-C). 50 mg of Co3O4 nanofibers were added to 100 mL solution of 80 mM H2O2 containing the dye (10 μM) in distilled water. Prior to UV illumination, the suspension was vigorously stirred by a magnetic stirrer in dark for 30 min to achieve an adsorption/desorption equilibrium, and the adsorption of the dye on the Co3O4 fibers was calculated. At specific time intervals, about 3 mL of the suspension were taken from the reaction chamber and analyzed with UV spectroscopy to study the degradation of the dye.

During UV exposure, stirring was maintained to keep the suspension homogenous. The concentration of dye in each degraded sample was determined using a UV spectrophotometer (Ocean optics USB-4000, USA) at a λmax of 463 nm in the case of MO and 663 nm for MB. A calibration curve was obtained by plotting absorbance at λmax vs. concentration (Figure 1). The percentage of degradation of MB and MO was obtained at different time intervals. The percentage photodegradation is given by equation 1.