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Sonochemical Facile Synthesis of Self-Assembled Poly(o‑phenylenediamine)/Cobalt Ferrite Nanohybrid with Enhanced Photocatalytic Activity Ufana Riaz,* Syed Marghoob Ashraf,† Rameez Raza, Kanika Kohli, and Jyoti Kashyap Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia (A Central University), New Delhi-110025, India S Supporting Information *

ABSTRACT: Although several photocatalytic materials have been discovered, developing an efficient visible light photocatalyst is still a great challenge in the photocatalysis field, because the existing inorganic semiconductors are only active in the ultraviolet (UV) range. With the aim to enhance the photocatalytic activity of CoFe2O4 in the visible region, the present preliminary study reports the synthesis of nanocomposites of CoFe2O4/poly(o-phenylenediamine) nanohybrid. The structure of the synthesized nanohybrid was confirmed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and transmission electron microscopy (TEM) analyses. Photocatalytic activity of the nanohybrid was investigated using Malachite Green (MG) as a model dye. Results revealed that enhanced photocatalytic activity was achieved in the visible range, because of the generation of holes in the narrow band gap of CoFe2O4 via sensitization with poly(o-phenylenediamine). A plausible pathway and mechanism for the photocatalytic degradation of Malachite Green (MG) is also discussed.

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INTRODUCTION Degradation of dyes via catalytic techniques such as microwave, photocatalysis, and sonophotocatalysis have been extensively reported as eco-friendly techniques for environmental remediation.1−4 However, the widely used semiconductor photocatalysts such as ZnO and TiO2 are only active in the UV range. Hence, the development of visible-light-driven photocatalysts has attracted remarkable attention. Although dye sensitization or doping of other elements makes the utilization of visible light possible, dopants usually act as recombination centers for the photogenerated electrons and holes.5−7 Therefore, research is now focused on the design and development of new single-phase effective photocatalysts under visible light irradiation. Among the newly developed photocatalysts, spinel ferrite structures (MFe2O4) (M = Mn, Cu, Co, Mg, Co, Ni, Fe) have been intensively investigated, because of their ease of preparation.8−11 CoFe2O4 nanoparticles prepared by sonochemical coprecipitation method have been reported to possess remarkably good structural, magnetic, and optical properties, which can help in developing cost-effective visible-light-driven photocatalysts for the degradation of environmental pollutants. © XXXX American Chemical Society

Lately, several studies have also been reported related to the novel process of hybridizing these spinels with delocalized conjugated materials such as carbon nanotubes,12 fullerene,13 graphene,14 and conducting polymers15,16 to enhance their photocatalytic activity in the visible region. Conjugated polymers with an extended electron system hold immense potential of photocatalytic activity, because of high absorption coefficients in the visible-light range and high mobility of charge carriers.17 Among the conducting polymers, polyaniline (PANI) has been receiving increased attention, because of its advantages of low cost and excellent environmental stability.18 Recently, efforts have been made to synthesize visible-lightdriven photocatalysts based on PANI nanocomposites such as PANI−TiO2,19,20 PANI−ZnO,21,22 and PANI−CdS/CdO.23,24 It has been established that a hybrid effect exists between semiconductors and PANI that causes a high separation efficiency of photogenerated electron−hole pairs, resulting Received: December 9, 2015 Revised: March 24, 2016 Accepted: May 11, 2016

A

DOI: 10.1021/acs.iecr.5b04596 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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dried and kept overnight in a vacuum oven at 100 °C. The assynthesized mixed hydroxide was sintered in a muffle furnace at 700 °C for 5 h to obtain CoFe2O4. After sintering, the obtained powder was ground using a mortar and pestle and dried under vacuum. The color of the synthesized CoFe2O4 was observed to be brown. Chemical Oxidative Polymerization of (o-Phenylenediamine). o-Phenylenediamine (0.35 g, 3.2 × 10−3 mol) was added to a 100 mL Erlenmeyer flask containing methanol and water in a 1:1 ratio (25 mL of each). Potasssium dichromate (1 g, 3.3 × 10−3 mol) was added to the reaction mixture, keeping the monomer:oxidant ratio at 1:1.133 and giving a dark brown solution. The flask was then kept on a magnetic stirrier that was fitted with a nitrogen inlet and maintained at a temperature of 10°−12 °C. The reaction was performed for 2 h. The color changed from cream to green. The synthesized poly(o-phenylenediamine) was then removed from the flask and washed several times with distilled water on a Buchner funnel. The removal of iron was confirmed by testing the filtrate with potassium ferrocyanide. Poly(o-phenylenediamine) was then dried in a vacuum oven for 72 h at 70 °C, to ensure complete removal of water and impurities. Yield: 88%. Ultrasonic Synthesis of CoFe2O4/Poly(o-phenylenediamine) Nanocomposites via In Situ Polymerization of oPhenylenediamine. CoFe2O4/poly(o-phenylenediamine) nanocomposites were synthesized using varying weight ratios of o-phenylenediamine (OPD) monomer, i.e., 1:0.25, 1:0.5, and 1:1 (presented later in this work). CoFe2O4 (1 g) and OPD (1 g) were mixed in distilled water and the reaction mixture was kept for sonication at 35 °C for 2.5 h. Initiator FeCl3 (monomer:oxidant ratio of 1:1) was dissolved in 20 mL of distilled water and added to the reaction mixture, dropwise, for a period of 30 min. The color of the solution changed from offwhite to green, indicating polymerization of POPD. The reaction was further continued for 3 h and the obtained nanocomposites were then purified to remove excess of ferric chloride. The presence of ferric chloride was checked using potassium ferrocyanide solution. The purified nanocomposites were dried in an oven for 72 h to ensure the complete removal of water. The nanocomposites were designated as CoFe2O4/ POPD 1:0.25, 1:0.5, and 1:1, according to the loading of OPD in the nanocomposites. Characterization. FT-IR spectra of nanocomposites were taken on FT-IR spectrophotometer (Shimadzu, Model IRA Affinity-1), in the form of KBR pellets. Ultraviolet−visible light (UV-vis) spectra were taken, using a UV-vis spectrophotometer (Shimadzu, Model UV-1800) using NMP as the solvent. XRD patterns of the nanocomposites were recorded on a powder diffractometer (Philips, Model PW 3710) (using a nickelfiltered Cu Kα radiation). Peak parameters were analyzed using Origin 6.1 software. TEM micrographs (TEM) were obtained using a Morgagni Model 268-D TEM system (FEI, USA). The samples were prepared by depositing an aqueous drop onto a carbon-coated copper grid, subsequently drying in air before transferring it to the microscope, operated at an accelerated voltage of 120 kV. Liquid chromatography−mass spectroscopy (LC-MS) was conducted using a Finnigan LCQ ion trap mass spectrometer equipped with an electro spray ionization (ESI) interface source and operated in negative polarity mode fitted with a Genesis, C-18 column (4.6 mm × 250 mm) containing 4 μm packed particles (Alltech, Deerfield, Germany). Acetonitrile and 0.03 M ammonium carbonate buffer (pH 7.7) were used as eluents. The pump program was set as follows: isocratic 20%

enhanced photocatalytic activity.25,26 Poly(o-phenylenediamine) (POPD), which is a polyaniline derivative containing quinoraline units, with strong electro activity and high environmental stability, is receiving considerable attention, since it can exhibit unique electroactive properties when combined with metal/semiconducting nanoparticles.27,28 Recently, Sundaram et al.29 have reported TiO2@PoPD core− shell nanocomposites for the degradation of Rhodamine B dye. It was reported that the combination of the synergetic and complementary behaviors of PoPD with TiO2 nanoparticles efficiently degraded Rhodamine B dye. To the best of our knowledge, no work has not been published on the synthesis and photocatalytic activity of CoFe2O4:POPD core−shell nanocomposites. In this paper, nanosized CoFe2O4:POPD nanocomposites have been synthesized via ultrasonic-assisted synthesis, because it affords a reliable, inexpensive, and promising technique to prepare nanocomposites at ambient conditions with chemical homogeneity achieved through atomic level mixing. The formation of the nanocomposite was confirmed by the spectral and morphological analyses. Malachite Green (MG) was chosen as a model dye for performing the degradation studies, because it is widely used as a biocide by the aquaculture industry to treat fungal and protozoal infections and as an additive by the food industry.30,31 Apart from this, it is also commonly used by the textile industry.32,33 Despite its extensive use, MG has always been a controversial dye, because of its toxic nature, as well as being a strong colorant to water streams, a potent carcinogen, and a source of respiratory infections.34 Hence, the treatment of waste waters containing MG dye is an essential concern for living beings as well as the environment. The degradation kinetics was studied to analyze the photocatalytic efficiency of these nanoparticles under ultraviolet (UV) and visible light. The exploration of the optimized parameters for the degradation of the dyes using nanosized CoFe2O4:POPD as a catalyst was also carried out to assess the effect of each parameter on the photocatalytic activity.

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EXPERIMENTAL SECTION Materials. Cobalt chloride (CoCl2·6H2O, Merck India) (molar mass = 237 g/mol, density = 1.92 g/cm3 (at 25 °C), solubility = 191 g/L (100 °C)), ferric chloride (FeCl3·6H2O, Merck India) (molar mass = 198.83 g/mol, density = 1.93 g/ cm3 (at 25 °C), solubility = 1600 g/L (at 10 °C)), ophenylenediamine (C6H8N2, Sigma−Aldrich, USA) (molar mass = 108 g/mol, density = 1.031 g/cm3, soluble in hot water), potassium dichromate (K2Cr2O7) (Merck India) (molar mass = 294.15 g/mol; density = 2.68 g/cm3 (at 25 °C)), and naphthalene sulfonic acid (NSA) (C10H8O3S, Sigma−Aldrich, USA) (molar mass = 208.24 g/mol) were used without further purification. N-methyl-2-pyrrolidone (NMP) (Merck India) and acetonitrile (Merck India) were also used without further purification. Synthesis of CoFe2O4. Nanosized magnetic CoFe2O4 was synthesized by emulsion route using CoCl2·6H2O and FeCl3· 6H2O in the weight ratio of 1:2. The solution of the former was mixed with NSA surfactant (0.5 g in 50 mL of H2O). Aqueous ammonia solution then was added into the mixture dropwise at 50 °C until a dark slurry was formed. The reaction mixture was stirred vigorously for 3h at 80 °C. The obtained hydroxide of cobalt and iron was then centrifuged and washed with ethanol and water to remove the excess surfactant. The sample was then B

DOI: 10.1021/acs.iecr.5b04596 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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band is characteristic of the BO6 vibration, where B denotes M3+ in the octahedral position (Oh) of a spinel structure.35 The IR spectrum of POPD (Figure 1, inset) showed NH stretching vibration peaks at 3406 for a secondary amine. The peaks at ∼1671−1631 cm−1 were assigned to the imine stretching vibrations of quinonoid and benzenoid forms of OPD units. The bands at 877, 800, and 690 cm−1 were characteristic of C− H out-of-plane bending vibrations of benzene nuclei in the phenazine skeleton. The presence of the above peaks confirmed the polymerization of poly(o-phenylenediamine). The IR spectrum of 1:0.25 CoFe2O4:POPD (Figure 1) shows a NH stretching peak centered at 3406 cm−1. An intense peak was observed at 1618 cm−1, while broad peaks were observed at 668 and 558 cm−1, indicating the strong interaction of the M−O bond with POPD. The change in the band’s position clearly indicated that the − NH group of POPD interacts with Fe2+ ions of the CoFe2O4. The characteristic peaks of the CoFe2O4 appeared to be suppressed, even at a lower loading of POPD, presumably due to the encapsulation of the nanoparticles of CoFe2O4 by POPD. However, upon increasing the loading of POPD, in the case of 1:0.5 CoFe2O4:POPD (Figure 1), it was observed that the peaks associated with POPD appeared at 1737 and 1433 cm−1, which were correlated to the imine stretching vibrations, while the lower bands corresponding to the M−O bond of CoFe2O4 were highly suppressed because of encapsulation by POPD chains. In the case of 1:1 CoFe2O4:POPD (Figure 1), an intense broad band was observed at 3408 cm−1, revealing the interaction between NH of POPD with OH of cobalt ferrite. It can thus be concluded that, at lower loadings of POPD, encapsulation of CoFe2O4 occurs, whereas, at higher loadings, an intense interaction between the two moieties is observed. The integrated areas of the peaks corresponding to NH of POPD and the M−O bond of the spinel in the nanocomposite are given in Table 1. It was observed that, as the loading of POPD increased in the nanocomposite, the M−O bond area decreased and that of the NH, as well as benzenoid and quinonoid of POPD, increased, which clearly confirmed the encapsulation of cobalt ferrite by POPD. UV-vis Studies. The UV-vis spectrum of pure CoFe2O4 (Figure 2a) was identified by bands at λmax = 216 and 254 nm, which were ascribed to Fe3+←O charge transfer of isolated Fe ions in tetrahedral (Td) and octahedral (Oh) coordination, respectively.36 Moreover, the absorption region between 200 nm and 300 nm was also assigned to the charge transfer between oxygen and Co2+ ions in Td symmetry.37 An absorption band at 588 nm and a shoulder at 625 nm can be attributed to the presence of CoFe2O4 nanoparticles, where Co2+ ions are in Td symmetry (588 nm) and Oh symmetry (shoulder at 625 nm), in coordination with Fe3+ ions.38 Moreover, the peak at 640 nm is also attributed to the presence of Co3+ in the Td position. Similar observations are made by Zacharaki et al.39 with cobalt spinel and modified cobalt spinel by XRD. The UV-vis analysis suggests that a mixture of ferrites (MFe2O4, where M is Co2+) is present as nanoparticles in two structures: (a) normal spinel ferrite, where Fe3+ ions are in the Oh position and M ions are in the Td position, (b) inverse spinel ferrite with Fe3+ in the Td and Oh positions, and M in the Oh position. The spectrum of pure POPD (Figure 2b) exhibited strong UV-vis absorption bands at 250 and 426 nm in NMP solution, which were assigned to the π−π* transition, resulting from the conjugation between the aromatic rings and the N atom, and an

acetonitrile: 80% buffer held for 2 min; grading to 100% acetonitrile over 10 min and held at 100% for 7 min. The diode array detector allowed for concomitant recording of spectra from 200 nm to 600 nm. The gradient HPLC separation was coupled with a ion trap mass spectrometer (Agilent Technologies, Model LC/MSD trap 6310). Photocatalytic Degradation of Malachite Green (MG) Dye Solution. A stock solution of MG dye of with a concentration of 200 mg/L was prepared and labeled as MG200; 200 mg of the nanohybrid was then added to 150 mL of this solution and kept in darkness for ∼1 h to establish equilibrium. The photocatalytic reactor lamp was then switched on to initiate the photocatalytic degradation reaction. During irradiation, the suspension was stirred continuously to keep the suspension homogeneous. Aliquots of the degraded sample were taken at 10 min and were centrifuged at a high speed (5000 rpm) using a centrifuge machine (REMI, Model R8C). Supernatent solutions were taken out and collected in clean dried conical flasks to determine the concentration of degraded dye. The dye concentration was monitored using a UV-vis spectrophotometer model (Shimadzu, Model UV-1800) at the λmax of MG dye (650 nm). A calibration graph based on Beer−Lambert’s law was obtained by plotting the absorbance against the concentration of dye in solution to determine the quantitiy of the dye degraded after different intervals of time. The removal percentage was calculated using the equation percent removal (%) =

C i − Cf × 100 Ci

where Ci is the initial concentration (mg/L) of dye taken and Cf is the concentration (mg/L) obtained after time t (in minutes).

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RESULTS AND DISCUSSION FTIR Analysis. The IR spectrum of CoFe2O4 (Figure 1) showed a peak centered at 3406 cm−1, indicating the presence

Figure 1. FTIR spectra of POPD, CoFe2O4, and CoFe2O4:POPD nanocomposites.

of a −OH stretching vibration while the OH deformation mode appeared between 1400−1630 cm−1. The presence of peaks at 668 and 558 cm−1 was attributed to the vibrations of the M−O bond, such as that of A−O−B spinel. The former band was attributed to the ABO3 vibration in the spinel lattice, where A denotes M2+ in the tetrahedral position (Td), and the second C

DOI: 10.1021/acs.iecr.5b04596 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. ∫ a dν̅ Values of CoFe2O4, POPD, and Its Nanocomposites ∫ a dν̅ sample cobalt ferrite POPD CoFe2O4:POPD 1:0.25 1:0.5 1:1

peak area, M−O bond (668 cm−1)

NH (3406 cm−1)

quinonoid (1529 cm−1)

benzenoid (1470 cm−1)

480

244.45

171.54

301 317 394

107 118 129

115.77 119.00 123.00

12.63

9.65 6.70 5.47

Table 2. ∫ a dν̅ Values of CoFe2O4 and Its Nanocomposites nanocomposite POPD CoFe2O4:POPD 1:0.25 1:0.5 1:1

2.5

∫ a dν̅ 2020

0.4 0.75 1.5

181 397 1094

absorbance at 426 nm

Figure 2. UV-vis spectra of (a) CoFe2O4 and (b) POPD and CoFe2O4:POPD nanocomposites.

intense band at 426 nm in the violet-to-blue region, assigned to a polaronic transition. Interestingly, in the nanocomposite, the polaronic peak showed a blue shift of 10 nm, which was attributed to the interaction between the POPD and CoFe2O4 nanoparticles (Figure 2b), that caused a shift in the polaronic transition of POPD. CoFe2O4 core−shell nanocomposites showed broad and stronger absorption than CoFe2O4 under the entire range of visible light (Table 2), because of the sensitizing effect of POPD. XRD Analysis. The XRD of POPD (Figure 3a) revealed several low-angle peaks, with maximum intensity observed at 2θ = 9.45°. The presence of sharp and intense peaks revealed a highly ordered and crystalline structure of POPD. The peaks with hkl values of (220) (first peak), (311) (third peak), (222) (fourth peak), (400) (fifth peak), (422) (sixth peak), and (511) (seventh peak) showed the cobalt ferrite phase while the second peak, (104), exhibited ferrite phase (Figure 3b, inset).

Figure 3. XRD of (a) POPD and (b) CoFe2O4:POPD nanocomposites.

The peaks in the diffractogram were in agreement with the expected sample peaks for cobalt ferrite, as given in the JCPDS database and relevant literature.40 The peaks were indexed for the (220), (311), and (400) diffraction planes, which confirmed the presence of a single CoFe2O4 phase instead of mixed CoO and Fe2O3 phases. The peak intensity of the (311) was relatively higher, because of a sintering effect that increased the crystallinity as well as the purity. The crystallite size (d) was calculated using the Scherrer equation, d= D

0.9λ β cos θ DOI: 10.1021/acs.iecr.5b04596 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research where β is the full width at half maximum and λ is the wavelength of Cu Kα radiation. The average crystallite size of CoFe2O4 was determined to be 15.3 nm, Table 3. In the case of Table 3. XRD Analysis of CoFe2O4 and Its Nanocomposites nanocomposite CoFe2O4 POPD CoFe2O4:POPD 1:0.25 1:0.5 1:1

peak (2θ)

area

height (a.u.)

fwhm (2θ)

crystallite size (Å)

35 9.45

258 343

225 1500

0.22 0.20

15.3 14.0

33.22 33.50 33.1

733 820 848

1200 1050 925

0.32 0.42 0.52

13.0 7.90 6.36

CoFe2O4:POPD 1:0.25 (Figure 3b), only the peaks related to CoFe2O4 were observed and were found to be slightly suppressed, which can be attributed to encapsulation by POPD particles. It is also clear from the XRD pattern that the CoFe2O4:POPD composite is similar to well-ordered diffractograms of the pristine components with reduced peak intensity, followed by the disappearance of low-angle diffraction peaks of POPD. For the CoFe2O4:POPD nanocomposites, it can be seen that the peak intensity of CoFe2O4 decreases and the peak at 33 Å becomes smaller and broader. This happens due to the destructive interference by the POPD chains. A more critical trend can be confirmed for the nanocomposites with the increased amount of POPD (i.e., 0.5−1 wt %). This is probably because the crystalline state of the POPD polymer deposited on CoFe2O4 attenuated the characteristic diffraction of CoFe2O4, affecting their detection. A similar observation has been reported by Liang and Li 40 for a polythiophene−TiO2 nanotube nanocomposite for the degradation of 2,3-dichlorophenols. In addition, it is clear from XRD analysis that, because of the POPD shell covering, all peaks were reduced in their intensity and appeared small. TEM Analysis. TEM analysis of the CoFe2O4 (Figure 4a) showed the formation of a dense agglomeration of distorted cubes with an average particle size of 200 nm. Agglomeration can be attributed to the formation of smaller particle size. The TEM image of POPD (Figure 4b) shows the formation of distorted spherical particles in the range of 42−50 nm. The TEM analysis of the CoFe2O4:POPD 1:0.25 nanocomposite (Figure 4c) showed the formation of a core−shell morphology, where CoFe2O4 is the core, with a POPD shell encapsulating these cores. The size of these nanoparticles was found to be in the range of 150−200 nm. As the loading of POPD increases up to 1 wt % (Figure 4e), the morphology clearly indicated a core−shell structure. The particle size was observed to be in the range of 60−80 nm in this nanocomposite. It appears that, as the amount of POPD increases in the composite, it retards the formation of largersized particles. This observatioon is consistent with the decrease in crystallite size with enhanced loading of the POPD in XRD analysis. It is revealed that ultrasonic-assisted synthesis produces nanocomposites that have a distorted core− shell structure. Analysis of the Thermal Stability of the Nanohybird. The TGA thermogram of POPD (see the Supporting Information) shows a steep decomposition curve. A weight loss of ∼10% is observed at 140 °C, which is due to the evaporation of volatile solvents, while 20 wt % decomposition is

Figure 4. TEM micrographs of (a) CoFe2O4, (b) POPD, (c) CoFe 2 O 4 :POPD 1:0.25, (d) CoFe 2 O 4 :POPD 1:0.5, and (e) CoFe2O4:POPD 1:1.

observed at 320 °C, which is due to the decomposition of unreacted monomer. A weight loss of ∼30% is observed at ∼550 °C, which confirms the high thermal stability of the polymer. The thermogram of CoFe2O4 (see the Supporting Information) reveals a weight loss of 6%, which is due to solvent evaporation; 10% weight loss is noticed at 160 °C, whereas 12% weight loss is observed at 550 °C. The stability of cobalt ferrite appears to be greater than that of POPD. Upon the loading of 25 wt % POPD in the nanohybrid, a three-step decomposition curve is obtained. A weight loss of ∼10% is noticed at 320 °C, while 20% weight loss is observed at 550 °C. For CoFe2O4:POPD 1:0.5, 5% weight loss is observed at 365 °C, while 15 wt % loss occurs at 550 °C. For the nanohybrid CoFe2O4:POPD 1:1, 10% weight loss is observed at 440 °C, whereas a 12 weight loss is observed at 640 °C. As the loading of POPD in CoFe2O4 is increased, the thermal stability is found to increase. Degradation Studies. The UV-vis spectrum of MG-200 in the presence of CoFe2O4 as the catalyst in a neutral medium (Figure 5a) showed the characteristic absorbance peaks at 325 and 625 nm. The peak at 325 nm was assigned to π−π* transitions of the benzene ring in MG dye, while the peak at 625 nm was typical of a substituted benzene. As the exposure time was increased from 15 min to 120 min, we observed that the absorbance maxima slightly decreased in both peaks. The peak at 325 nm showed a decrease in the absorbance value from 1.9 to 0.4 in 120 min. Similarly, the peak at 625 nm E

DOI: 10.1021/acs.iecr.5b04596 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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intensity of the peaks in the UV and vis regions confirmed the degradation of the MG dye. It appears that, upon the addition of POPD, the decrease in the absorbance intensity of the peak in the visible region was enhanced, which confirmed the catalytic behavior of POPD under UV irradiation. In the case of nanocomposites, a drastic decrease in the absorption peak intensity of the dye solution was observed. This happens because the photocatalytic activity of CoFe2O4 is enhanced by the presence of POPD, which acts as a sensitizer. The mechanism is explained in a later section. Kinetics of Degradation. The plot of C/C0 versus time, using CoFe2O4 as a catalyst under UV light (Figure 6a) showed

Figure 5. UV-vis spectra of MG-200 in the presence of (a) CoFe2O4, (b) POPD, (c) CoFe2O4:POPD 1:0.25, (d) CoFe2O4:POPD 1:0.5, and (e) CoFe2O4:POPD 1:1.

showed a decrease from 1.5 to 0.183 in 120 min. The peaks revealed appreciable reduction in the presence of CoFe2O4 as a catalyst. The spectrum of MG dye solution in the presence of POPD as a catalyst (Figure 5b) showed a decrease in the absorbance value from 1.9 to 0.3 in 120 min for the peak observed at 325 nm while the peak at 625 nm, showed a decrease from 1.5 to 0.1 in the same span of time. Interestingly, the photocatalytic activity of the pristine components appeared to be comparable. The UV-vis spectrum of MG-200 ppm in the presence of CoFe2O4:POPD 1:0.25 as a catalyst (Figure 5c) showed a decrease in the absorbance value from 1.9 to 0.08 in 90 min in the case of the peak observed at 325 nm, while the peak at 625 nm showed a decrease in the absorbance values from 1.5 to 0.08 in 90 min. The spectrum of MG-200 ppm in the presence of CoFe2O4:POPD 1:0.5 as a catalyst (Figure 5d) also showed significant reduction in the absorbance values from 1.9 to 1.2 in 15 min and 0.02 in 90 min, in the case of the peak observed at 325 nm, while the peak at 625 nm showed a decrease from 1.5 to 0.02 in 90 min. The UV-vis spectrum of MG-200 ppm in the presence of CoFe2O4:POPD 1:1 as a catalyst (Figure 5e) showed a large reduction in the absorbance value from 1.9 to 0.124 in 60 min for the peak at 325 nm, while, for the 625 nm peak, the absorbance values show a decrease from 1.5 to 0.07 in 60 min. Complete degradation occurred at 90 min and no peak was observed in this time interval. The decrease in the absorbance

Figure 6. Plot of C/C0 versus time in a neutral medium for MG-200 ppm: (a) peak at 325 nm and (b) peak at 625 nm.

85% degradation for the 325 nm peak in 200 ppm dye solution, whereas at 625 nm (Figure 6b), the peak showed 90% degradation in 120 min. Similarly, using POPD as a catalyst under UV light (Figures 6a and 6b), the percentage degradation was observed to be 91% and 95%, respectively, for the 325 and 625 nm peaks. When the nanocomposite was used as a catalyst, higher degradation was observed (Figures 6a and 6b). The 325 nm peak revealed 87% degradation for the 200 ppm MG dye solution, while the 625 nm peak showed 95% degradation in 90 min. Similarly, using CoFe2O4:POPD 1:0.5 as a catalyst (Figures 6a and 6b), 80% degradation was observed for the 325 nm peak, while 95% degradation was observed for the 625 nm peak, whereas 95% and 99% degradation was observed using CoFe2O4:POPD 1:1 as a catalyst (Figures 6a and 6b). Complete degradation was observed in the case of CoFe2O4:POPD 1:1. F

DOI: 10.1021/acs.iecr.5b04596 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Effect of Catalyst Concentration, Dye Concentration, and Recyclability. Figure 8a shows the change in degradation

The plots of ln C/C0 versus time (Figures 7a and 7b) revealed the degradation kinetics of MG using CoFe2O4,

Figure 8. (a) Variation of degradation rate constant with (a) catalyst concentration (mg/L) and (b) dye concentration (ppm).

Figure 7. Plot of ln C/C0 versus time in a neutral medium for MG-200 ppm using CoFe2O4, POPD, and CoFe2O4,:POPD nanocomposites: (a) peak at 325 nm and (b) peak at 625 nm.

rate constant with increasing nanoparticle concentration. The rate constant (k) increased nonlinearly as the nanocomposite concentration increases, up to 200 mg; beyond this value, it was observed to become constant (see Figure 8a). This happens because an increase in the catalyst concentration increases the number of active sites and the reactive surface area, leading to enhanced dye degradation. The k-value slightly decreased as the initial concentration of the dye was increased from 100 ppm to 250 ppm and further to 350 ppm (Figure 8b). The decrease in the k-values was observed to be dependent on the amount of loading of POPD. The greatest amount of degradation was achieved when CoFe2O4:POPD 1:1 was used as a catalyst. This catalyst also revealed only a slight variation in the k-values upon increasing the dye concentration, which confirms that this nanohybrid revealed the greatest degradation ability among other nanohybrids. The results of recyclability (Figure 9) showed a fair decline of ∼4% in the fifth cycle in the photocatalytic activity, because the presence of radicals might cause partial oxidation of CoFe2O4. It can be noticed that the decrease of the photocatalytic activity of CoFe2O4:POPD is less pronounced, compared to pure POPD and CoFe2O4, revealing their stability and also their reusability as an effective photocatalyst. Degradation Pathway. Catalytic degradation of the MG dye was further confirmed through analyzing the partially degraded samples of the dye by LC-MS method. MS analysis of fractionated dye samples after 30 min of retention time was performed, and the results are shown in Table 4. The first

POPD, and its nanocomposites. The kinetics was found to be of pseudo-first-order in all the cases. The rate constant (k) values using CoFe2O4 as a catalyst was observed to be 0.016 and 0.019, respectively, for the 325 and 625 nm peaks. The kinetics was noticed to increase rapidly when CoFe2O4:POPD was used as a catalyst. The k-values using CoFe2O4:POPD 1:0.25 were observed to be 0.028 and 0.033, whereas for CoFe2O4:POPD 1:0.5, it was observed to increase up to 0.036 and 0.050, respectively, for the peaks of 325 and 625 nm. The rate of degradation was found to be highest when CoFe2O4:POPD 1:1 was used as a catalyst. The k-value was found to be 0.06 for the 325 nm peak, whereas for the 635 nm peak, it was found to be 0.09. The peak in the visible region (i.e., 625 nm) showed higher degradation, compared to the peak in the UV region. This confirms that the degradation of the substituted benzene ring occurred faster, compared to that of the benzene ring. The dye concentration measured after equilibrium was used as the initial concentration, and it is reasonable to conclude that dye adsorption on the catalyst surface should have little contribution to the variation of dye concentration during the photocatalytic process. The obtained photodecoloration depicted that the nanocomposites exhibit remarkable photocatalytic activity under irradiation with UV light. The control (MG dye solution without catalyst) did not exhibit the photodecoloration upon irradiation with UV light, up to 4 h. G

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(diphenylmethanone), was obtained via the degradation of F-2. Elimination of the NO group through attack of OH• and H• free radicals formed intermediate F-4, (m/z 100) (cyclohexanol), which, upon further radical attack, yielded low-molarmass fragments of alcohols (i.e., butanol and propanol). Mechanism of Degradation. In order to demonstrate that the dyes were photodegraded rather than adsorbed on the surface of the photocatalysts at the end of the photocatalytic reaction, the photocatalytic systems were irradiated with and without visible light after the adsorption equilibrium, respectively. The dye concentration was decreased under visible light irradiation, while no noticeable change in the dye concentration was found after 9 h of stirring without visible light irradiation. Therefore, the observed photobleaching in the photocatalysis process was proposed to be due to the oxidative photodegradation of the dye molecules rather than adsorption. The chemical interaction between the negatively charged backbone of POPD and the positively charged groups of MG molecules occurs, which leads to attachment of the dye moieties to the surface of the nanocomposite. Since the band gap of the CoFe2O4:POPD nanocomposite is low, upon visible irradiation, the excited-state electrons in POPD migrate to the conduction band (CB) of CoFe2O4 and the photogenerated holes in the valence band (VB) of CoFe2O4 can directly transfer to the highest occupied molecular orbital (HOMO) of POPD. The ultrasonic irradiation prevents polaron and bipolaron formation by obviating e−/h+ recombination. Sonication causes fast mass transport to and from the POPD surface, which also prevents e−/h+ recombination. The e− in the CB of CoFe2O4 reacts with O2 and H2O molecules sticking at the surface of POPD particles and forms O2− and OH• ions and free radicals, while the h+ oxidize water to form H+ and OH− radicals and degrade the dye. Because conducting polymers are charge transporting materials, the photogenerated charges can migrate easily to the surface of the nanocomposite and photodegrade the MG molecules via the reactive oxidant species, i.e., OH• and H• free radicals. Because of the higher band gap of CoFe2O4, e−/h+ recombination is completely obviated and, because of concomitant sensitization by POPD, higher photodegradation rates are obtained in the nanocomposite.

Figure 9. Degradation of MG, as a function of the number of cycles.

Table 4. Intermediates Found at Different Retention Times sample

intermediates with % abundance retention time (30 min)

CoFe2O4 POPD CoFe2O4:POPD 1:1

100 (100%), 74 (25%), 60 (20%), 301 (20%) 100 (100%), 211 (60%) 64 (100%), 181 (10%)

intermediate with 100% abundance was taken as the main degradation product at the retention time. Intermediates (100% abundance) with low m/z values ranging from 64 to 100 were obtained. The intermediates with their decreasing m/z values, with corresponding molecular structures, are shown in Scheme 1. They are successively labeled as F-1, F-2, F-3, F-4, F-5, and FScheme 1. Degradation Pathway of Malachite Green (MG) Dye

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CONCLUSION

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ASSOCIATED CONTENT

Photocatalytic degradation of Malachite Green (MG) dye was carried out using CoFe2O4 and POPD nanocomposites. POPD coupled with CoFe2O4 was found to enhance the rate of degradation. MG dye revealed 96% degradation in the presence of the CoFe2O4:POPD nanohybrid. The methodology adopted in this study demonstrates the efficiency of POPD in sensitizing CoFe2O4 for rapid degradation of pollutants. Future research is needed for the development of more efficient catalysts, which are of low cost and can be regenerated easily. The magnetic properties and the catalyst regeneration studies are under investigation in our laboratory and will be published soon.

6. Above m/z 301, no prominent intermediate was found. Therefore, we have assumed F-1 to correspond to m/z 310. The intermediates revealed that degradation proceeded via elimination of methyl groups, amino group, asymmetric cleavage of benzene rings, all through attack of OH• and H• free radicals. Unpreferential attack of OH• and H• free radicals on carbons of benzene rings resulted in their cleavage and oxidation. F-1, (m/z 301) (4-((4-iminocyclohexa-2,5-dien-1ylidene) (phenyl) methyl)-N,N-dimethyl anilinium), was obtained from the parent dye via elimination of a dimethyl group, cleavage by attack of OH• and H• free radicals. This fragment then degraded into F-2, (m/z 211) (4-nitrosophenyl) (phenyl)methanone, via oxidation through attack of the OH• free radical on the bridge group. Consequently, F-3, (m/z 181)

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04596. Thermal analysis data of POPD and its nanocomposites with cobalt ferrite. The thermal stability shows an H

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increase with the increase in the loading of POPD (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. † Now retired.

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ACKNOWLEDGMENTS Dr. Ufana Riaz wishes to acknowledge the University Grants Commission (UGC) for granting the UGC Major Research Project vide Sanction No F. No( 41-199/2012(SR) and the Department of Science and Technology (DST)-science and engineering research board DST-SERB, India vide sanction no. SB/S-1/PC-070/2013 for granting major research project. One of the coauthors Miss Jyoti Kashyap acknowledges University Grants Commission (UGC), New Delhi for providing financial support through Basic Scientific Research (BSR) fellowship. The authors also thank the sophisticated analytical instrumentation facility (SAIF) at All India Institute of Medical Sciences (AIIMS) for the HRTEM facility and Prof.R.Nagarajan, Department of Chemistry, University of Delhi for XRD facility.

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