One of the main groups of organic compounds is azo materials. These components have the most application in azo dye synthesis that involves an extensive spectrum of colors. Azo dyes are widely used in textile industries. The azo functional group(-N=N-) in azo dyes is a carcinogen, toxic and bio recalcitrant [1-2]. Textile wastewater is infected with azo dyes even after conventional treatment. Various chemical and physical treatment techniques and processes have been used to remove dye pollutants from contaminated water. The adsorption [3-5] and advanced oxidation processes [6-7] are successful methods for dye removal. The advanced oxidation process is based on the in situ production of highly reactive hydroxyl radicals (.OH) that react with azo dyes and degrade them. Photo-Fenton process is an example of an advanced oxidation process. Photocatalysis uses a semiconductor under illumination light (ultraviolet or visible) to generate electron/hole pairs with the ability to degrade azo dyes. Li et al. reported alumina supported hematite with high activity and stability. They evaluated the catalyst activity with orange II. Fe2O3/ Alumina showed higher activity than Fe2O3/ silica and bare hematite because of reduced particle size of hematite and the enhanced surface adsorption of dye on the catalyst.
Marcio Rodriques and his coworkers  have synthesized zero valent iron nanoparticles in variable synthesis conditions and used them in the degradation of the azo dye disperse red 1. Nano zero valent iron particles yielded high color removal in a short time (around 98% in 10 min).
Magnetite is another compound of iron which was used in the degradation of dye pollutants [10-12].
Cobalt oxides such as CoO or Co3O4 are p-type semiconductors with interesting electronic and magnetic properties. They have found applications as an efficient catalyst, high temperature solar selective absorber, pigment for glasses and ceramics etc. Cai and his coworkers  synthesized Co3O4 /BiVO4 composite by the impregnation method. The photocatalyst has exhibited enhanced photocatalytic activity for phenol degradation. CO3O4 nanoparticles were used as photocatalyst for degradation of methyl orange, Diclofenac Sodium , Cr(VI) and dye removal.
Nanosize spinel ferrite particles recently have received considerable attention because of their wide applications. CoFe2O4, one of the most significant spinel ferrite exhibited remarkable photocatalytic properties .
Meso-macroporous nanospheres of CoFe2O4 were applied as adsorbent and photocatalyst for Cr(VI) removal and anionic dye degradation (Methyl Orange), respectively .
CoFe2O4 nanoparticles assembled on mesoporous graphitic carbon nitride were tested in the photocatalytic degradation of Malachite Green and 93% degradation efficiency was obtained . The superparamagnetic cobalt ferrite nanoparticles offered heterogeneous and recoverable catalyst. The Catalyst separation from reaction media and recycling it are of practical problems. To overcome the problem of catalyst separation and making recyclable photocatalyst, coating of nanoparticles on the inert substrate can be an appropriate option. In this project, photocatalyst nanoparticles were stabilized on the glass by doctor bade technique.
Although, a lot of researches have been devoted to investigating the photocatalytic performance of cobalt ferrite, Iron oxides, and cobalt oxides nanoparticles, there are rare articles about the comparison of CoFe2O4 with its single metals oxide . This comparison allows choosing the most suitable option for achieving more efficient water purification in the shortest possible time.
In this work, the aim is a comparison between the performance of CoFe2O4 as a composite with Co3O4 and FeO as (single oxides) individual oxides in photocatalytic degradation of azo dyes. CoFe2O4, FeO and Co3O4 nanoparticles were synthesized by the same route (wet chemical method). The structural, optical and morphological properties of particles were investigated by XRD, FT-IR, DRS and FESEM analysis. The dye degradation performance of CoFe2O4, FeO and Co3O4 were evaluated by photodegradation of Reactive Red 4 and Acid Black 1. The kinetics of dye degradation by particles were investigated.
The precursor materials, cobalt (II) chloride hexahydrate (CoCl2.6H2O) and iron (III) chloride hexahydrate (FeCl3.6H2O) were obtained from Merck Chemical Company. Sodium hydroxide (NaOH), Oleic acid (C18H34O2), Acetic Acid (C2H4O2), Terpineol (C10H18O), ethyl cellulose, Acid Black 1 (C22H14N6Na2O9S2) and Reactive Red 4 (C32H19ClN8O14S4Na4) were purchased from Sigma-Aldrich Chemical Co. Distilled water was used throughout.
Synthesis of Co3O4, FeO and CoFe2O4 nanoparticles by wet chemical route
To synthesize CoFe2O4 nanoparticles, as mentioned in our previous paper  Cobalt (II) chloride hexahydrate (5 mmol), 1.22 g and iron (III) chloride hexahydrate (10 mmol), 2.72 g were separately dissolved in double distilled water (25 mL) and placed in an ultrasonicator for 2 min. Two solutions were mixed and sodium hydroxide (25 mL, 3 M) was added and stirred up to pH = 12. Oleic acid as a chelating agent (1 mL) was added to the above suspension. The temperature of the suspension was raised to 80 °C with continued stirring for 1 h. The product was cooled to room temperature and the precipitate was separated by centrifuge, washed twice with double distilled water and ethanol. The precipitate was dried at 75 °C (dark brown powder), annealed at 600 °C for 10 h and black powder was obtained. The same experimental procedure as CoFe2O4 nanoparticles preparation was performed except adding cobalt(II) chloride hexahydrate solution and iron(III) chloride hexahydrate solution for preparing FeO and Co3O4 nanoparticles, respectively. Fig. 1 shows a Flow chart for wet chemical route preparation of CoFe2O4 nanocomposite, Co3O4, and FeO nanoparticles.
Coating on glass using nanoparticles pastes by Doctor Blade method
A 0.8 g of each nano-powders prepared by wet chemical route was grinded by adding 130 µL of acetic acid and stirred for 5 min. 130 µL double distilled water was added and the solution was stirred for 1 min and this process repeated 5 times. Then 130 µL Ethanol was added and the solution was stirred for 1 min, and this process repeated 15 times, too. After this 15 ml ethanol, 3 mL terpineol and 0.4 g ethyl cellulose were added to 5 mL ethanol and the paste solution was sonicated for 1 min. The solvent was evaporated by stirring in the air and nanopowder/ ethyl cellulose terpineol paste was used for coating on the glass by Doctor Blade for CoFe2O4, Co3O4and FeO nanoparticles separately. The photocatalyst-coated glass was air dried, and sintered at 300 oC in the air to improve the contact between the film and the substrate. Fig. 2 shows Glass slides coated with a paste of CoFe2O4, FeO and Co3O4 nanoparticles prepared by a wet chemical route using Dr. Blade method.
FT-IR absorption spectra of samples were obtained using KBr disks on an FT-IR 6300. Powder X-ray diffraction analysis was performed on a D8 Advance, BRUKER diffractometer in the diffraction angle range 2θ=5-80° with Cu Kα radiation (1.5406 A°). Diffuse reflectance spectra (DRS) were collected with a V-670, JASCO spectrophotometer and transformed to the absorption spectra according to the Tauc relationship. Field emission scanning electron microscopy (FESEM) images were taken on a Hitachi, S-4160 scanning electron microscope. The azo dyes solutions concentration was determined using its absorbance by a Cary 500 UV–Vis spectrophotometer.
Photocatalytic activity test
Photocatalytic degradation of Reactive Red 4 (RR4) and Acid Black 1 (AB1) in water was performed at room temperature exposed to UV light irradiation so as to evaluate the catalytic performance of photocatalysts. A 250 W Hg lamp was used as the light source. For all the degradation experiments, the photocatalyst coated on a glass slide was appointed in a Petri dish which contained 25 ml dye solution which exposed at O2 gas for 30 min. The area of the photocatalyst coated on glass was 11 cm2. The mixture of dye and photocatalyst coated on glass was stirred in the dark condition to allow mixture to establish sorption and desorption equilibrium of dye molecules by the photocatalyst nanoparticles. The light source was placed about 20 cm away from the liquid surface of the mixture. To determine dye degradation percent by photocatalyst first standard solutions with different concentration of Reactive Red 4 and Acid Black 1were prepared and their UV-Vis spectra were recorded (inserted in Fig. 3) Then the graph of absorbance in maximum wavelength (λmax for AB1=616 & λmax= 534 for RR4) vs concentration was plotted and the equation of the most fitted line was obtained (Fig. 3). This equation was used to determine the concentration of dye by UV absorbance intensity of dye which degraded with the photocatalyst. The equation of calibration curves of Acid Black 1 and Reactive Red 4 has been inserted in Fig. 3. Then the mixture of dye and photocatalyst coated on glass was irradiated under a UV lamp to initiate photocatalytic degradation of azo dyes. During the reaction, at a determined interval time (15min) small amount of solution was analyzed by Cary 500 UV–Vis spectrophotometer. Relative degradation of each azo dye can be calculated as follows (Eq.1):
Relative degradation (% ) = (C0-Ct)/C0 ×100 (1)
Which Co and Ct were the dye concentrations at an initial time and after a Reaction period of t (min) under UV-light irradiation, respectively.
The photocatalytic degradation kinetics of azo dyes using photocatalysts nanoparticles coated on glass was appraised using the first order and second order equations given in the equations (Eq. 2) and (Eq. 3) and the plots were drawn and the first and second order constants were calculated.
Where C0 is the initial concentration of azo dye solutions, Ct is the concentration of dye solutions at time t, k1app is the first order constant, k2, is the second order rate constant, t is the Ultra-Violet light exposure time. The regression correlation coefficient (R2) of the first and second order plot was calculated. Then the equation with greater regression correlation coefficient was chosen as the kinetic order of dye degradation.
RESULT AND DISCUSSION
X-ray Diffraction Analysis
To characterize the crystalline structure of the samples, the XRD patterns of CoFe2O4, Co3O4 and FeO were obtained as shown in Fig. .4. All peaks from X-ray diffraction pattern of CoFe2O4 clearly matched well with face centered cubic (fcc) CoFe2O4 (JCPDS card 22-1086) [23-26], corresponding to Miller indices (111), (220), (311), (400), (422), (511) and (440). The reflection from the XRD pattern displayed the spinel structure of CoFe2O4 with preferred orientation along (311) plane . Unfortunately, the peak of impurity at 2θ = 36.93 was observed in the sample, which is related to NaCl crystals produced as a by-product during synthesis. For Co3O4, the diffraction peaks at 22.1, 36.4, 42.8, 44.8, 52.6, 65.6, 70.2 and 77.6 correspond to (111), (220), (311), (222), (400), (422), (511) and (440) planes of Co3O4 (JCPDS card no. 43.1003), respectively. For FeO, the diffraction peaks at 41.05, 49.8 and 73.17 correspond to (111), (200) and (220) planes of FeO (JCPDS card no.02.1186), respectively. Other peaks in the FeO diffraction pattern are related to sodium iron oxide (NaFeO2) (JCPDS card no.20-1115) and halite (NaCl) (JCPDS card no.75-0306) presented in the sample. The average crystallite sizes of spinel CoFe2O4, FeO and Co3O4 were calculated from X-ray line broadening using Scherrer’s equation and were found to be about 47 nm, 50 nm, and 40 nm, respectively.
FT-IR Spectroscopic Characterization
Fig. .5 shows the FTIR spectra of CoFe2O4, Co3O4, FeO nanoparticles annealed at 600 °C recorded between 3500 cm−1 and 400 cm−1. The O−H stretching vibrations interacting through H bonds are observed at around 3430 cm−1 in three samples, 2927 cm−1 and the absorption band present at about 1623 and 1634 cm−1is due to the bending of the absorbed water molecules . The absorption band at 1113 cm_1 is the characteristic of CoFe2O4system and this may be due to the residual FeOOH. The absorption bands present at about 585 cm−1and 410 cm−1 is due to the stretching vibrations of metal oxide in octahedral group complex Co(II) −O2- and Fe(III) −O2- tetrahedral group complex of the CoFe2O4 phase respectively which proves the existence of spinel ferrite [29- 30]. The peak at 666 cm−1 in FTIR spectrum of Co3O4 is attributed to Co−O vibrations where cobalt ions are in a tetrahedral position. The peak at 574 cm−1 is associated with Co−O vibration in the spinel lattice where cobalt cations are in octahedral positions. The band at 880 cm−1 is related to CoOOH or water of hydration [31-34]. In the FTIR spectrum of FeO, the peak observed at 1435 cm−1 is probably related to carbon dioxide vibrations adsorbed on the surface of nanoparticles. The strong absorption band at 570 and 484 cm−1 are evidence of Fe−O−Fe bonds.
UV-Vis diffuse reflectance Spectroscopic Characterization for optical properties
UV-Vis absorption spectroscopy was used to investigate the optical properties of CoFe2O4, FeO and Co3O4 semiconducting nanoparticles. It should be noted that the optical properties of the nanoparticles mainly depend on the particle size. The band gap and surface roughness affect the absorbance of a semiconducting material [35-37]. Fig. 6 shows UV-Vis diffuse reflectance spectra and Tauk plots of CoFe2O4, FeO and Co3O4 nanoparticles. The peak at 332 nm in the absorption spectrum of CoFe2O4 is related to the transition of an electron from the valence band (VB) to the conduction band (CB) [38-40]. Co3O4 is a p-type semiconductor. There are two absorption peaks (λ=648 and 385nm) being obviously found in Fig. 6 for Co3O4 absorption spectrum which indicates ligand-metal charge transfer events O(II)→Co(III) and O(II) → Co(II), respectively . Tauc plot is used to calculate the band gap of semiconducting materials . The Tauc formula (Eq. (4)) depicting the relationship between the absorption coefficient and the incident photon is as follows:
αhv= A(hv–Eg)n (4)
where α is the absorption coefficient, A is a constant and n is an index which assumes the values 1/2, 3/2, 2 and 3 depending on the nature of electronic transition, where n=1/2 for direct band gap semiconductors. An extrapolation of the linear region of the plot (αhv)2 vs hv gives the value of the optical band gap Eg as shown in Fig.6. The measured band gap of CoFe2O4 was found to be 1.32 eV. For Co3O4, the band gap was measured in 2.17 eV. The energy band gap of FeO nanoparticle was 1.6 eV.
Field emission scanning electron microscope (FESEM) was employed to investigate the morphology of samples (Fig. 7). It was found that the CoFe2O4 sample consisted of homogenous connected nanospheres (Fig. 7. c & f). CoFe2O4 nanoparticles had a diameter of about 48 nm. FESEM picture of Co3O4 shows that nanoparticles were separated and unsymmetrical octahedral (Fig. 7. b and e) and the size of particles was about 73nm. With respect to FESEM picture of FeO sample in (Fig. 7. a & d), it was found that FeO nanoparticles were grown in a cylindrical shape and the smallest dimension of their section was about 57 nm.
Photocatalytic degradation of azo dyes (AB1 & RR4) by CoFe2O4 nanoparticles
The photocatalytic degradation of RR4 and AB1 was performed under UV light irradiation by using CoFe2O4 nanoparticles coated on a glass slide as a photocatalyst. Fig. 8 shows the results of Reactive Red 4 and Acid Black 1 decomposition under UV light irradiation in the presence of CoFe2O4 coated on a glass slide.
The process of photocatalytic degradation of azo dyes over CoFe2O4 catalyst can be described as follows . The first step involved adsorption of dye onto the surface of CoFe2O4 nanoparticles. Exposure of dye adsorbed CoFe2O4 nanoparticles to UV light led to the generation of electron-hole(e--h+)pairs in CoFe2O4 as indicated in the equation . The photo-generated electrons in the conduction band of CoFe2O4 interacted with the oxygen molecules adsorbed on CoFe2O4 to form superoxide anion radicals (.O2-)(Eq. b). The holes generated in the valence band of CoFe2O4 reacted with surface hydroxyl groups to produce highly reactive hydroxyl radicals (Eq. c). These photo-generated holes could lead to dissociation of water molecules in the aqueous solution, producing radicals (Eq. d). The highly reactive hydroxyl radicals (.OH) and superoxide radicals (.O2-) could react with an azo dye adsorbed on CoFe2O4 nanoparticles and lead to its degradation as represent in (Eq. e and f).
CoFe2O4 +hυ → e- (CB) + h+(VB) (a)
O2 + e- → .O-2 (b)
h+ + OH- → .OH (c)
h+ + H2O → H+ + .OH- (d)
.OH- + azo dye → Degradation products (e)
.O2- + azo dye → Degradation products (f)
The maximum absorbance of Reactive Red 4 decreased with increasing UV irradiation time until 105 min in presence of CoFe2O4 nanoparticles. CoFe2O4 photocatalyst decomposed Reactive Red 4 about 61% (Fig. 8.b). The slope of azo dyes in the curve of Relative concentration vs irradiation time was not constant therefore kinetic rate order could not be of zero order (Fig. 8.a). So the photocatalytic degradation kinetics of azo dyes was appraised with the first and second order equations given in (Eq.2, 3) and the corresponding plots were drawn, as shown in Fig. 8(c & d), the photocatalytic degradation kinetics of RR4 was first order and calculated rate constant was about 0.01(min-1). Acid Black 1 was degraded by using photocatalyst under UV light in 120 min about %80 (Fig. 8. b). Degradation of Acid Black 1 by CoFe2O4 nanoparticles composite coated on glass followed first order rate kinetic and the rate constant was obtained around 0.012(min-1) (Fig. 8. d). In this research, we made a comparison between the RR4 and AB1 degradations. It is noticeable that due attention to relative degradation graphs in (Fig. 8.b) the photocatalytic performance of CoFe2O4 in RR4 degradation was better than AB1 till 90 min. It was obviously found that CoFe2O4 performance in degradation of AB1 was more efficient than RR4 and the rate constant for AB1 was greater.
Photocatalytic degradation of azo dyes (AB1 & RR4) by Co3O4 nanoparticles
The photocatalytic activty of Co3O4 was also investigated through RR4 and AB1 degradation under UV light irradiation (Fig. 9). For both azo dyes the photocatalyst till 90 min had efficiency in dye degradation. The maximum absorbance of Reactive Red 4 and Acid Black 1 decreased with increasing UV irradiation time. The photocatalytic degradation of azo dyes over Co3O4 nanoparticles had the same procedure mentioned for CoFe2O4 at section (3.5). The Co3O4 nanoparticles degraded RR4 about 28% but 63% of AB1 was degraded (Fig .9.b). Degradation of RR4 using Co3O4 nanoparticles followed second order rate kinetic with rate constant equaled to 0.0004 (ppm-1.min-1) while the AB1 degradation followed first-order kinetic rate and rate constant was equal to 0.11(min-1) (Fig. 9.c, d).
Fig 9. a) Relative Concentration, b)Relative Degradation, c & d) kinetic parameters of Reactive Red 4 and Acid Black 1 degradation in presence of Co3O4 under UV light irradiation.
Photocatalytic degradation of azo dyes (AB1 & RR4) by FeO nanoparticles
The photocatalytic performance of FeO nanoparticles was investigated for degradation of RR4 and AB1 under UV light irradiation. The relative concentration, relative degradation, and kinetic parameters are summarized in Fig. 10.
The degradation of RR4 under UV light irradiation in presence of FeO photocatalyst continued until 120 min and finally, 65% of RR4 was degraded. The RR4 degradation by using FeO nanoparticles coated on a glass slide as photocatalyst had second order rate kinetic and rate constant was equal to 0.001ppm-1.min-1. The photocatalytic behavior of FeO nanoparticles coated on a glass slide in the degradation of AB1 was greatly different. UV-Vis absorbance spectra of AB1 showed a steady and slow degradation rate until 45 min UV irradiation but after 45 to 60 min the speed of degradation increased severely. Probably the reason for high-speed degradation was related to oxidation of iron from Fe2+ to Fe3+ and contribution of two iron species may have led to prompt of degradation. Another possible reason for the increase in speed was a photo-Fenton phenomenon. It seemed that during t=45-60 min the photocatalytic process and photo-Fenton phenomena had synergistic effects on degradation of dye. In the Fenton process, hydroxyl radicals are produced in the presence of an iron catalyst with degradation of hydrogen peroxide, according to the following Equations [i-g].
.OH+ .OH→ H2O2- (i)
Fe2++H2O2→Fe3+ + .OH+HO- (g)
Hydrogen peroxide is produced from a combination of hydroxyl groups. As seen, in this reaction, Fe+3 is produced which has low ability to catalyze hydroxyl radical. After UV ray irradiation in this reaction, Fe+3 is reduced to Fe+2, therefore, the production of hydroxyl radicals is accelerated. This process is called as to Photo-Fenton Process:
Fe3+ +H2O+hν→Fe2++HO. +H+ (h)
This reaction takes place after reaction (g) and continues with photo radiation. The efficiency of the Fenton reaction can be enhanced by using UV/vis irradiation. The main advantage of UV/vis irradiation is that Fe2+species can be regenerated more quickly than the standard Fenton process, giving one more hydroxyl radical ready for the organic pollutants degradation (Fig. 11).
Relative concentration and degradation graphs of AB1 in a period of 45-60 mins show the rapid change that confirmed our probable suggestions (Fig. 10. a, b). According to the Fig. 12 the color of Acid Black 1 in 60 min UV irradiation over FeO nanoparticles converted to white, that confirmed the photo-Fenton process.
Degradation of AB1 obeyed first-order kinetic (0-45 min). The rate constant of the first period (0-45min) was calculated 0.008 min-1.
Comparison of the photocatalytic degradation of AB1 and RR4 by CoFe2O4, Co3O4, and FeO
In degradation of AB1, FeO nanoparticles coated on glass had better performance in comparison with CoFe2O4 and Co3O4 nanoparticles. The relative degradation percent of AB1 decreased in the order of FeO>CoFe2O4>Co3O4. Degradation of AB1 by above-mentioned catalysts followed first order rate kinetics. The degradation rate of AB1 by photocatalysts decreased in the order of FeO > Co3O4> CoFe2O4. For degradation of RR4, FeO nanoparticles showed photocatalytic activity in a longer time than CoFe2O4 and Co3O4 and relative degradation percent decreased in the order of FeO>CoFe2O4>Co3O4. Degradation of RR4 by CoFe2O4 followed first order rate kinetic but the degradation of RR4 by FeO and Co3O4 followed second order rate kinetics. The rapidity of RR4 degradation by the aforementioned catalysts decreased in the order of CoFe2O4 > FeO >Co3O4. In the degradation of RR4, FeO had relatively better performance in comparison with CO3O4 and CoFe2O4. Table 1 summarizes degradation condition of azo dyes (AB1 & RR4) over photocatalysts (CoFe2O4, Co3O4, and FeO). Two main factors to determine photocatalytic performance are the band gap of photocatalyst and surface area (related to the particle size). For FeO, the further photo-Fenton process helps to degradation of azo dyes.
The CoFe2O4 composite and individual iron and cobalt oxide photocatalysts were synthesized by a wet chemical method that FTIR and XRD analysis results confirmed the formation of CoFe2O4, FeO and Co3O4. The morphological and optical properties of the photocatalysts were characterized by field emission scanning electron microscopy (FESEM) and diffuse reflectance spectroscopy (DRS), respectively. The photocatalysts nano-particles were coated on the glass by Doctor Blade method. Among the catalysts prepared, the FeO photocatalyst exhibited higher photocatalytic activity than Co3O4 and CoFe2O4 in the photocatalytic degradation of azo dyes such as RR4 and AB1 in water, which was due to the high specific surface area and photo-Fenton phenomena. The reaction in the degradation of AB1 by FeO was a photo-Fenton phenomena-like reaction. The degradation efficiency depended on free radical generation rate from Fe+2 and Fe+3.
The authors wish to thank the University of Isfahan for financial support of this work.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.