Document Type : Research Paper
Authors
1 Department of Physics, Faculty of Science, University of Kufa, Al-Najaf, Iraq
2 Department of Biomedical Engineering, Al-Khwarizmi College of Engineering, University of Baghdad, Baghdad, Iraq
3 Department of chemistry, College of science, University of Kufa, Al-Najaf, Iraq
4 Dental Technology Department, College of Medical Technology, Al-farahidi University, Baghdad, Iraq
5 Pharmacy of College, University of Hilla, Babylon, Iraq
6 Chemistry Department, College of Sciences, University of Diyala, Iraq
Abstract
Keywords
INTRODUCTION
Nanomaterials with small size and high surface area exhibit attractive properties that are very different from those of bulk, therefore they have a special position in many scientific field such as engineering, pharmaceutical, aerospace, energy production, civil construction, etc. [1-3].
Spinel ferrites—as a category of interesting nanomaterials—have received tremendous attention in the scientific communities [4-6]. Spinel ferrites with the general formula of AFe2O4, A is a di-valent ions, are crystallized in a face centered cubic structure that consists of tetrahedral and octahedral sites for the accommodation of A2+ and Fe3+ cations, respectively [7, 8]. Ferrites have been used for many applications such as magnetic imaging, magnetic data storage, and photocatalysis [9-12]. The properties of the ferrites can be tuned by using of various synthesis route, introducing dopant elements, modification of composition, and alteration in cation distribution in crystalline structure [7, 10, 13].
Substitution of Co2+ or Fe3+ with di- or tri-valent cations results in substantial changes in the magnetic and photocatalytic behavior of the cobalt ferrite [14, 15] . In this way, substitution of Fe3+ with the larger ions from rare earth elements group is of the great interest [16, 17]. Incorporation of larger ions leads to the structural distortion and internal strains that induce alteration in the magnetic, optical, and catalytic properties of the cobalt ferrite nanomaterial [18, 19].
Specifically, photocatalytic activity is one of the most promising properties of the cobalt ferrite, on which this work is focused. Photocatalytic process defines as the use of photo-induced nanomaterial for both environmental remedies and hydrogen production [20-22]. As for environmental applications, the photocatalytic process has opened up the doors for removal of the pollutants from air, water, and soil [23, 24]. It is important to have an efficient method for getting rid of the pollutants from water resources, because they are known to be a threat to human and other life form on earth. Common pollutants include toxic chemicals (such as surfactants, fertilizers, dyes, oil, and petrochemical compounds) which are being profusely discharged into the environment [25].
Seeking for more efficient photocatalyst material with fewer adverse impacts on the environment has been a major concern in the research communities [26]. Due to the chemical stability, non-toxic, and low production cost, cobalt ferrite has been recognized as a great candidate for the environmental remedy application [27]. Moreover, effective exploiting of the photocatalytic process necessitates the recovery and reuse of the photocatalyst particles. To meet this need, the magnetic properties of the ferrite compounds provide the magnetically separation of the photocatalyst particles from the media [28-30]. For example, Vani and co-workers studied the photocatalytic activity of the Tb3+ substitution in cobalt ferrite for degradation of crystal violet [31]. Toloman et al. synthesized Ni substitution of CoFe2O4 nanoparticles and studied their photocatalytic activity for visible light degradation of Rhodamine B [32].
Herein, Yb3+ substitution in CoFe2O4 (CoYbxFe2-xO4) nanoparticles have been synthesized through sol-gel auto-combustion method. Investigation of the magnetic properties of the synthesized nanoparticles was carried out by VSM analysis. The photocatalytic activity of the nanoparticles was studied for the visible light degradation of methyl orange (MO) solution.
MATERIALS AND METHODS
Synthesis of CoYbxFe2-xO4 nanoparticles
The simple and straightforward sol-gel auto-combustion reaction was used to synthesize of Yb3+ substitution in B-site of the cobalt ferrite nanoparticles (CoYbxFe2-xO4). For this purpose, first given amount of citric acid was dissolved into 50 mL of deionized water as a complexing agent. The amount of citric acid was determined to be in oxygen balance with the metal nitrate precursors. Then, different amount of nitrate precursors, that is Co(NO3)2.6H2O, Yb(NO3)3.5H2O, and Fe(NO3)3.9H2O, were added to the aqueous solution of citric acid. The obtained solution was heated to 180 °C under vigorous stirring until evaporation of the water to form a dark brown viscous solution. The heating was continued to ignite the viscous solution. Finally, the solid was collected and calcined at 500 °C for 4 hours. The different molar ratio of nitrate precursors (Co:Yb:Fe) were used, as follows: 1:0:2 (x = 0), 1:0.1:1.9 (x = 0.1), and 1:0.2:1.8 (x = 0.2). According to the synthetic purpose, CFO, 0.1-CYFO, and 0.2-CYFO samples stand for the CoFe2O4, CoYb0.1Fe1.9O4, and CoYb0.2Fe1.8O4 nanoparticles, respectively.
Characterization
The phase structure and crystallinity of the synthesized nanoparticles were determined using X-ray diffraction (XRD) patterns in Philips Pro PMD XRD diffractometer (Cu Kα, λ= 1.54 Å).. The morphology of the prepared nanoparticles was studied using field emission scanning electron microscope (FESEM) in TESCAN Mira3 equipped with a detector for microanalysis of the samples by energy dispersive X-ray spectroscopy (EDX). The optical properties of the nanoparticles were studied by diffuse reflectance UV-Vis spectroscopy (DRS) using JASCO UV/Vis/NIR V-670 spectrophotometer. The magnetic properties of the Yb3+ substituted CoFe2O4 nanoparticles was investigated at the room temperature using vibrating sample magnetometer analysis by BHV-55 VSM.
Photocatalytic activity
The photocatalytic activity of the different synthesized CoYbxFe2-xO4 nanoparticles was studied using degradation of methyl orange (MO) dye under visible light irradiation. All the photocatalytic reactions were done at the same irradiation time (105 min) and concentration of MO solution (50 mL of 50 mg.L-1). An ordinary white color LED lamp (50 W) was used as a source of the visible light, and the distance between the light source and the container of the MO solution was kept constant at 30 cm. The MO solution was allowed to reach the adsorption/desorption equilibrium with the nanoparticles under 15 min of stirring in the darkness. Then, the illuminating step was started for 105 min, and every 15 min a given amount of the MO solution (5 mL) was collected to evaluate the level of the photocatalytic degradation. The photocatalyst nanoparticles were separated from the dye solution using bar magnet. The efficiency of the visible light degradation was obtained using UV-Vis spectroscopy at maximum wavelength of MO solution (λmax=465 nm).
In addition, the effect of various parameters on the level of the MO degradation was studied, including: the effect of different amount of the loaded photocatalyst, pH of MO solution, and radical scavenging agents.
RESULTS AND DISCUSSION
Structure and crystalline phase
The XRD patterns represent the crystalline structure for the different concentration of Yb3+ incorporated into the network of CoFe2O4 nanoparticles, shown in Fig. 1. As can be seen, the diffraction planes are corresponded to the cubic phase of the spinel of cobalt ferrite (JCPDS file no. 001-1121). There is no extra peak, which reveals that the Yb3+ ions are well incorporated into the CoFe2O4 structure. Due to the higher ionic radius of the Yb3+ (0.98 Å) with respect to the Fe3+ (0.64 Å), one can be expected that the crystallite size decreases by substituting of Yb3+ for Fe3+ in the CoFe2O4 network. Incorporation of larger ions leads to the lattice distortion and a decrement in the crystallinity rate. As an overall result, the average crystallite size decreases with increase the concentration of Yb3+ ions [33, 34]. For that, the Scherrer equation [35] was employed to determine the average crystallite size for the synthesized CoYbxFe2-xO4 nanoparticles. As expected, the values of the calculated crystallite size show a decrease with increase the concentration of Yb3+ ions within the structure of the CoFe2O4. The calculated crystallite size values for the CoFe2O4, CoYb0.1Fe1.9O4, and CoYb0.2Fe1.8O4 nanoparticles are 25.3 nm, 21.18 nm, and 20.68 nm, respectively.
Morphology
FESEM images for the synthesized CoYbxFe2-xO4 nanoparticles are shown in Fig. 2. Clearly, the pure CoFe2O4 nanoparticles have the larger size nano-spherical particles ranging between 30-60 nm (Fig.2a). However, due to decrease of crystallite size caused by introducing of Yb3+, the FESEM images (Fig.2b, c) exhibit smaller nanoparticles for the Yb3+ substituted CFO nanoparticles. The particle size decreased with increasing the concentration of the Yb3+ into the CFO structure.
EDX spectra, shown in Fig.3, represent the elemental measurements to investigate the composition of the synthesized nanoparticles along with relative amount of the components. The insets to the Fig. 3 shows the amount (wt%) of each constituents existed within the nanoparticles.
Magnetic properties
The room temperature magnetization of the different synthesized CoYbxFe2-xO4 nanoparticles was studied using VSM analysis. Fig. 4 shows M-H curves for the synthesized nanoparticles containing different concentration of Yb3+. All the synthesized CoYbxFe2-xO4 nanoparticles exhibit ferromagnetic behavior. However, the saturation magnetization (Ms) decreases by increasing the concentration of substituted Yb3+ in B-site of CoFe2O4. The highest (65.52 emu/g) and lowest (40.66 emu/g) Ms belong to the CoFe2O4 and CoYb0.2Fe1.8O4 nanoparticles, respectively. Given to the fact that the Yb3+ has the lower magnetic moment (4.5 µB) compared to that of Fe3+ (5.9 µB), the magnetization trends are easily justified [36]. The inset to the Fig. 2 discloses the zoom on the part of the M-H curves to represent the Hc for the synthesized nanoparticles. The decrease in the Hc values revealed that the magnetic order was disrupted by substituting of Yb3+ for Fe3+. Due to the fact that the Fe3+ ions in the CoFe2O4 structure distribute between A and B sites, introduction of the Yb3+ to the B site can reduce the interaction of Fe3+ ions. As a result, the Hc decreases with increasing the concentration of Yb3+ within the CoFe2O4 crystalline structure [37]. Also, Table 1 summarizes the obtained magnetic data for all the synthesized nanoparticles.
The magnetic moment per formula unit in Bohr magneton (µB) [38] was obtained using the following equation:
where M is the molecular weight of the certain composition, Ms is the saturation magnetization.
Optical properties
The DRS spectra, shown in Fig.5, describe the optical behavior of the synthesized CoYbxFe2-xO4 nanoparticles in the UV-Vis region. As seen, all the nanoparticles have the substantial absorption in the range of 400-700 nm. The absorption intensity decreased for the Yb3+ substituted cobalt ferrite compared to the pure CoFe2O4 nanoparticles. Also, the absorption threshold is shifted toward shorter wavelength with increasing the concentration of Yb3+. This observation can be explained by dependence of absorption character on the particles size, lattice distortion and impurity centers [32, 39, 40]. According to the XRD and FESEM results, the particle size decreased with increasing the concentration of Yb3+ substituted for Fe3+ in CoFe2O4.
The band gap of the synthesized nanoparticles was calculated by Tauc method, plotting (αhν)2 versus hν and then extrapolating of the curves (Fig. 5b). The band gap values showed a blue shift associated with increasing concentration of Yb3+, which is attributed to the reduced particle size [32]. The band gap values for CoFe2O4, CoYb0.1Fe1.9O4, and CoYb0.2Fe1.8O4 are 1.4 eV, 1.71 eV, and 2.07 eV, respectively.
Photocatalytic activity
The photoactivity of the synthesized CoYbxFe2-xO4 nanoparticles was studied for the degradation of MO solution under visible light irradiation. Fig. 6a shows that the photocatalytic efficiency of the synthesized nanoparticles is of the following order: 0.1-CYFO > 0.2-CYFO > CFO, confirming the superior photocatalytic efficiency of the CoYb0.1Fe1.9O4 nanoparticles compared to the other synthesized nanoparticles. This result is attributed to the reduced recombination rate of the electrons/holes caused by substitution of the Yb3+ ions. However, more amount of Yb3+ induced the rapid recombination of charges, which is due to the repeated trapping of the charges [31]. These result also are in accordance with the DRS analysis, which showed the lower band gap value for the pure CoFe2O4. The narrower band gap increases the recombination of the electrons and holes [31, 41]. By visible light illuminating for 105 min, the 91.7% of the MO was degraded using 0.1-CYFO nanoparticles, whereas the pure CFO provided the MO degradation by 54.7%.
The kinetics of the photocatalytic reaction for all the synthesized nanoparticles is shown in
Fig. 6b, revealing the first order kinetic rate for the nanoparticles— that is, -ln (C/C0) = kt where k (min-1) is rate constant, t (min) is the irradiation time, C, and C0 are the concentration of dye before and after the performing the photocatalytic reaction. By plotting -ln(C/C0) versus irradiation time, the rate constant of the reaction using the nanoparticles was calculated from the slop of the curve. The 0.1-CYFO nanoparticles indicated the highest constant rate of 0.298 min-1. The other samples including 0.2-CYFO and CFO have the kinetic rate of 0.173 min-1 and 0.096 min-1, respectively.
Also, Fig. 7 exhibits the absorption spectra of the MO solution exposed to the photocatalytic degradation using CoYb0.1Fe1.9O4 nanoparticle at the different irradiation time. Evidently, the absorption of MO solution decreased with proceeding of the photocatalytic reaction.
Effect of dosage of photocatalyst
The photocatalytic degradation of MO solution was carried out using the different dosage of the 0.1-CYFO nanoparticles (0.01, 0.03, 0.05, 0.07, and 0.09 g), as shown in Fig. 8.
The photodegradation of MO solution increased with increasing the amount of the photocatalyst. The highest photoactivity was obtained by using 0.05 g of the 0.1-CYFO nanoparticles. As can be seen from the Fig. 8, further amount of the nanoparticles caused to the dramatic decrement in the photodegradation level of the MO solution. This result is attributed to the limitation of the light beam to penetrate onto the turbid dye solution caused by dispersion of exceeded amount of photocatalyst nanoparticles [42].
Effect of pH
The photocatalytic efficiency of the 0.1-CYFO nanoparticles was studied at different pH of the MO solution. Due to the fact that the MO is an anionic dye, the positively charged surface of the photocatalyst nanoparticles is in a favor of the adsorption of the dye molecules on the photocatalyst surface [43]. Therefore, one can be expected that under acidic conditions, the degradation efficiency of the MO is higher than that of in alkaline media.
Fig.9 shows the pH dependence of the photodegradation degradation of the MO solution. As can be seen, there is an enhancement in the photocatalytic degradation level of the MO solution under acidic condition. At pH of 5, the photodegradation of the MO solution approached to more than 95%. However, an increase in the pH of MO solution to 9 resulted in a significant decrease in the photocatalytic efficiency.
Reusability studies
The photocatalytic stability of the synthesized 0.1-CYFO nanoparticles was studied at 7 consecutive reaction cycles. After each reaction, the photocatalyst was collected using a bar magnet and washed several times by deionized water/ethanol solution to remove the residual adsorbed dye. Then, the photocatalyst was heated at 100 C for 1 hour.
Fig.10 represents that the synthesized 0.1-CYFO nanoparticles have a great photocatalytic stability. The decrement of the photocatalytic efficiency is only 7.9% after 7 successive reaction cycles. Also, it can be noted that the loss of the photodegradation efficiency is not negligible after forth reaction. In addition, the variations of the constant rate of the photocatalytic reactions at 7 reaction cycles are depicted in Fig.10. The constant rate of the photocatalytic degradation using 0.1-CYFO decreased by 13% after 7 successive reaction cycles.
Mechanism of photocatalytic degradation
The photocatalytic degradation of MO solution was studied in the presence of different radical scavenging agents to find the mechanism of photocatalytic degradation over the 0.1-CYFO nanoparticles. In this case, isopropanol [44], EDTA [45], and sodium azide [45] were used as scavenging agents for quenching of the hydroxyl radicals, photo-generated holes, and superoxide radicals, respectively. Fig. 11 shows that the photocatalytic degradation of MO solution is decreased using isopropanol and sodium azide, revealing that the dominant oxidative species formed by light induced of the 0.1-CYFO nanoparticles are hydroxyl and superoxide radicals.
CONCLUSION
To sum up, we have synthesized Yb3+ substituted CoFe2O4 nanoparticles using simple and facile sol-gel auto-combustion method. Different concentration of Yb3+ ions was incorporated into the crystalline structure of the CoFe2O4 nanoparticles, and the effect of the Yb3+ substitution for Fe3+ on magnetic and photocatalytic behavior of the CoFe2O4 was investigated. The Ms and Hc values were decreased by incorporation of Yb3+, which is due the lower magnetic moment of the Yb3+ ions. However, the photocatalytic activity was benefited by the Yb3+ substitution, which was attributed to the contribution of Yb3+ in reducing the recombination rate of charge carriers. So that, the Yb3+ substituted CoFe2O4 nanoparticles disclosed the higher photocatalytic efficiency compared to the pure CoFe2O4 nanoparticles. During 105 min illumination under visible light, more than 91% of MO solution was degraded using the CoYb0.1Fe1.9O4 nanoparticles.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.