Direct Synthesis of Magnetic CoFe2O4 Nanoparticles as Recyclable Photo-Fenton Catalysts for Removing Organic Dyes.

Herein, CoFe2O4 nanoparticles were directly synthesized through a solution combustion method using ferric nitrate, cobalt nitrate, and glycine as raw materials. The effects of glycine on the phase composition and magnetic properties of the CoFe2O4 products were investigated. When the fuel/ferric nitrate ratio was 0.8, the obtained product was pure CoFe2O4 with an average particle size of 25 nm. Furthermore, the saturation magnetization is 77.3 emu/g, which is about 95.7% that of CoFe2O4 bulk materials at room temperature and good for recycling. The photo-Fenton catalytic properties of CoFe2O4 were investigated for assessing its efficacy in removing dyes. It could degrade the 20 ppm MB in 75 min. To improve the photo-Fenton catalytic performance, NH4HCO3 and glucose were employed as additives. Due to the pores formed by NH4HCO3 and glucose, the G-CoFe2O4 and N-CoFe2O4 could degrade the 20 ppm MB in 40 and 25 min, respectively. The results indicated that these additives can effectively improve the catalytic activity of CoFe2O4. The modified CoFe2O4 is a promising alternative recyclable photo-Fenton catalyst for removing organic dyes.

Introduction

Organic dyes generated by industrial waste pose a significant threat to human health and the environment. Therefore, developing effective technologies to degrade organic dyes in wastewater is extremely important for a sustainable society.[1,2] Many technologies have been employed for organic dye degradation, including adsorption, chemical precipitation, biological processes, and advanced oxidation processes.[3−6] Photo-Fenton catalysis is an advanced oxidation process that attracted significant attention for the production of oxide species (·OH), which are efficient oxidants with high redox potential.[7,8] In particular, photo-Fenton catalysts based on transition metal oxides (e.g., Fe, Co, and Cu) have been studied extensively because they are inexpensive, environmentally benign, magnetically recyclable, and exhibit visible light responses.[9−13] Among these metal oxides, spinel-type cobalt ferrite (CoFe2O4) has been intensively studied for degradation of organic dyes due to its excellent stability, separation, and catalytic activity.[14]

Thus far, a variety of methods have been proposed to synthesize CoFe2O4, including solvothermal methods,[15] sol–gel methods,[16] and chemical synthesis methods.[17] However, such methods typically involve complicated procedures, including long reaction duration, repeated washing, and calcination, to obtain the final products. A facile and efficient synthesis method would be important for practical applications. To prepare magnetic CoFe2O4 nanoparticles, we developed a solution combustion synthesis (SCS) method producing oxide nanomaterials with a high yield. This method is essentially an exothermal redox reaction between fuels and oxidizers.[18−20] In general, the oxidizers are metal nitrates and the fuels are glycine, citric acid, urea, etc.[21] Additionally, the energy generated by exothermal reactions can be used to sustain the reaction system. Furthermore, the apparatuses used for this process are simple and cost-effective.[22] The magnetic properties of a material heavily depend on its particle size and fabrication method.[23] Various undesirable phases may exist in products, weakening their magnetic properties.[24] With the advantages and characteristics of SCS, it is possible to prepare pure CoFe2O4 nanoparticles in a single step without further processing.

We present a direct method of the preparation of pure CoFe2O4 nanoparticles via SCS reactions between metal nitrates and glycine. The entire synthesis process is completed in 10–15 min. The effects of glycine on the synthesis of CoFe2O4 nanoparticles and the magnetic properties of the obtained nanoparticles were extensively studied. The photo-Fenton catalytic and recycling performances of the synthesized CoFe2O4 were investigated for dye degradation. To improve photo-Fenton catalytic performance, NH4HCO3 and glucose were introduced as additives.

Results and Discussion

Previously reported studies have indicated that the phase and microstructure of SCS products depend on the fuel-to-oxidant ratio (φ).[25] First, the XRD patterns of the prepared products with different φ values were studied. As shown in Figure a, the diffraction peaks closely match those of cubic spinel CoFe2O4. For φ = 0.6, 0.7, and 0.8, no additional peaks can be detected for any impurity phases, and the strong diffraction peaks indicate that the CoFe2O4 products have good crystallinity. For φ = 0.9, additional peaks were located at 32 and 61.5°, which are characteristic peaks of CoO. These XRD results confirm that the parameter φ affects the phase composition of the SCS products. This is due to the different chemical energy liberated from a different fuel-to-oxidant ratio, and then, the chemical energy transforms the metal ions to the corresponding metal oxide with a different oxidation state.

Figure 1

(a) XRD patterns of the products with different φ values. (b) Local amplification of the products with different φ values.

To gain a better understanding of the formation of CoFe2O4 (φ = 0.8) during SCS, we studied the process using TG-DSC. Figure a presents the results. At 174 °C, there is an exothermic peak accompanied by a huge weight loss. It is noteworthy that the SCS process is a redox exothermic reaction between an oxidizer and fuel. In this reaction system, the ferric nitrate and cobalt nitrate acted as oxidizers, and the glycine acted as the fuel (reducer). Additionally, the glycine also acted as a complexing agent.[26] When all the raw materials were dissolved in water, the materials were in their atomic or molecular states. The glycine complexes the Fe3+ and Co2+ to form a gel, as shown Figure b. During heating, water experienced volatilization and an exothermic reaction between the oxidizer and fuel occurred at approximately 174 °C. The reaction is described in eq . The huge weight loss can be attributed to the gases (CO2, H2O, and N2) released during the SCS reaction.

Figure 2

(a) TG-DSC curves for the synthesis of CoFe2O4 particles (φ = 0.8). (b) Glycine complex of Fe3+ and Co2+.

Figure presents SEM images of the CoFe2O4 products with different φ values. All products exhibit a flocculent structure. The microstructure of the product with φ = 0.8 is presented in Figure . It is clear that this CoFe2O4 product has a two-dimensional flocculent structure (Figure a). To obtain additional information, high-resolution TEM was conducted, as shown in Figure b. One can see that the product is composed of CoFe2O4 particles approximately 25 nm in size.

Figure 3

SEM images of CoFe2O4 products with different φ values: (a) φ = 0.6, (b) 0.7, (c) 0.8, and (d) 0.9.

Figure 4

(a) TEM image and (b) high-resolution TEM image of the CoFe2O4 product (φ = 0.8).

Figure presents the hysteresis loops measured at room temperature for CoFe2O4 products with different φ values, and all the samples exhibit typical ferromagnetic characteristics. The CoFe2O4 product for φ = 0.8 has the highest magnetization saturation (Ms) value of 77.3 emu/g, which is 95.7% of the value of bulk CoFe2O4 materials value (80.8 emu/g) at room temperature.[27] The Ms values for φ = 0.6, 0.7, and 0.8 are 54.2, 59.5, and 29.9 emu/g, respectively. The Ms values for magnetic nanoscale materials depend heavily on their chemical composition and crystallinity.[28] Therefore, the high saturation magnetization value with φ = 0.8 is attributed to high crystallinity and a pure CoFe2O4 phase. For φ = 0.9, the product is composed of both CoFe2O4 and CoO, resulting in the lowest saturation magnetization. For φ = 0.6, 0.7, and 0.8, the coercive field is closely based on the pure CoFe2O4 phase in the samples. It is noteworthy that the coercive field for the φ = 0.9 sample is twice as large as that for the other values. This indicates that CoO in samples can enhance the coercive field value. This enhancement is attributed to the reduction of interparticle interactions between CoFe2O4 particles.

Figure 5

Magnetic hysteresis loops of the CoFe2O4 products with different φ values: (a) 0.6, (b) 0.7, (c) 0.8, and (d) 0.9.

The photo-Fenton catalytic activity of CoFe2O4 (φ = 0.8) was investigated based on the degradation of MB. The absorbance spectra of 100 mL of MB solutions (20 ppm) were recorded between 0 and 75 min in the presence of 5 mg of CoFe2O4 particles. The absorbance spectra are presented in Figure . The absorbance and concentration of the MB decrease gradually with increasing reaction time.

Figure 6

Absorbance spectra of 100 mL solutions of MB (20 ppm) in the presence of 5 mg of CoFe2O4 particles.

Various studies have indicated that catalytic materials with large SSAs based on the presence of mesopores can improve chemical activity.[29,30] Therefore, we adopted NH4HCO3 and glucose used as additives to form additional pores. Samples prepared with NH4HCO3 (4 g) and glucose (2 g) were labeled as N-CoFe2O4 and G-CoFe2O4, respectively. Because the CoFe2O4 particle preparation is an exothermic process, NH4HCO3 and glucose were decomposed during reaction. Additionally, the particles were prepared in air, meaning that the O2 in the air could participate in the reactions. These reactions are defined in eqs and 3.

All products in reactions are in a gaseous phase. A significant amount of gases escape during the preparation process, forming mesopores in the CoFe2O4. Figure presents the SEM micrographs of N-CoFe2O4 and G-CoFe2O4. Compared to the CoFe2O4 sample (Figure ), the N-CoFe2O4 and G-CoFe2O4 samples contain more pores. The SSAs, pore diameters, and pore volumes of the prepared samples are listed in Table . Based on the incorporation of additives for pore-forming, the N-CoFe2O4 and G-CoFe2O4 samples exhibit enhanced SSAs, pore diameters, and pore volumes. Additionally, the dispersant effect of gases can prevent the agglomeration of nanoparticles.[31] The energy generated by the reactions can be removed from the reaction system by the gases, which can inhibit grain growth. TEM micrographs of N-CoFe2O4 and G-CoFe2O4 are presented in Figure .

Figure 7

SEM micrograph of (a) G-CoFe2O4 and (b) N-CoFe2O4.

Table 1

BET parameters, Crystallite Size, and Saturation Magnetization of Samples

sample	SSA (m2/g)	pore diameter (nm)	pore volume (cm3/g)	crystallite size (nm)	Ms (emu/g)
CoFe2O4	2.72	10.56	0.00562	25	77.3
N-CoFe2O4	4.13	11.24	0.00883	20	48.0
G-CoFe2O4	52.58	11.99	0.152445	10	42.9

Figure 8

TEM micrographs of (a, b) N-CoFe2O4 and (c, d) G-CoFe2O4.

The TEM micrographs of N-CoFe2O4 (Figure a) and G-CoFe2O4 (Figure c) are consistent with the SEM micrograph (Figure a,b). The sizes of the N-CoFe2O4 and G-CoFe2O4 particles are smaller than those of the CoFe2O4 samples (Figure b). The average sizes of the N-CoFe2O4 and G-CoFe2O4 particle are 20 and 10 nm, which are observed in Figure b and Figure d, respectively. Past research indicated that glucose could be used as a template to fabricate two-dimensional nanosheets.[32] Typically, two-dimensional materials with large SSA and ultrathin characteristics provide unique physical and chemical properties.[33,34] The G-CoFe2O4 sample has a two-dimensional nanosheet structure, the highest SSA, and the smallest grain size overall. XPS spectra of the prepared samples are presented in Figure . There are no significant differences between the CoFe2O4, N-CoFe2O4, and G-CoFe2O4 samples. A survey scan of all products indicates the presence of Co 2p, Fe 2p, C 1s, and O 1s phases.[35,36] These results indicate that the phases of the prepared samples are unaffected by the additives (NH4HCO3 and glucose). However, based on an increase in pores, the Ms values of the N-CoFe2O4 and G-CoFe2O4 are reduced. The Ms values of the N-CoFe2O4 and G-CoFe2O4 samples are 48.0 and 42.9 emu/g, respectively.

Figure 9

XPS spectra of the prepared samples.

The photo-Fenton catalytic activity of the N-CoFe2O4 and G-CoFe2O4 samples was investigated based on the degradation of MB under the same conditions. As shown in Figure , 5 mg of G-CoFe2O4 degrades the MB in 40 min and 5 mg of N-CoFe2O4 degrades the MB in 25 min. One can see that the catalytic activity of CoFe2O4 is significantly enhanced by NH4HCO3 and glucose. Based on eqs and 3, this enhancement can be attributed to the large number of pores formed in the products. A mesoporous structure with a high SSA is suitable for MB molecular transfer and the generation of highly accessible multichannel reaction sites for adsorption and degradation. Degradation activity is defined by reactions (eqs –8) between the surface Fe3+ on the CoFe2O4 and H2O2.

Figure 10

Absorbance spectra of 100 mL solutions of MB (20 ppm) in the presence of 5 mg of (a) G-CoFe2O4 and (b) N-CoFe2O4 particles.

Furthermore, based on its magnetic behavior, the catalyst is easy to recycle. As shown in Figure , N-CoFe2O4 particles can be rapidly separated from the solution using an external magnet. This indicates that N-CoFe2O4 particles can be used as a recyclable catalyst. To analyze the chemical stability of the N-CoFe2O4 particles, the recycling catalytic degradation of N-CoFe2O4 particles was observed under the same conditions described above for four cycles, as shown in Figure . One can clearly see that the MB degradation efficiencies over three cycles of 25 min are 98, 96, and 95%. Therefore, the degradation activity of the N-CoFe2O4 particles exhibits no significant decrease over three cycles. These results demonstrate that magnetic CoFe2O4 nanoparticles have excellent potential as recyclable photo-Fenton catalysts for the removal of organic dyes. This facile strategy can offer an effective technique for the preparation of other binary catalysts with mesoporous structures for treatment of wastewater.

Figure 11

(a) N-CoFe2O4 nanoparticles dispersed in solution. (b) N-CoFe2O4 nanoparticles separated from the solution via magnetism.

Figure 12

Recycling photo-Fenton catalytic activity of N-CoFe2O4 for the degradation of MB solutions.

Conclusions

CoFe2O4 nanoparticles with a cubic spinel structure were successfully synthesized via SCS in one step. The fuel-to-ferric nitrate ratio determined the phase composition and magnetic properties of the CoFe2O4 nanoparticles. When this ratio was 0.8, the as-prepared product was pure CoFe2O4 with an average particle size of 25 nm. The saturation magnetization was 77.3 emu/g, which is 95.7% of the value of bulk CoFe2O4 at room temperature. Based on the presence of pores formed by NH4HCO3 and glucose additives, the photo-Fenton catalytic performance of the CoFe2O4 was effectively improved. Additionally, the synthesized magnetic CoFe2O4 catalyst exhibited excellent recyclability. The method presented in this article provides a simple and highly efficient method for the mass production of magnetic CoFe2O4 catalysts with mesoporous structures as well as other binary catalysts.

Experimental Section

Synthesis

In our experimental process, all used reagents were of analytical grade. Ferric nitrate (Fe(NO3)3·9H2O) was used as an Fe source, and cobalt nitrate (Co(NO3)2·6H2O) was used as a Co source. First, 8 g of ferric nitrate, 3 g of cobalt nitrate, and various amounts of glycine were mixed in 100 mL of distilled water in a 500 mL glass. The tunable parameter (φ) was the molar ratio between glycine and ferric nitrate (φ = 0.6, 0.7, 0.8, and 0.9). Based on the chemical propellant theory, which is the underlying principle of SCS, the stoichiometric ratio was φ = 1. The glass was then heated in the air using an electrical furnace. The heated temperature is 150 °C. As heating continued, free water evaporated and the solution formed a gel. Next, an instantaneous combustion exothermic reaction occurred. The entire process only required around 10–15 min, and the final product was prepared directly. The reaction process is illustrated in Figure .

Figure 13

Photos of the SCS reaction process: (a) original solution, (b) formed gel, and (c) final product.

Characterizations

The prepared products were characterized using X-ray diffraction (XRD) (Rigaku D/max-RB12) and X-ray photoelectron spectroscopy (XPS; PerkinElmer). Thermal characteristics were investigated under flowing air at a heating rate of 10 °C/min using thermogravimetry and differential scanning calorimetry (Rigaku, DT-40, Tokyo, Japan). The morphology and particle size of the products were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Magnetic properties were measured using vibrating sample magnetometry at room temperature. The specific surface area (SSA) was analyzed via the Brunauer–Emmett–Teller (BET) method using an automated surface area and pore size analyzer (QUADRASORB SI-MP, Quantachrome Instruments, Boynton Beach, FL).

Catalytic Activity Evaluation

The photo-Fenton catalytic activity of CoFe2O4 was investigated based on the degradation of methylene blue (MB) in water with 0.5 mL of H2O2 under simulated sunlight irradiation (150 W halogen lamp). The initial concentration of MB was 20 ppm, and 5 mg of the prepared product was used as a catalyst. The MB solution volume was 100 mL and was ultrasonically treated for 30 min in the dark. The concentration of MB over irradiation time was tested at a special wavelength (664 nm) by using a UV-1200 spectrophotometer.