Microwave-Hydrothermal Synthesis of Ferric Oxide Doped with Cobalt

Ferric oxides have drawn significant interest due to their unique properties, relatively low cost, and due to their potential applications in different fields. In this work, cobalt (Co) doped iron oxide (Fe ₂O ₃) powders, with crystalline size 36.97 nm were successfully prepared using a microwave- hydrothermal process for the first time and characterised using different techniques. The morphology of the samples was characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive analysis of X-ray spectroscopy (EDAX), Fourier transform infrared (FT-IR) spectroscopy and ultraviolet-visible (UV-Vis) spectroscopy. The images show monodispersed particles with a sharp-edged square morphology. It was found that the average size was about 33.3 nm for Fe₂O₃ and 36.97 nm for Co- Fe₂O₃ . The Co atomic percentage dopants were approximately 5.73%. The nanosized synthesised materials in this study may find an application in the areas of removal of toxic metal and dyes research.

• Nanostructures
• Ferric Oxide
• Doping
• Cobalt
• Characterisation

1. Introduction

Iron is found in nature in different chemical compounds. Normally, iron has eight electrons on its valence shield, and because of oxygen’s electronegativity it can form bivalent and trivalent combinations. These have many applications in different fields such as drug delivery systems [1] , cancer treatment [2] , magnetic resonance imaging [3] , rechargeable lithium batteries, catalysis gas sensors and biosensors [4] .

Iron oxides nanocrystals have attracted increasing attention for their outstanding new properties such as their biocompatibility, catalytic activity and low toxicity. In addition, they can be easily separated and removed from a solution by simply using an external magnet. There are three different forms of Iron oxide; mainly FeO, Fe₂O₃ and Fe₃O₄. Fe₂O₃ is the most common oxide of iron and it has four crystallographic phases; namely α-Fe₂O₃ (hematite), β-Fe₂O₃, γ-Fe₂O₃ (maghemite) and ε-Fe₂O₃ [5] [6] .

Much effort has been devoted to preparing nanoparticles and many methods have been reported for fabricating metal oxides such as forced hydrolysis [7] , combustion [8] , anhydrous solvent [9] , so-gel [10] , wet chemical synthesis [11] , microwave-hydrothermal synthesis [12] and spray pyrolysis [13] . Among them, the microwave- hydrothermal process for fabrication of nanoparticles is a new technique. It is a combination of hydrothermal and microwave processes. There are many advantages for using this method such as savings in energy and time and the low temperature requirements for the synthesis of anaphase materials.

Doping of transition metal ions into Fe₂O₃ can improve the properties of nanocrystalline materials by narrowing the energy-band gap and inhibiting electron-hole recombination [4] . So far there are no reports for using the microwave-hydrothermal method for the preparation of Co-Fe₂O₃ nanoparticles; therefore this technique was used in this study to fabricate Co-doped Fe₂O₃ nanopowders. The physical properties of the prepared nanoparticles were then studied.

2. Experiment

2.1. Chemicals and Materials

Nonahydrate ferric nitrate (Fe(NO₃)₃∙9H₂O), hexhydrate cobalt nitrate (Co(NO₃)₂∙6H₂O), and absolute ethanol (C₂H₅OH) were purchased from Sigma-Aldrich (Poole, UK). Hexamethylenetetramine (HMT) was purchased from Fisher Scientific (Loughborough, UK) Distilled water was employed for preparing all the solutions.

2.2. Instruments

The microwave digestion system was sourced from CEM Corporation (North Carolina, USA), and the centrifuge from Hettich (Kirchlengern, Germany). A Scholar 171 magnetic stirrer plate was sourced from Corning stirrer (Tewksbury, USA). The oven came from F.LLI GALLI Company (Milano, Italy). The transmission electron microscopy (TEM) was from JEOL Ltd. (Welwyn Garden City, UK), and the scanning electron microscope (SEM) and energy dispersive analysis of X-ray spectroscopy (EDAX) equipment were a Cambridge S360 from Cambridge Instruments (Cambridge, UK). The FT-IR spectra were PerkinElmer RX FTIR ×2 with diamond ATR, and DRIFT attachment from PerkinElmer (Buckinghamshire, UK). The UV-Vis analysis was collected by the Shimadzu UV-2550 spectrophotometer double beam (Nakagyo-Japan).

2.3. Preparation of Co-Fe 2 O 3 Nanoparticles

The nanoparticles were prepared by the microwave-hydrothermal method using a typical procedure described in previous work [14] with some modification: 4.05 g of Fe(NO₃)₃∙9H₂O and 1.2 g of hexamethylenetetramine (HMT) were dissolved in 30 mL of distilled water and ethanol mixture (1:1, v/v) and stirred vigorously (1100 rpm) until dissolved. Then, 2.32 g of Co(NO₃)₂∙6H₂O was added to the mixture with constant stirring. After 30 minutes, the mixture was transferred into a Teflon-lined stainless-steel autoclave that was placed in the microwave at 160˚C for 90 minutes. The mixture was left to cool down to room temperature. The resulting precipitate was collected by centrifugation for 10 minutes. The Co-Fe₂O₃ nanoparticles were washed with distilled water and ethanol. Finally, the prepared nanoparticles were dried in an oven at 60˚C for 24 hours. Undoped Fe₂O₃ was also prepared using the same procedure without adding Co(NO₃)₂∙6H₂O.

2.4. Characterisation of the Fabricated Materials

The surface morphology of the prepared nanoparticles was characterised using scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In addition, the compositional analysis was studied using energy dispersive analysis of X-ray spectroscopy (EDAX). The FT-IR spectra were collected in the attenuated total reflectance (ATR) mode in the range of 500 - 4000 cm⁻¹. For UV-Vis absorption measurements, the powder samples were dispersed in deionised water with a fixed concentration (5 mg∙4 mL⁻¹) [15] . The nanomolar suspensions were prepared by milling in order to minimise the reflection of light [16] .

3. Results and Discussion

3.1. Preparation of the Co-Fe 2 O 3 Nanoparticles

Iron oxide nanoparticles (Fe₂O₃) have attracted intensive attention because they are common in nature, and are consequently eco-friendly and inexpensive [6] . In this study, a microwave-hydrothermal method was utilised to fabricate nanosized materials to decrease energy consumption, decrease preparation time from days to minutes, and simplify procedures.

Due to the competition between electron-hole pair recombination, metal doping is the perfect modification method to prevent recombination and charge carrier trapping. In literature, Mg is the most studied P-dopant for Fe₂O₃, besides Ca and Ti, which can be used for P-doping Fe₂O₃ [17] . In this work, a new doping method using Co will be discussed. Cobalt is one of the transition metal ions and it can result in higher photocatalytic activity compared with undoped Fe₂O₃. Moreover, it can improve optical activity by narrowing the energy-band gap and extend absorption to the visible region [18] .

In this study, Fe (NO₃)₃∙9H₂O and Co (NO₃)₂∙6H₂O were used as iron and cobalt sources, respectively. He- xamethylenetetramine (HMT) was used as a molecular building block for self-assembled molecular crystals [19] [20] . Figure 1 shows an image of the fabricated Co-Fe₂O₃ powders.

3.2. Characterisation of the Fabricated Materials

3.2.1. SEM Analysis

SEM analysis was used to study the surface morphology of the prepared nanoparticles. Figure 2 represents the SEM images of Fe₂O₃ and Co-Fe₂O₃ nanoparticles in different magnifications. They demonstrate that the grain size is homogeneous, polygonal and agglomerates. By comparing the micrographs, it was found that no significant morphological differences can be viewed. The agglomeration is ascribed to the removal of nanostructure-stabilising ions by washing with water [21] . Moreover, SEM analysis gives only the average grain size of the samples, which simply represents the fact that each grain is formed by aggregation of a number of nanocrystals.

3.2.2. EDAX Analysis

Energy dispersive X-spectroscopy (EDAX) was used to identify elements that exist in the prepared nanosised powders. Figure 3 shows the EDAX patterns and compositions of Fe₂O₃ and Co-Fe₂O₃. The results confirm that all the elements appear at their corresponding keV values. It was observed that there was a new peak in Co- Fe₂O₃, representing Co, Figure 3(b), which confirms doping of iron oxide (Fe₂O₃) with cobalt. Table 1 shows the atomic percentages of the nanoparticle elements, which were iron, oxygen and cobalt. It was found that the Co atomic dopant percentage was nearly 5.73%.

3.2.3. TEM Analysis

The TEM morphologies and microstructures of the prepared Fe₂O₃ and Co-Fe₂O₃ powders are shown in Figure 4. Clearly, they were composed of uniformly dispersed particles, which indicates that high disparity and uni-

Figure 1

Image of the fabricated Co-Fe 2 O 3 powder

Figure 2

SEM images of (A) (C) and (E) Fe2O3 and (B, D, and F) Co-Fe2O3 using different magnifications

Table 1

The atomic percentages for Fe 2 O 3 and Co-Fe 2 O 3

Sample	Atom%			Total
	Fe %	O %	Co %
Fe2O3	36.02	63.98	0	100.00
Co-Fe2O3	29.58	64.69	5.73	100.00

formity are achieved using this route. In addition, it was found that Co-Fe₂O₃ maintains the cubic particle structure of the undoped Fe₂O₃. The monodispersed particles had a sharp-edged square morphology with an average size range of about 33.3 nm for Fe₂O₃ and 36.97 nm for Co-Fe₂O₃. It was concluded that the average size of nanoparticles was increased slightly due to Co doping in the Fe₂O₃ lattice.

3.2.4. FT-IR Analysis

Figure 5 shows the FT-IR spectra of Fe₂O₃ and Co-Fe₂O₃. The bands cantered at 3335 cm⁻¹ and 1574 cm⁻¹ are ascribed to the O-H bonding stretching and bending vibrational modes, respectively [22] . It suggests the presence of very small amount of free and adsorbed water on the surface of the samples. In addition, a peak at around 563 cm⁻¹ is ascribed to the stretching between iron and oxygen in Fe₂O₃. The absorption band located at 523 cm⁻¹ in the Co-Fe₂O₃ nanoparticles samples was attributed to metal dopant-oxygen stretching modes. Similar observations have been documented in literature [23] [24] . The absorption bands around 1030 cm⁻¹ and 1110 cm⁻¹ are caused by the vibration of crystalline Fe-O modes, which are characteristic of Fe₂O₃ [25] . On doping,

Figure 3

EDAX spectra of (a) Fe 2 O 3 and (b) Co-Fe 2 O 3 .

(b)Figure 4

TEM micrographs of Fe 2 O 3 (A) (C) and (E) and Co-Fe 2 O 3 (B) (D)and (F)

the band at 537 cm⁻¹ shifts toward a lower frequency suggesting the possible formation of a Co-O-Fe bond. The decrease in the intensity of bands suggests the possible interaction of dopants with surface hydroxyl groups of Fe₂O₃ [26] .

3.2.5. UV-Vis Spectroscopy

It has been believed that these narrow band gap values are beneficial for the efficient utilisation of visible light for photocatalysis. The UV-Vis spectrophotometer was used to investigate the absorption regions of Fe₂O₃ and Co-Fe₂O₃, as can be seen in Figure 6. The energy band gaps (E) were calculated using the following equation [16] :

Figure 5

FT-IR spectra of Fe 2 O 3 and Co-Fe 2 O 3

Figure 6

The UV-Vis spectra for (a) Fe 2 O 3 and (b) Co-Fe 2 O 3

As can be seen in Figure 6, there was a decrease in the spectra at the absorption edge of ≈668 nm, and 586 nm for Fe₂O₃ and Co-Fe₂O₃ respectively (indicated by an arrow) [16] . The energy band gaps were calculated to be 1.86 eV for Fe₂O₃ and 2.01 eV for Co-Fe₂O₃. The reported values of the indirect band gap of Fe₂O₃ are in the range of 1.38 - 2.09 eV, the results are in good agreement with previous reports [27] . Moreover, it was observed that the absorption edge is extended towards the visible region for Co-Fe₂O₃ compared with Fe₂O₃. This phenomenon arises due to the transfer of charge from the dopant (Co²⁺) to the conduction or valence band of Fe₂O₃, which enhances visible light absorption and promotes the photocatalytic activity [27] .

4. Conclusion

Nanocrystaline transition metal doped Fe₂O₃ powders were successfully fabricated using the microwave-hydro- thermal method. The properties of the fabricated materials were investigated using different techniques, and the morphology of the fabricated nanoparticles was analysed by SEM analysis and TEM analysis. TEM images showed the Co-Fe₂O₃ powders formed 36.97 nm crystals. Moreover, the product was characterised using an EDAX analysis that confirmed doping of iron oxide with cobalt. Work is currently in progress to use nanosized fabricated materials in this work in water purification and the removal of pollutants from wastewater.

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