Structural, Optical and Magnetic Studies of Cr2O3 Nanoparticles Prepared by Microwave-Assisted

Document Type : Research Paper

Author

Department of Chemistry, Faculty of Science, Golestan University, Gorgan, Iran

Abstract

In this paper, quasi-rectangular Cr2O3 nanoparticles were prepared by microwave-assisted solution using Cr(NO3)2.6H2O and salicylic acid at the presence of NaOH in optimum conditions for the power of 360 W, the temperature of 50 ºC and the time duration of 30 min. The as-prepared chromium precursor was annealed in an air atmosphere at two different temperatures, 500 ºC and 600 ºC, for 3 h. Quasi-rectangular Cr2O3 nanoparticles were prepared and characterized by Fourier transform infrared (FT-IR), and ultraviolet–visible (UV-Vis) spectroscopy, X-ray diffraction analysis (XRD), Transmission electron microscopy (TEM) and Vibrating sample magnetometer (VSM). The sharp peaks in FT-IR spectra and high intensity peaks in XRD patterns confirm that the single and pure phase existence of the Cr2O3 nanoparticles with an average nanoparticle size of 25 nm. The optical band gap (Eg) values of the as-prepared Cr2O3 nanoparticles calculated were in the range of 1.6 and 1.8 eV, respectively. The TEM images revealed a highly homogeneous quasi-rectangular morphology without any agglomeration. The magnetic properties of the Cr2O3 nanoparticles show weak ferromagnetic behavior.

Keywords



INTRODUCTION

Chromium (III) oxide (Cr2O3) is one of the most important transition metal oxide because of its good catalytic, magnetic, electrical and gas sensing properties as well as being one of the most stable pigments [1-12]. In addition, Cr2O3 nanoparticles have high mechanical and physicochemical properties with a wide band gap (Eg ≈ 3.4 eV) [13-15]. There are many methods to prepare Cr2O3 nanoparticles such as sol-gel, microwave, hydro- and solvo-thermal, thermal decomposition, combination membrane, pulse laser deposition, mechano-chemical and casting methods [1-12, 16-18]. Microwave-assisted synthesis is a new technique for the synthesis of various transition metal oxide nanoparticles. Recently, it is being of interest as an appropriate tool due to its short duration, highly pure nanoparticles preparation, and improved physico-chemical properties [1, 6, 19, 20]. Recent studies have shown problems for the production of Cr2O3 nanoparticles such as morphology control, purity and high thermal stability [21]. Yahyazadehfar et al. [22] prepared Cr2O3 nanoparticles using microwave-assisted synthesis in optimum conditions with highly homogeneous morphology of crystalline nanostructures, existence of single phase, high thermal stability and high specific surface. Deppak et al. [1] prepared Cr2O3 nanoparticles using Cr(acac)3 and ethylene glycol by microwave-assisted synthesis in high yield (95%). Su et al. [6] prepared spherical Cr2O3 nanoparticles using K2CrO4 as a precursor by microwave-assisted synthesis.

In this paper, continuing the previous work on the transition metal nanoparticles [23-28], Cr2O3 nanoparticles with quasi-rectangular shape (Scheme 1) were synthesized by microwave-assisted solution synthesis as an-easy-to-use and effective method. The Cr2O3 nanoparticles were then characterized by FT-IR, UV-Vis, XRD, TEM and VSM.

EXPERIMENTAL

Materials and Methods

Cr(NO3)2.6H2O, salicylic acid and NaOH were purchased from Merck company and used without further purification. FT-IR and UV-Vis spectra were carried out using a Perkin-Elmer and Jasco spectrophotometer, respectively. The X-ray patterns were conducted with a Bruker AXS diffractometer D8 ADVANCE in the range 2θ = 10o–80o. TEM images were recorded on transmission electron microscope Philips with CCD camera Olympus Veleta. The magnetic properties were investigated by Vibrating sample magnetometer.

Synthesis of Cr2O3 nanoparticles

A mixture of Cr(NO3)2.6H2O and salicylic acid (1:3 molar ratio) were dissolved into 30 mL of distilled water and stirred for 20 min at 50 ºC. Then the solution was located under microwave irradiation at optimized conditions (the power of 360 W, temperature of 50 ºC, and time duration of 30 min). An aqueous solution of NaOH was added dropwise to this solution until the pH reached 12. After cooling to r.t. the solid products were obtained by decanting, and were dried at 100 ºC. Then they were divided into two equal parts and heated at the temperatures of 500 and 600 ºC for 3 h. After that, the as-prepared Cr2O3 nanoparticles with quasi-rectangular shape were washed twice with distilled water and being dried at 80 ºC in a furnace for 24 h. Finally, the products were characterized by FT-IR, UV-Vis, XRD, TEM and VSM.

RESULTS AND DISCUSSION

FT-IR and UV-Vis spectra

FT-IR spectra of the Cr2O3 nanoparticles prepared at 500 and 600 ºC are shown in Fig. 1. Two weak and sharp absorption bands at about 410 and 442 cm-1 and two strong and sharp absorption bands at 555 and 620 cm-1 confirmed the Cr-O single bonds [29-31]. The first three bands at 410, 442, and 555 cm-1 can be attributed to the bending mode of Cr-O and the band at 620 cm-1 confirmed the presence of crystalline α-Cr2O3 [32]. The high intensity of the peaks of Cr2O3 bands indicates the good crystalline nature of the α-Cr2O3 materials [33]. The weak broad band at about 1626 cm-1 may be attributed to the presence of water molecules adsorbed at the surface of the α-Cr2O3 nanoparticles [29].The relatively strong broad band at 3398 cm-1 for the as-prepared Cr2O3 nanoparticles at 500 ºC and 3424 cm-1 for the as-prepared Cr2O3 nanoparticles at 600 ºC confirmed the O-H stretching mode of water molecules [29]. The other band at 950 cm-1 is due to the combination of lattice modes [34].

UV-Vis spectra of the Cr2O3 nanoparticles prepared at 500 and 600 ºC are shown in Fig. 2. A broad absorption peak appeared at about 390 nm and confirmed the d3 electronic transition of Cr3+ ion in the six coordination geometry and octahedral symmetry, corresponding to the 4A2g → 4T1g [9, 34]. However, a weak shoulder at about 480 nm appeared for the Cr2O3 nanoparticles prepared at 500 ºC. The optical band gap ‘Eg’ of the as-prepared Cr2O3 nanoparticles was calculated using Tauc’s equation [13, 35]:

(αhυ)2 = A(hυ − Eg)

Where A is a constant; α is the absorption coefficient; hυ is the photon energy and Eg is the optical band gap energy. The energy band gap of 1.6 and 1.8 eV for the Cr2O3 nanoparticles prepared at 500 and 600 ºC have been determined using the results of UV-Vis spectrophotometer (Fig. 3) which are lower than those reported by several authors [35, 36].

XRD patterns

Fig. 4 shows the XRD patterns of the as-prepared Cr2O3 nanoparticles at 500 and 600 ºC. It can be seen that the diffraction peaks were indexed in the rhombohedral structure with an R-3c space group and could be indexed to those of pure Cr2O3 (JCPDS card # 38-1479) [3-8, 10, 21]. The peak broadening of the patterns confirmed very small crystallines which according to Scherrer’s equation and the sharp peak appeared at 2θ ≈ 37 (110), the average crystalline size is 20 nm.

TEM images

Fig. 5 demonstrates the TEM images of the as-prepared Cr2O3 nanoparticles at 500 and 600 ºC. The images show that Cr2O3 nanoparticles mainly exhibit quasi-rectangular shape (minimum 10 and maximum 40 nm) and almost uniform in size and shape. When the temperature decomposition was increased to 600ºC, the as-prepared Cr2O3 nanoparticles became agglomerated.

VSM

The magnetic properties of the as-prepared Cr2O3 nanoparticles at 500 and 600 ºC were studied as a function of applied magnetic field and shown in Fig. 6. It was observed that the M vs. H graph is linear and no saturation magnetization can be seen even at high magnetic field [9, 11, 12], indicating paramagnetic behavior while bulk Cr2O3 is antiferromagnetic [12].

CONCLUSION

Quasi-rectangular Cr2O3 nanoparticles were synthesized and characterized. FT-IR and XRD results confirmed the single-phase existence and purity of Cr2O3 with the rhombohedral structure. The particle size was examined by TEM and indicated the nanosize of the samples. The average size of the sample is about 20 nm. The magnetic measurement indicated the paramagnetic nature of the as-prepared quasi-rectangular Cr2O3 nanoparticles.

ACKNOWLEDGMENTS

This work was supported by Golestan University.

CONFLICTS OF INTEREST

The authors announce that there are no conflicts of interest.

 

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