Thermal Treatment Method for Synthesis and Characterization of the Octahedral Magnetic Nanostructures of Co₃O₄ from a New Precursor

Materials and methods

Sodium hexanitrocobaltate (III) and hexamethylenetetramine (HMTA) were purchased from Merck. All chemicals and solvents were of high purity and used without any further purification. Elemental analyses were performed by using a Leco, CHNS-932 elemental analyzer. Fourier transform infrared (FTIR) spectra were recorded on a FT-IR JASCO 460 spectrophotometer in the region of 4,000–400 cm⁻¹ using KBr pellets. Electronic absorption spectrum was recorded on a JASCO 7580 UV–Vis–NIR double-beam spectrophotometer. Quartz cuvettes of 10 mm path length were used for spectrophotometric experiments. X-ray diffraction (XRD) patterns were recorded by a Philips X-ray diffractometer using Ni-filtered Cu Kα radiation. Scanning electron microscope (SEM) images were obtained using a LEO instrument model 1455VP. Prior to taking images, the samples were coated by a very thin layer of Pt to make the sample surface a conductor and prevent charge accumulation and obtaining a better contrast. Room temperature magnetic properties were studied using an alternating gradient force magnetometer (AGFM) device, made by Meghnatis Daghigh Kavir Company in an applied magnetic field sweeping between ±10,000 Oe.

Synthesis approaches

Synthesis of trans-Na[Co(HMTA)₂(NO₂)₄].H₂O

Aqueous sodium hexanitrocobaltate(III) solution was prepared by dissolving Na₃[Co(NO₂)₆] (2.02 g, 5 mmol) in 20 mL double distilled water. HMTA solution was prepared by dissolving HMTA (1.4 g, 10 mmol) in 20 mL of double distilled water. Then sodium hexanitrocobaltate (III) solution was added to the HMTA solution, subsequently the reaction mixture was refluxed for 5 h and then stirred in air for 4 h. The resulting precipitate, trans-Na[Co(HMTA)₂(NO₂)₄].H₂O (Figure 1), was filtered and washed with cold water to remove the ionic impurities. Anal. Calc. for NaCoC₁₂H₂₆N₁₂O₉ (MW=564.33 g/mol): C, 25.54; H, 4.64; N, 29.78. Found: C, 25.34; H, 4.61; N, 30.20%. FTIR (KBr pellet, cm⁻¹): 3,430, 1,655 (H₂O), 2,960, 1,470, 1,266 (CH₂), 2,713, 1,402, 835 (N–O), 1,240, 1,009 (C–N), 686 (Co–N). UV–Vis (dimethylformamide, DMF, λ_max/nm (ε/M⁻¹cm⁻¹)): 620 (410), 314 (3,140).

Figure 1:

Structure of trans-Na[Co(HMTA)₂(NO₂)₄].H₂O.

Analysis of trans-Na[Co(HMTA)₂(NO₂)₄].H₂O complex

The FTIR spectrum of trans-Na[Co(HMTA)₂(NO₂)₄].H₂O can be separated into three different regions (Figure 2(a)). The first high-energy region, in the range of 3,450–2,700 cm⁻¹, corresponds to ν(O–H) of free water molecules around 3,430 cm⁻¹ [22], ν(C–H) of methylene groups in HMTA molecules at 2,960 cm⁻¹ and ν(N–O) of nitrite groups around 2,713 cm⁻¹ [23]. In the second region in the range of 2,500 to 1,500 cm⁻¹, the following bands are observed: an absorption band of δ(O–H) at 1,655 cm⁻¹ and a series of bands at 1,470, 1,266, δ(C–H), 1,402, 835, 812 cm⁻¹δ(N–O)_, 1,240 and 1,009 cm⁻¹ν(C–N). The absorption band at 686 cm⁻¹ is assigned to the Co–N vibration (Table 1) [22].

Figure 2:

(a) FTIR spectra for trans-Na[Co(HMTA)₂(NO₂)₄].H₂O. (b) Electronic spectrum of Na[Co(HMTA)₂(NO₂)₄].H₂O (1×10⁻⁴ M) in DMF.

The electronic spectrum of trans-Na[Co(HMTA)₂(NO₂)₄].H₂O was recorded in DMF solution (Figure 2(b)). The absorption bands in the visible region (520–680 nm) are assigned to ligand field (d–d) transitions. Also, a sharp band in the UV region at 314 nm is assigned to the intraligand transition [24].

Analysis of Co₃O₄ nanostructures

As shown in Figure 3 with increasing the reaction temperature from 600 to 700°C the morphology of product was not changed but with increasing the temperature the agglomerated octahedral nanostructures were destroyed and formed smaller octahedral nanostructures; the average size of nanostructures at 600°C was about 100 nm, although at 700°C the octahedral nanostructure size was about 40 nm.

Figure 3:

SEM images of as-prepared (a) thermal treatment in 600°C, (b) 700°C.

Figure 4(a) shows that the XRD pattern of as-prepared Co₃O₄ after thermal treatment at 700°C. All of the diffraction peaks can be classified as pure cubic phase (space group: Fd3m) with lattice parameters a = b = c = 8.084, being very close to the values in the literature (JCPDS Nos. 74–2120), and no typical peaks of impurities were distinguished.

Figure 4:

(a) XRD pattern (b) FTIR spectra (c) VSM Magnetization versus applied at 300K of as-synthesized Co₃O₄ nanostructures on 700 °C.

The crystallite size measurements were also carried out using Scherrer equation:

where K is the so-called shape factor, which usually takes a value of about 0.9, λ depicts the wavelength of Cu Kα radiation and β is the breadth of the observed diffraction line at half intensity maximum (FWHM). By using the (311) peak, the average crystallite size of the obtained Co₃O₄ was about 39 nm.

Figure 4(b) depicts the FTIR spectrum of as-synthesized Co₃O₄. The absorption peaks at 586, 666.24 and 1,445 cm⁻¹ are allocated to the ν(Co–O) modes, which proves the formation of Co₃O₄ nanostructures. The peaks at 3,390 and 1,628 cm⁻¹ are ascribed water molecules on the external surface of the sample in handling to record the spectra [25].

The hysteresis loops (Figure 4(c)) measured at room temperature using an AGFM device show a ferromagnetic behavior of the Co₃O₄ nanostructures. The remnant magnetization (M_r) is 2.3 emu/g, the coercive field (H_c) is 305.3 Oe and the magnetization at saturation (M_s), which is determined by the extrapolation of curve of H/M versus H, is estimated to be only 8.69 emu/g [20].

Scheme 1(a) describes the synthetic approaches for preparation of the Co-complex and Scheme 1(b) illustrates that the HMTA capped the Co and caused the orientation growth.

Scheme 1:

(a) Schematically synthesis of Co-complex (b) Schematically mechanism of formation octahedral Co₃O₄ nanostructures.