Modified Sol-Gel Processing of NiCr₂O₄ Nanoparticles; Structural Analysis and Optical Band Gap

Structural analysis

The vibration spectrum of NiCr₂O₄ nanoparticles (Figure 1) was obtained by exposing the sample to infrared radiation (FT–IR, Perkin Elmer spectrum RX1, ϑ=4,000 to 400 cm⁻¹) and recording the variation of the absorption with frequencies of metal-oxygen (M-O) and metal-metal (M-M) bonds. Sequentially absorptions at 616 and 495 cm⁻¹ confirm the formation of M-O (Cr-O and Ni-O) bonds. Moreover, a single and sharp absorption at around 883 cm⁻¹ indicates the metal-metal (Ni-Cr) vibrational frequencies.

Figure 1:

FTIR spectrum of NiCr₂O₄ nanoparticles.

The formation of NiCr₂O₄ nanoparticles with an appropriate structure was analyzed using Rigaku X-ray diffractometer (XRD, PTS 3003, Cu-Kα 30 kV & 20 mA, angle  =10–80°). Figure 2 shows the XRD pattern of NiCr₂O₄ nanoparticles. The XRD pattern of NiCr₂O₄ nanoparticles is clearly supported with literature (JCPDS 75–1728) without any minor phase. The mean nanoparticles diameter was calculated using Scherrer equation (1) from XRD pattern according to the line width of the (311) plane reflection peak,

where θ and λ are the angle and wavelength (1.5418 Å), respectively and β is the width of the XRD peak at half height. k is a shape factor (~0.9 for spherical-shaped nanoparticles). The particle size was calculated at about 24 nm. This is obviously supported by SEM size measurements (~27 nm).

Figure 2:

XRD pattern of NiCr₂O₄ calcined at 1,073 K for 4 h.

A general complementary technique for structural analysis of NiCr₂O₄ nanoparticles observed by 3D scanning of the sample using a scanning electron microscope (SEM, KYKY-EM 3200). The surface scanning of the nanoparticles (Figure 3) indicates that a narrow distribution of the particles with an average grain size about 27 nm.

Figure 3:

SEM of the NiCr₂O₄ calcined at 1,073 K for 4 h.

Magnetic property

Vibrating sample magnetometer (VSM) system was used to measure the magnetic properties of NiCr₂O₄ nanoparticles (Figure 4) as a function of magnetic field. The magnetization parameter obtained ~0.2 emu/g in 8kOe applied field at 25 °C. It seems that the NiCr₂O₄ nanoparticles are paramagnetic; however, the nickel chromite in bulk is known as ferromagnetic materials [28].

Figure 4:

VSM curve of the NiCr₂O₄ nanoparticles.

Diffuse reflectance spectroscopy

The optical band gap of NiCr₂O₄ nanoparticles carried out by diffuse reflectance spectroscopy (DRS, SCINCO S4100). DR spectra of synthesized NiCr₂O₄ nanoparticles were obtained between 200 and 1,000 nm with a DRS spectrophotometer shown in Figure 5. This curve shows a sharp absorbance peak around 310  nm.

Figure 5:

DRS of NiCr₂O₄ nanoparticles.

The band gap, E_g, and absorption coefficient of a semiconductor are related to Tauc model [29]. In general, for direct band gap determination, plot of (αhν)² versus hν is linear at higher values of (hν)², but tends to deviate from linearity as hν approaches a lower value. Band gap value was obtained by extrapolating the straight portion of the graph, as indicated by solid line. The optical absorption curve of NiCr₂O₄ nanoparticles obtained from raw data’s of DRS results is shown in Figure 6. The optical absorption curve has been measured and the absorption edge shows that the band gap of NiCr₂O₄ is about 1.7 eV; hence, NiCr₂O₄ nanoparticles are semiconductor material applicable for photoelectric devices.

Figure 6:

The optical band gap of NiCr₂O₄ nanoparticles.

Colorimetric coordinates (L*.a*.b*)

The most frequently used modern system is the CIE (L* a* b*) space which provides an effective means for visualizing color: the L* – axis represents lightness-darkness (ranging from 0 to 100), the a* – axis redness-greenness, and the b* axis yellowness-blueness. The color parameters (L*.a*.b*) of NiCr₂O₄ nanoparticles identified by Reflectance Spectrophotometer (RS, Ihara-spcam spectrophotometer) adopted by the Commission Internationaled’Eclairage (CIE) where L* is the luminance or lightness component, ranging from 0 to 100, a* (from green to red) and b* (from blue to yellow) are two chromatic components (–120 to+120) [30]. The L*.a*.b* color parameters of NiCr₂O₄ nanoparticles obtained in this study from reflectance spectroscopy are shown in Table 1. The relatively high values of L* (31.58) along with low a* and b* confirm full light absorption (Figure 7).

Figure 7:

Arrangement of color attributes in the CIE 1976 (L*.a*.b*) color space.