Simple Thermal Decompose Method for Synthesis of Nickel Disulfide Nanostructures

Introduction

Transition metal chalcogenide nanomaterials, e. g., CdS, ZnO, ZnS, SnO, SnS, CoS, CuS, CuS₂, FeS₂, CoS₂ and NiS₂ have received considerable attention over the years owing to the novel properties observed when compared to their bulk counterparts, which result from a quantum confinement effect [1–6]. It is known that metal sulfides exhibit interesting electronic properties, and thus have several technological applications. Many unique and interesting properties, in contrast to their bulk species, have been exhibited for this class of materials such as higher luminescence efficiency, superior mechanic toughness and lowered lasing threshold [7–10]. Among the family of metal sulfides, nickel sulfides have attracted much interest not only because the nickel sulfide system contains a number of phases, but also because of their multiple applications as a possible transformation toughened. So, different phases and morphologies of nickel sulfides sometimes coexist [11–19]. In this experimental work, we used a simple thermal decompose method for synthesis of NiS₂ nanostructures. A new nickel complex was used to prepare this material. Using metal complex and surfactant led to increase steric effect and hence the very small particles were obtained.

Experimental

Results and discussion

XRD pattern of the as-synthesized nanoparticles is shown in Figure 1. It can be seen that all of the peaks are related to NiS₂ structure with anorthic phase (JCPDS = 73-0574). As shown in this figure, the main peaks are placed at 27.5, 31.5, 35.5, 39, 45.5, 53.5, 56.5, 59, 61.5, 73, 75 and 77. Positions are related to (111), (200), (012), (112), (202), (133), (122), (320), (231), (131), (024) and (241) Miller indices. The broking of peaks in this spectrum is the nanostructure nature of the product. There are no peaks related to the other structures that show the product has high purity.

Figure 1:

XRD pattern of NiS₂ nanostructure.

The particle size and morphology of the synthesized product were characterized by SEM image (Figure 2). As shown in this figure the product is mainly composed of very tiny particles. The main reason of these small particles is the presence of PEG surfactant agent. In fact this material capped the surface of particles and prevent to aggregation in high value. Also besides the separated particles, aggregated ones were formed in the product. The main reason for this is the high surface to volume ratio of the nanoparticles. In fact by decreasing the particles size, this ratio is increased and hence the surface energy will be enhanced and therefore some of the nanoparticles aggregate together.

Figure 2:

SEM images of NiS₂ nanoparticles.

To investigate whether the surface of the nanostructures capped with the organic molecules, FT-IR analysis was used. As shown in Figure 3, there is a peak located at 1,140 cm^–1 that relate to the C–O stretching model of the PEG. Also there are two small peaks at 2,890 cm^–1 and 2,930 cm^–1 that belong to C–H stretching models of the PEG carbon chain. So it can be said that the nanostructure surfaces are capped with PEG molecules. The broad peak at 3450 cm^–1 is attributed to water molecules on the nanostructure surfaces.

Figure 3:

FT-IR spectra of NiS₂ nanoparticles.

Figure 4 Shows PL spectra of the NiS2 nanoparticles. As shown in this figure, this material shows a blue shift respect to bulk one that is due to the presence of nanoparticles. In fact, by decreasing the particles the conduction and valance band gap are expanded and hence the band gap is increased. The calculated band gap of this product was 2.34 eV that is larger than bulk one.

Figure 4:

PL spectrum of NiS₂ nanoparticles.

Conclusion

In this work, NiS₂ nanostructures were synthesized successfully via a simple thermal decompose method. PEG was used as the surfactant to decrease the particle size via steric effect. Also using metal complex around the nanostructures capped with ligand and therefore nanostructure particles cannot close together. FT-IR result showed that PEG molecules capped the nanostructures surface. Optical properties were analyzed by PL spectra. It was found that PL showed a blue shift that can be attributed to small particle size of the synthesized product.