Synthesis and Characterization of Cadmium Sulfide Nanoparticles via a Simple Thermal Decompose Method

Introduction

II–VI binary compound semiconductors have drawn considerable attention due to their interesting electronic and optical properties. They play important roles in both basic science and application fields. It has been well accepted that size and morphology of nanomaterials are crucial issues for their application and great efforts have been devoted to achieve size and morphological controllable syntheses of semiconductor nanocrystals [1]. Cadmium sulfide (CdS), a direct band gap material with E_g of 2.42 eV at room temperature, has vital applications in the optoelectronic devices [2–4]. And it receives a wide range of research interest because of their unique properties and their wide variety of potential applications. For instance, potential applications for laser light-emitting diodes, solar cells, non-linear optical, optoelectronic and electronic devices are in discussion [5–9]. During the past decades, various methods have been applied to fabricate CdS nanocrystalline, such as electrochemically induced deposition [10, 11], thermal decomposition [12], laser-assisted catalytic growth method [13], ultrasound irradiation [13], solvothermal method [14–17] and hydrothermal method [18, 19].

Most of the products have different morphologies such as dendrites [20], flakes [21], spheres [22], nanorods [23, 24], nanowires [25, 26], triangular and hexagonal plates [27], flower-like shape [28] and sea-urchin-like shape [29]. In this method, the organic surfactant, such as polyethylene glycol (PEG), coated on the surface of the particles, plays key roles in determining not only the size but also the shape of the products during the synthetic procedures; and the obtained particles are relatively stable and can be re-dispersed in nonpolar solvents easily.

Experiment

Preparation of CdS nanoparticles

CdS nanoparticles were synthesized in a three-neck flask under argon atmosphere. In a typical synthesis process, the [Cd(oct)₂]–PEG complex was prepared by reaction of 0.5 g of [Cd(oct)₂] and 8 ml of PEG. The mixed solution was placed into the flask under stirring and then 0.28 g sulfur was added to the solution. The mixture was heated up to 170 °C approximately 120 min. The color of the solution changed from green to black. The black solution was cooled to room temperature. The nanoparticles were separated upon the addition of excess ethanol and centrifuged. The samples were washed with absolute ethanol and dichloromethane and dried in vacuum oven at room temperature. Scheme 1 shows the schematic of CdS nanoparticles formation.

Scheme 1:

Schematic of CdS nanoparticles preparation.

Results and discussions

X-ray powder diffraction (XRD) pattern of the as-prepared sample is shown in Figure 1. The XRD pattern is consistent with the spectrum of CdS. The reflections of CdS can be indexed well to hexagonal CdS (space group: P63mc; JCPDS No. 06-0314). The crystallite sizes of the as-synthesized CdS, Dc, was calculated from the major diffraction peaks of the base of (110) using the Scherrer formula: Dc=K λ/β cos θ; where K is a constant (ca. 0.9) (33); λ is the X-ray wavelength used in XRD (1.5418 Å); β is the Bragg angle; θ is the pure diffraction broadening of a peak at half-height, that is, broadening due to the crystallite dimensions. The diameter of the nanoparticles calculated by the Scherrer formula is 31 nm.

Figure 1:

XRD pattern of CdS nanoparticles.

The morphology of the product is examined by SEM analysis, as shown in Figure 2. The product is composed from very tiny particles. The size of particles examined by SEM was about 30–40 nm.

Figure 2:

SEM image of CdS nanoparticles.

The main reason for decreasing the particle size of CdS nanostructure is the presence of PEG and octanoate that act as surfactant agent. In fact they cap the particle surfaces and prevent from aggregation. The TEM image of the product is given in Figure 3. The size of the nanocrystals obtained from the TEM is 30 nm.

Figure 3:

TEM image of CdS nanoparticles.

To investigate whether the surface of the nanoparticles was capped with organic surface the FT-IR of the as-synthesized samples was performed. Typically, Figure 4 shows the FT-IR spectra of CdS nanoparticles. The spectra of CdS nanoparticles show weak stretch vibration at 1150 cm^–1 attributing to the C–O stretching model of the PEG, two weak stretch vibrations at 2,920 and 2,885 cm^–1 attributing to the C–H stretching models of the PEG carbon chain indicating PEG molecules are absorbed on the surface of nanoparticles [31–38]. There was no evidence of free precursor, [Cd(oct)₂], in the sample, so the PEG serves as the capping ligand that controls growth.

Figure 4:

FT-IR spectra of CdS nanoparticles.

PL measurement was carried out at room temperature with wavelength of 392 nm (Figure. 5). The PL spectrum consists of one strong peak at 393 nm that can be ascribed to a high-level transition in CdS semiconductor crystallites. It has been reported that this kind of band edge luminescence arises from the recombination of excitons and/or shallowly trapped electron–hole pairs.

Figure 5:

PL spectra of CdS nanoparticles

Conclusion

CdS nanostructures with very tiny particles were synthesized via a simple thermal decomposition method with a new precursor. The octanoate complex and PEG acted as surfactant and increased steric effect and subsequently the particles size decreases. SEM and TEM analyses proved that the synthesized CdS was composed from very tiny particles. FT-IR spectra showed that PEG was capped on the nanoparticles surface. The optical properties were obtained from PL spectra. A strong peak was seen in 393 nm that can be attributed to recombination of excitons and/or shallowly trapped electron–hole pairs.