Trioctylphosphine as Both Solvent and Stabilizer to Synthesize CdS Nanorods
© to the authors 2009
Received: 21 March 2009
Accepted: 4 June 2009
Published: 17 June 2009
High quality CdS nanorods are synthesized reproducibly with cadmium acetate and sulfur as precursors in trioctylphosphine solution. The morphology, crystalline form and phase composition of CdS nanorods are characterized by transmission electron microscopy (TEM), high-resolution TEM and X-ray diffraction (XRD). CdS nanorods obtained are uniform with an aspect ratio of about 5:1 and in a wurtzite structure. The influence of reaction conditions on the growth of CdS nanorods demonstrates that low precursor concentration and high reaction temperature (260 °C) are favorable for the formation of uniform CdS nanorods with 85.3% of product yield.
KeywordsCdS nanorods Trioctylphosphine Size control Photoluminescence
Nanoscale one-dimensional semiconductor materials have drawn much attention due to their unique mechanical, optical, and electronic properties [1–3]. Among metal chalcogenides, cadmium sulfide is of particular interest because of its intrinsic direct band gap (2.5 eV), which has shown great potentials in bioimaging , solar energy conversion [5, 6] and photocatalysis . During the past few years, much effort has been devoted to the development of synthetic approaches for one-dimensional CdS nanomaterials. Various methods, such as anodic aluminum oxide (AAO) template-assisted synthesis , hydrothermal process , solution–liquid–solid (SLS)-assisted growth [10, 11], seeded-type growth , colloidal micellar [13, 14] and single-source molecular precursor route [15, 16] have been developed and extensively studied. However, harsh conditions, complicated reagents, usage of sacrificial templates or guiding catalysts are required in these processes, which may complicate the application of the nanostructures. Very recently, the synthesis of CdS nanorods has been achieved via thermal decomposition of two precursors in a mixture of binary surfactants [17–20]. However, all these synthetic schemes require delicate control of amount of alkylphosphonates or surfactant ratios. Until now, the monosurfactant can be used for the formation of CdS nanorods [15, 16, 21]. Although, many methods have been reported for synthesis of colloidal CdS nanorods, only Yong et al.  reported the successful synthesis of CdS nanorods using a hot colloidal method, starting with oleylamine as the single surfactant. Non-catalytic and alkylphosphonates-free, solution-based synthesis methods are more effective for low-cost and large volume production than other methods for synthesizing colloidal nanorods. In this paper, we describe a new synthesis that favors growth of CdS nanorods under nonextreme conditions. Only tri-n-octylphosphine (TOP) both as solvent and stabilizer was required for the synthesis. The method is simple and suitable for the large-scale preparation of high-quality CdS nanorods.
Materials and Methods
TOP (90%) was purchased from Alfa Aesar Chemicals and used without further purification. Sulfur powder, Cd(CH3CH2COO)2 · 2H2O and solvents used were purchased from Beijing Chemical Reagents Co.
Synthesis of CdS Nanorods
In brief, cadmium acetate (40 mg) was first dissolved in 3 mL hot TOP under N2flow. Separately, elemental sulfur powder (24.0 mg) was added to TOP (1 mL) and mixed vigorously until dissolved. Next, the sulfur solution was injected to the cadmium solution and mixed thoroughly at 260 °C. The resulting yellow solution was stirred at 260 °C for reaction 6 h, cooled to 50 °C and finally methanol was added to give a fine deposit of CdS nanorods, which was separated by centrifugation and dissolved in toluene. The above centrifugation and isolation procedure was then repeated several times for purification of the CdS nanorods.
Characterization of CdS Nanorods
The size and shape of these CdS nanorods were examined using transmission electron microscopy (TEM) and high resolution TEM (HRTEM). TEM and electron diffraction (ED) were obtained with Jeol-200CX (operated at 120 kV), and HRTEM image was obtained with FET TECNAI F30 (operating at 300 kV), respectively. The sample for TEM was prepared by placing a drop of toluene dispersion of nanords on the amorphous carbon-coated copper grids. The structure of the nanorods was investigated by X-ray diffraction (XRD) using a Rigaku D/MAX 2400 X-ray diffracometer with Cu Kαradiation (λ = 1.5405 Å). Prior to the XRD measurements, the samples were prepared by spreading several drops of CdS nanords on the glass substrate. The surface of CdS nanorods was measured by using a VG ESCALAB-5 X-ray photoelectron spectrometry (XPS) system. Optical absorption spectra were collected at room temperature on a PE Lambda 35 Ultraviolet-visible (UV–Vis) spectrometer using 1-cm quartz cuvettes. The toluene solvent was used for the background sample. Room temperature photoluminescence (PL) measurement was carried out on a SPEX Fluorolog-2 spectrometer of front face collection with 500-μm slits. PL spectra were collected between 400 and 700 nm at room temperature with 360 nm excitation powers. The Fourier-transform infrared spectra (FTIR) of the samples were recorded using a Bruker Equinox 55.
Results and Discussion
In conclusion, we have demonstrated the formation of CdS nanorods by using TOP as single-coordinating solvent. It is worth mentioning that the growth conditions are simple and also can be easily adopted for large-scale preparations. Also, no additional solvents and stabilizers are needed. This process may bring conveniences to explore the capping mechanism of nanocrystallites surface. Attempts to grow other II–VI semiconductor nanorods with the present synthesis scheme are being pursued. Theoretical calculations are also underway to gain an insight into the mechanism of rod formation.
The work is supported by the 111 Project B07012. The authors would like to express our sincere thanks to Prof. H J Gao and C M Shen from Institute of Physics (Chinese Academy of Sciences) for their assistance in the experiments and helpful discussions.
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