Shape and phase control of CdS nanocrystals using cationic surfactant in noninjection synthesis
© Zou et al; licensee Springer. 2011
Received: 4 March 2011
Accepted: 6 May 2011
Published: 6 May 2011
Monodispersed CdS nanocrystals with controllable shape and phase have been successfully synthesized in this study by adding cationic surfactant in noninjection synthesis system. With the increase of the amount of cetyltrimethylammonium chloride (CTAC) added, the shape of the CdS nanocrystals changed from spherical to multi-armed, and the phase changed from zinc-blende to wurtzite. It was found that halide ion Cl- plays a key role in the transformation, and other halide ions such as Br- can also induce similar transformation. We proposed that the strong binding between Cd2+ and halide ions reduced the reactivity of the precursors, decreased the nuclei formed in the nucleation stage, and led to the high concentration of precursor in the growth stage, resulting in the increase of size and phase transformation of CdS nanocrystals. In addition, it was found that the multi-armed CdS nanocrystals lost quantum confinement effect because of the increase of the size with the increase of the concentration of CTAC.
Colloidal semiconductor nanocrystals have received considerable attention because of their size-dependent properties and applications in fields such as optoelectronic devices [1–3] and biological fluorescence labeling [4, 5]. Among those colloidal semiconductor nanocrystals, CdS nanocrystals with controllable size have been successfully synthesized in noncoordinating solvents [6–8], and their physical properties have been extensively investigated. Apart from the size control of the nanocrystals, more and more attention has been devoted to the control of the shape and phase of the nanocrystals [9–15], which can also influence the physical properties of the nanocrystals . Based on the successful synthesis of CdS nanocrystals with various morphologies, the nanocrystals have been introduced into nanocrystal photovoltaic devices [17, 18].
For nanocrystal photovoltaic application, semiconductor nanorods or branched nanocrystals are preferable to quantum dots because they naturally provide direct electron transport path which can enhance charge collection efficiency . Peng and co-workers [9, 10] have successfully developed a high temperature organometallic method to synthesize CdS nanorods, which has been expanded to synthesize CdS nanowires [11–13]. In this method, alkylphosphonic acid is used as a ligand. Hyeon and colleagues  proposed an alternative method to synthesize rod-shaped CdS nanocrystals based on injection of S-oleylamine into hot Cd-oleylamine precursors. Yong et al.  investigated the synthesis of rod-shaped CdS nanocrystals in oleylamine systematically and obtained CdS nanocrystals of different shapes by adding different surfactants. Although great advancement has been made in the synthesis of rod-shaped CdS nanocrystals, there still exist some drawbacks in the current methods. First, the chemicals used, such as alkylphosphonic acid and oleylamine, are quite expensive. Second, the current methods are based on hot injection which limits the yield of CdS nanocrystals. Low-cost synthesis of rod-shaped CdS nanocrystals for photovoltaic application in high yield is still a challenge.
Recently, we have proposed a noninjection method to synthesize high-quality size-controllable CdS quantum dots in noncoordinating solvents without nucleation initiators . All the source chemicals used are air-stable and inexpensive. Moreover, hot injection is avoided in this method, and the size of CdS nanocrystals can be easily tuned by changing the concentration of oleic acid (OA) which serves as a ligand. However, the CdS nanocrystals obtained have cubic zinc-blende structure which is highly isotropic, while rod-shaped CdS nanocrystals always have hexagonal wurtzite structure. Therefore, improvements are needed to synthesize rod-shaped CdS nanocrystals using this noninjection method.
In this article, we propose the shape and phase control of CdS nanocrystals based on a noninjection method. We found that addition of cationic surfactant cetyltrimethylammonium chloride (CTAC) could induce the phase transformation of the CdS nanocrystals from cubic to hexagonal structure, resulting in the formation of multi-armed CdS nanocrystals. It was found that halide ion Cl- plays a key role in the transformation, and other halide ions such as Br- can also induce similar transformation. Compared with alkylphosphonic acid and oleylamine, CTAC is quite economic. Therefore, we believe that this approach is suitable for large-scale production of branched CdS nanocrystals for photovoltaic application.
Cadmium oxide (CdO, 99.5%) and sulfur (99.5%) were purchased from Shanghai Chemical Reagent Ltd.(Shanghai China). OA (tech. 90%), 1-octadecene (ODE, tech. 90%), and cetyltrimethylammonium hydroxide (CTAOH) were purchased from Aldrich (St. Louis, MO, USA). CTAC and cetyltrimethylammonium bromide (CTAB) were purchased from Aladdin Chemistry (Shanghai China). All the chemicals were used as received without any further purification.
A mixture of CdO (1 mmol), OA (6 mmol), and 15 mL ODE was degassed at room temperature for 15 min in a three-necked flask and then heated to 280°C under Ar flow. After the CdO was totally dissolved and the solution turned clear to form a Cd-oleate precursor solution, the temperature of the solution was lowered to about 30°C. 0.5 mmol S and cationic surfactant (CTAC, CTAB, or CTAOH) with different amounts were added into the flask. Then, the mixture was heated to 240°C at a rate of approx. 20°C/min and reacted at this temperature for 60 min under a flow of argon gas. The resultant nanocrystals were collected by precipitation with excess ethanol followed by centrifugation, and the product could be redispersed in hexane for characterization.
Transmission electron microscopy (TEM) images were collected using a Philips CM200 transmission electron microscope operating at 160 kV. High-resolution TEM (HRTEM) measurements were carried out using a JEOL 2010F equipment operating at 200 kV. A sample for TEM analysis was prepared by drying a drop of nanocrystal hexane solution on a carbon-coated copper grid and letting it dry in air. X-ray powder diffraction (XRD) was conducted on an X'Pert pro X-ray diffractometer. The samples were prepared by drop casting the nanocrystal hexane solution on glass slides. Ultraviolet-visible (UV-Vis) absorption spectra were recorded on a U-4100 Spectrophotometer.
Results and discussion
Recently, other research groups also found that halide ions could influence the growth of CdS nanocrystals significantly. Saruyama et al.  reported zinc-blende CdS seed nanoparticles could be transformed to wurtzite nanorods through an Ostwald ripening process induced by Cl- and the surfactants OA and oleylamine. Tai et al.  showed that wurtzite CdS nanoparticles could be synthesized by the addition of NaCl in an ultrasound-assisted microwave synthesis system. Therefore, it is believed that the presence of Cl- favors the formation of wurtzite CdS. According to the "hard-soft acid-base model,"  Cd2+ is a soft acid, and Cl- is a soft base. Soft acids bind strongly to soft base, so Cl- binds strongly to Cd2+, which reduces the reactivity of the precursors. As a result, the nuclei formed in the nucleation stage decrease, and the precursors remained for the growth of the nuclei increase, leading to the formation of larger CdS nanocrystals. Earlier study has proven that the structure of CdS nanocrystals is size dependent [31–33]. The zinc-blende structure is dominant at small diameter (approx less than 4 nm), and wurtzite structure is more important at large diameters (approx greater than 5 nm). Therefore, the phase transformation from zinc-blende to wurtzite happens with the addition of CTAC or CTAB. Moreover, at high precursor concentration, relative difference between the growth rates of different faces can lead to rod-shape nanocrystals . However, in contrast with above mentioned study, we obtained nanocrystals with more complex structure consisting of zinc-blende cores and wurtzite arms. The formation of zinc-blende cores can be attributed to the magic-sized nuclei formed at the nucleation stage. It has been found that these nuclei all have zinc-blende structure which is more suited than the corresponding wurtzite structure to the formation of closed-shell formations and thus are more stable . Unfortunately, we failed to separate these nuclei because of the high reaction rate. It was observed that multi-armed CdS nanocrystals had been formed when the reaction temperature reached 200°C. When the nanocrystals is more than 4 nm in diameter, the growth of wurtzite phase is more favored. At high precursor concentration, the faster growth rate of (0001) plane leads to the growth of wurtzite arms. As a result, the multi-armed CdS nanocrystals with zinc-blende cores and wurtzite arms are formed.
We report the successful shape and phase control of CdS nanocrystals using cationic surfactant CTAC in noninjection synthesis. Multi-armed CdS nanocrystals with zinc-blende cores and wurtzite arms are synthesized economically, which are suitable for nanocrystal photovoltaic application. It is confirmed that Cl- must be responsible for the phase transformation from zinc blende to wurtzite, and other halide ions such as Br- can also induce similar transformation. It is believed that this scheme can be extended to synthesize other semiconductor nanocrystals with complex structure.
transmission electron microscopy
X-ray powder diffraction.
The authors express their appreciation for the support provided by the Fundamental Research Funds for National Basic Research Program of China (973 Program) (No. 2007CB613403), the Central Universities (Program No. 2010QNA4001), and the Natural Science Foundation of Zhejiang Province (Y1080068).
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