Synthesis of size-controlled monodisperse Pd nanoparticles via a non-aqueous seed-mediated growth
© Zhang et al.; licensee Springer. 2012
Received: 27 April 2012
Accepted: 8 June 2012
Published: 19 June 2012
We demonstrated that stepwise seed-mediated growth could be extended in non-aqueous solution (solvothermal synthesis) and improved as an effective method for controlling the uniform size of palladium nanoparticles (Pd NPs) in a wide range. The monodisperse Pd NPs with the size of about 5 nm were synthesized by simply reducing Pd(acac)2 with formaldehyde in different organic amine solvents. By an improved stepwise seed-mediated synthesis, the size of the monodisperse Pd NPs can be precisely controlled from approximately 5 to 10 nm. The as-prepared Pd NPs could self assemble to well-shaped superlattice crystal without size selection process.
KeywordsNanocrystalline materials Crystal growth Pd Non-aqueous seed-mediated growth Self-assembly
As an important catalyst and a promising alternative of Pt-based catalysts, Pd has received continual interest in the past decades [1–4]. For example, Pd nanoparticles (Pd NPs) can be used as efficient catalysts for a large number of carbon-carbon coupling reactions [5, 6] as well as hydrogenation/dehydrogenation reactions [7, 8]. Pd NPs have also been demonstrated to exhibit efficient electrocatalytic activities in fuel cell reactions [9, 10]. As one of the solid heterogeneous catalysts, the performance of Pd strongly depends on the size and/or shape, especially in nanometer scale [11, 12]. Therefore, synthesis of monodisperse Pd NPs with uniform shape has been an important subject of chemical research.
Many methods, especially solution-based synthesis with various protecting agents, have been developed to prepare monodisperse Pd NPs [13–15]. However, most of the reported capping agents, which were used to stabilize Pd NPs and prevent their aggregation, have drawbacks under harsh catalytic reaction conditions because a ‘clean’ surface of the nanoparticles was difficult to get due to the presence of bulky or strongly binding surfactants around these nanoparticles . Recently, several groups have succeeded in synthesizing monodisperse Pd NPs with a size smaller than 5 nm by applying oleylamine as the capping agent [17, 18]. However, it remains a challenge to control the size of the Pd NPs in a wider range while keeping their uniformity.
Seed-mediated growth was widely adopted in synthesis of nanoparticles with different size, but most of them were applied in aqueous solution [19–22]. There were few research reports concerning the seed-mediated growth in non-aqueous solution. In this communication, the stepwise seed-mediated growth was extended in non-aqueous solution (solvothermal synthesis) and improved as an effective method for controlling the uniform size of Pd NPs in a wide range. The seeds were prepared by simply reducing palladium(II) acetylacetonate (Pd(acac)2) with formaldehyde in different organic amine solvents. The size of the monodisperse Pd NPs can be precisely controlled from about 5 to 10 nm. In addition, we concluded that amine groups played critical role in stabilizing the Pd NPs.
The synthesis was carried out using commercially available reagents. Palladium(II) acetylacetonate, octadecylamine (98%) and n-octylamine (99%) were purchased from Alfa Aesar (Beijing, China), while oleylamine (80% to 90%) was from Acros Organics (Shanghai, China). Butylamine and formaldehyde were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals were used as received without further purification.
Synthesis of 5-nm monodisperse Pd NPs
Typically, 5.0 mg Pd(acac)2 was dissolved in the mixture of 10 mL of organic amine (such as oleylamine, octadecylamine, n-octylamine and butylamine) and 0.2 mL formaldehyde to form growth solution. The growth solution was then transferred to a 25-mL Teflon-lined autoclave and maintained at 200°C for 3 h. The product (about 5 nm Pd NPs) was separated by centrifugation (12,000 rpm for 10 min). The product was then dispersed in hexane.
Stepwise seed-mediated solvothermal growth
As for the preparation of Pd NPs of about 8 nm, the as-prepared 5-nm Pd NP solution without separation was employed as the seed, and a seed-mediated growth was carried out by mixing 2-mL seed solution and the growth solution, followed by solvothermal treatment at 200°C for 3 h. Similarly, about 10-nm Pd NPs were synthesized using unseparated 8-nm Pd NPs as growth seeds by keeping the same growth condition.
The morphology and structure of the products were characterized by scanning electron microscope (SEM, Hitachi S-4800, Japan), transmission electron microscope (TEM, JEM-2100, Japan) and X-ray powder diffraction (XRD, Panalytical X-pert diffractometer with Cu-Kα radiation; PANalytical B.V., Netherlands). The samples for IR spectra measurements (Nicolet 330) were obtained by depositing the ethanol suspensions of Pd NPs on KBr substrate followed by solvent evaporation.
Results and discussion
In summary, monodisperse Pd NPs were prepared via a facile solvothermal synthesis using organic amine as solution and stabilizing agent. The size of the Pd NPs could be controlled from about 5 to 10 nm by an improved stepwise seed-mediated method. The as-prepared Pd nanoparticles could self assemble to well-shaped superlattice crystal without size selection process. Organic amine was proved to play a critical role in the formation of monodisperse Pd NPs.
Fourier transform infrared
- Pd NPs:
scanning electron microscopy
transmission electron microscope.
This work was supported by the National Basic Research Program of China (grant no. 2011CBA00508), the National Natural Science Foundation of China (grant no. 21171141), and the National Fund for Fostering Talents of Basic Science (grant no. J1030415).
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