Zhang et al. Nanoscale Research Letters 2012, 7:312 http://www.nanoscalereslett.com/content/7/1/312 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 fur- ther 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, octade- cylamine, n-octylamine and butylamine) and 0.2 mL for- maldehyde 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 dis- persed 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. Structural characterizations The morphology and structure of the products were characterized by scanning electron microscope (SEM, Hitachi S-4800, Japan), transmission electron micro- scope (TEM, JEM-2100, Japan) and X-ray powder dif- fraction (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 Figure 1a-d shows the representative TEM images of Pd NPs prepared using oleylamine, octadecylamine, n- octylamine and butylamine, respectively. It can be seen that all of the samples consist of Pd NPs with a uniform size. These nanoparticles tended to arrange in a two- dimensionally hexagonal close-packed array due to their high uniformity. The average sizes of the as-prepared particles are 5.4, 5.0, 4.7 and 5.8 nm, respectively. By comparing the Pd NPs prepared in oleylamine or octa- decylamine, no obvious difference can be observed, indi- cating that the formation of the Pd NPs is not affected by the double bond in oleylamine. With the chain length of the organic amines changing from eighteen to four, the size of Pd NPs increase from about 5 to 5.8 nm, Page 2 of 6 which may be caused by the weaker protection of the or- ganic amines with shorter chain length. The result indi- cated that the organic amines performed as the capping- agent to protect the Pd NPs from aggregation. It should be pointed that a proper reduction rate would play a key role for the preparation of NPs with a narrow size distri- bution. A strong reducing agent (e.g., NaBH4, LiBH4) makes the nucleating process very fast, so it is difficult to obtain NPs with a uniform size [23]. Many reports showed that a weaker reductant, such as amine-borane complexes, was suitable for the syntheses of monodis- perse metallic NPs [17,18,23]. In the present case, for- maldehyde was chosen as the reducing agent, and the products showed that it also performed an effective re- ducing rate for the preparation of Pd NPs with a narrow size distribution. To reveal the growth mechanism of the as-prepared Pd NPs, the Fourier transform infrared (FT-IR) was employed to investigate the stabilizing ligands existed in the surface of the Pd NPs for stabilizing ligands or cap- ping agents usually play a key role in the formation of monodisperse nanocrystals. In the FT-IR spectra of the as-prepared Pd NPs (Figure 2), the peak located at about 1389 cm−1 is assigned to C-N stretching, indicating the presence of organic amine. The peaks located at about 2,924 and 2,853 cm−1 are assigned to C-H symmetric and asymmetric stretching of methylene. The peak located at about 3,429 cm−1 is assigned to N-H stretch- ing, and 1,622 cm−1 to N-H rocking vibrations of the or- ganic amines [24]. It should be pointed that an N-H stretching of free amines should locate at about 3,400 cm−1. The peak, which shifted to 3,429 cm−1, revealed the covalent bonding of the N-H to the Pd surface. These FT-IR spectra provided supportive evidence that organic amine exactly existed on the surface of the as- prepared monodisperse Pd NPs [24]. Therefore, we speculate that organic amine performs as a solvent and the stabilizer for formation of monodisperse Pd NPs. Formaldehyde was used to control the reacting rate which made the system tend to nucleate gently. The size of the Pd NPs could be easily controlled by the stepwise seed-mediated growth in non-aqueous solu- tion. Instead of H2O applied in the aqueous seed- mediated growth for the syntheses of Au and Pd NPs. The mixture of formaldehyde and organic amine was used as the solvent in the present study. The Pd NPs pre- pared in the organic amine solvents could be conveni- ently employed as the seed for further growth due to their uniform size and good dispersion. Figure 3a illu- strated the synthetic procedure for the stepwise seed- mediated solvothermal growth. The monodisperse Pd NPs prepared in n-octylamine were employed as the seeds for the stepwise growth in the mixture of Pd(acac)2 (5.0 mg), n-octylamine (10 mL) and formaldehyde (0.2