Synthesis of nanosize-controllable copper and its alloys in carbon shellsw

A very simple method for synthesizing nanosize-controllable (4–40 nm) metals (Cu, Ag, Pd, Rh and Ni) or alloys within carbon shells has been developed. The thickness of the carbon shell that prevents the core metals from being aggregated or oxidized ranges from 3–5 nm. In addition, the synthesized metal nanoparticles exhibit a correlation between dpZ/R (Z = valence of metal and R = atomic radius (nm) of metal) and OH (on cyclodextrins (CDs)) to M (metal) ratios of 2.3–15. This has significant implications in that many metal nanoparticles with selected sizes can be obtained at a known OH/M ratio. A dependence of melting temperature depression on the size of the nanoparticles (Cu, Ag and CuAg) within carbon shells (M@C) has been experimentally observed for the first time. Potential applications of the M@C nanoparticles in significantly enhanced heat dissipation of electrooptical devices have also been found in preliminary studies. These carbon coated size-controllable metals, multi-metals or alloy nanoparticles may form a new class of materials that certainly have many other applications in the areas of nanotechnology. Surfaces of dispersed metals with very fine particle sizes are generally active and favorable for catalysis. As particle sizes of a material are reduced to a scale of less than 30 nm, a considerable increase in surface atoms (to total atoms) may lead to significant changes in physical and chemical properties. A major consideration is whether nanosize metals can be synthesized with controllable sizes, shapes and surface properties. In general, nanosize metals synthesized by known methods have limited size and dispersion properties and are susceptible to oxidation or aggregation. It is, however, rather challenging to design a new and simple method for synthesis of size-controllable metal nanoparticles that are protected by an inert shell. Experimentally, we have found that copper present in chemical-mechanical-polishing wastewater can be recovered by chelating with b-cyclodextrin (b-CD). More remarkably, nanosize Cu can be obtained by carbonization of the b-CD-copper complex. If Cu(II) ions can be chelated with b-CD molecules to which hydroxyl groups have been appended, Cu(II) may be reduced and encapsulated in the carbon core as the b-CDs are carbonized. This phenomenon suggests the discovery of a simple method for synthesis of nanosize metals (e.g., Cu, Ag, Pd, Rh, Ni and bimetals) with any desirable size encapsulated within the carbon shells. Nanosize-controllable Cu, Ag, Pd, Ni, CuPd alloy and Cu–Ag bimetals can be synthesized with a very simple method. Briefly, a-, bor g-CDs (Wako, 498%) and metal nitrates (e.g., Cu(NO3)2 3H2O, Pd(NO3)2 2H2O, Ni(NO3)2 6H2O, Rh(NO3)3 xH2O or AgNO3) (Fluka, 499%) at OH (on CDs)to-metal (OH/M) ratio of 2.3–15 were mixed, dried at 373 K for 24 h, and carbonized under ultra-high purity nitrogen (99.99%) at 573–673 K for 2 h to yield the core–shell (M@C) nanoparticles. Structures of the samples were characterized by X-ray diffraction (XRD) (Bruker, D8 Advance) scanning from 10 to 801 (2y) at a scanning rate of 31 min . The Raman spectra of the shell carbon of the core–shell nanoparticles were determined with a 532 nm laser (Nd:YAG) at 298 K. The melting temperatures of the Cu@C, Ag@C and CuAg@C nanoparticles were measured on a thermogravimetric analyzer (TGA) coupled with differential scanning calorimetry (DSC) (TA instrument, SDT Q600) under ultrahigh purity nitrogen (100 mL min ). The morphology of the samples was studied by high-resolution transmission electron microscopy (HR-TEM) (Philips, Tecnai F20) at an accelerating voltage of 200 kV and nano-beam diffraction (NBD) patterns were also determined. The Cu K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) spectra of the core-shell nanoparticles were determined on the Wiggler BL17C beamline at the Taiwan Synchrotron Radiation Research Center. The EXAFS data of copper in the Cu@C, Cu/Ag@C and CuPd@C nanoparticles were analyzed using the UWXAFS 3.0 and FEFF 8.0 simulation programs. Fig. 1(a) shows the XRD pattern of the copper nanoparticles synthesized at an OH/Cu molar ratio of 15. The broadened peaks at 2y = 43.31 (Cu(111)) and 50.41 (Cu(200)) indicate the existence of nanosize metallic copper. Copper(I) and (II) oxide are not observed in the XRD of the carbon encapsulated Cu nanoparticles (Fig. 1). It should be noted that carbon observed at 2y = 20–301 is in the amorphous state. The XRD patterns for Ag, Pd, bimetal Cu/Ag (1 : 1) and alloy CuPd (1 : 1) also suggest the existence of their corresponding nanoparticles (Fig. 1(b)–(e)). The well dispersed and uniformsize nanoparticles of Cu, Cu/Ag and CuPd encapsulated in the amorphous carbon shells can be seen in Fig. 2. The nano-beam diffraction (NBD) patterns (shown in the upper right corner of Fig. 2(a) and (b)) of the Cu@C nanoparticles also implicate Department of Environmental Engineering, Sustainable Environmental Research Center, and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan. E-mail: [email protected]; Fax: +886 6 2752790; Tel: +886 6 2763608 Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA. E-mail: [email protected]; Fax: +1 8