Fine Crystal Structure Transition of Cobalt Nanoparticles Formed in a Microfluidic Reactor Yujun Song,*,†,‡ Tao Zhang,† Wantai Yang,§ Sacharia Albin,# and Laurence L. Henry| School of Materials Science and Engineering, Beijing UniVersity of Aeronautics and Astronautics, 100083, Center for AdVanced Micro Structures and DeVices, Louisiana State UniVersity, Baton Rouge, Louisiana 70806, College of Materials Science and Engineering, Beijing UniVersity of Chemical Technology, Department of Electrical and Computer Engineering, Old Dominion UniVersity, Norfolk, Virginia 23529, and Department of Physics, Southern UniVersity and A & M College, Baton Rouge, Louisiana 70813
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3766–3772
ReceiVed April 16, 2008; ReVised Manuscript ReceiVed June 15, 2008
ABSTRACT: The X-ray diffractions of cobalt nanoparticles synthesized by a microfluidic reactor process at low temperature suggest glasslike phases with only one broad peak. An X-ray absorption fine-structure (XAFS) technique is used to investigate fine crystal phase transition or changes in atomic arrangement and interatom spacing, during the Co nanoparticle formation by the microfluidic reactor process. The results indicate that the dynamic energy and retention time in microfluidic channels, and Ostwald ripening and quenching processes will affect the atom arrangement and the interatom spacing significantly. The hierarchies of surface energies for the main cut planes in different Co crystal structures can be used to elaborate these effects very well. The atom rearrangement and the interatom spacing shift during the nanoparticle formation are very important in the control of the nanoparticle properties via microfluidic processes, particularly for those sensitive to their fine crystal structures, such as magnetic and optical properties.
1. Introduction Phase control of nanoparticles is one of the key issues in nanoparticle synthesis since the physical and chemical properties of materials depend directly on the atom arrangement in the nanoparticles.1–9 A slight change of the atomic arrangement in the nanoscale materials may cause significant changes in the physical and chemical properties, particularly for those properties that are sensitive to the nearby atomic interaction and the atomic spacing, such as magnetic and optical properties.2,7,9 Phase transition in nanoparticles is caused by atomic rearrangement during the nanoparticle formation. The arrangement of atoms in nanostructures coherently relates both to the free energy difference among phases and to the phase transition activation energy.2,3,5 Thermodynamically, the phase transition activation energy should be overcome before the atoms move to their new positions during the transformation.3 There are two kinds of transformations in phase transition according to the distance of the atom movement, that is, diffusionless transformation and diffusional transformation. Long-range diffusion is required in diffusional transformation for the nucleation and growth of the new phases, while the atoms move only short distances, on the order of the interatomic spacing, in diffusionless transformation in order to form the new phases.2,3 There are many unique phase transitions and nuclei growths that are observed in the nano regime that are different from what occurs in their bulk species, such as lower melting temperature,10–12 unique growth along certain crystal surfaces to form different shapes, etc.13,14 And it is still not well understood why and how these unique transitions and nuclei growth occur. Monitoring of phase transition and nuclei growth in nanoparticle formation is critical in understanding the atomic arrangements in the nano regime. Synthesis procedures via solution phase reduction of metal salts * Corresponding author. E-mail:
[email protected]. † Beijing University of Aeronautics and Astronautics. ‡ Louisiana State University. § Beijing University of Chemical Technology. # Old Dominion University. | Southern University and A & M College.
are often controlled kinetically rather than thermodynamically, leading to the possibility of controlling the atomic arrangement by manipulating the reaction kinetics parameter in the formation of nanoparticles.15 During the nanoparticle formation through bottom-up processes, nanoparticles are formed by an initial nucleation stage in which tiny seed particles precipitate spontaneously from solution full of solutes (i.e., atoms or molecules) and a subsequent growth stage in which the newly formed seeds capture dissolved atoms or molecules.1 It is clear that the formation of tiny seeds in the nucleation stage and the consuming of the dissolved solutes by the formed tiny seeds in the growth stage are dominated by solute diffusion. Although the nucleation usually occurs at the nonequilibrium defects in heterogeneous nucleation, the growth of nuclei to nanoparticles can be treated as quasi-homogeneous since the surrounding of the nuclei is isotropic in the homogeneous solution. However, the transformed volume will not fit perfectly into the space originally occupied in the solution, causing formation of amorphous local areas by homogeneous nucleation. In addition, the interfacial layer between the formed solid particle and the solution is not always isotropic since the formation energy for different crystal planes is usually very different (see Supporting Information on the surface energies of different cut planes). During the nucleation and growth stages, the nuclei and the particles are active and usually in metastable stages that indicate that the shape, crystal structures, or the crystallite quality will be in unstable status. Thereby, Ostwald ripening occurs very often in the nanoparticle growth stage, represented by the depletion of surrounding smaller nanoparticles during the growth of larger nanoparticles.16 There is no doubt that the atomic rearrangement is taking place during the Ostwald ripening process. Quenching of the formed nanoparticle solution will cause the particles to precipitate from the metastable nanoparticle solution, which may introduce the aggregation of particles. The aggregation of individual particles will reduce the free energy of the system, causing fine structure changes in the nanoparticles. These processes are very important to control the fine phase
10.1021/cg8003992 CCC: $40.75 2008 American Chemical Society Published on Web 08/29/2008
Crystal Structure Transition of Cobalt Nanoparticles
Crystal Growth & Design, Vol. 8, No. 10, 2008 3767
structures of the formed nanoparticles, and the resulting chemical and physical properties. However, little attention has been given to them. Cobalt can be used as a typical example for investigation of these processes since it has three stable phases, with nearly similar formation free-energies, whose fine crystal structures depend on the phase transition and the interaction among the three phases. This feature favors formation of the isotropically arranged atoms or amorphous structures by precisely controlling the growth kinetics. Due to the easy control of the kinetic parameters (such as flow rates, reaction temperature, growth time and quenching sequence) at different stages in the nanoparticle formation, the microfluidic reactor process is used as a synthesis technique for this investigation. This process has been utilized in the synthesis of nanoparticles for size, shape and crystal structure control.4,8,17–22 However, in our previous study, the XRD could not distinguish differences in the fine crystal structures of cobalt nanoparticles synthesized at different microfluidic kinetics at room temperature by giving only one similar broad peak, similar to that of amorphous metal or alloy.23,24 Thus, in this paper, techniques such as X-ray absorption finestructure (XAFS) at two regimes, or X-ray absorption near K-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS), are used to monitor the change in atomic arrangement and the shift in atom distance using the sensitivity of the technique as a local probe of those changes.25–35 We used them to monitor the fine crystal structure transition during the nanoparticle formation using Co nanoparticles as a model, controlled by a microfluidic reactor. The sizes and shapes of nanoparticles were also correlated with these phase transitions, characterized by transmission electron microscopy.
2. Experimental Section 2.1. Materials. Tetrahydrofucan (THF, 99.90% pure packaged under nitrogen), CoCl2 (99.9%, anhydrous), lithium hydrotriethyl borate as 1 M solution in THF, 3-(N,N-dimethyldodecylammonium)-propanesulfonate (SB12), acetone (reagent anhydrous, water
