An Efficient and Low-Cost Method for the Purification of Colloidal Nanoparticles

Abstract

Efficient purification is critical to the fundamental and practical exploitation of nanoparticles in the technological domain. Whether for lighting, bio-imaging, coatings, photovoltaics, or displays, a routine, low-cost purification method that is directly applicable post-synthesis and in reuse/recycling would be of tremendous benefit to the commercialization of nanoparticle-based devices. Moreover, purification is essential to understanding the functional properties of nanoparticles, which are strongly dependent on purification history. In nonaqueous media, the most common technique for nanoparticle purification is the precipitation–dissolution method. This technique requires significant time and materials, including expensive centrifuges that do not scale well in an industrial setting. Meanwhile, the efficiency of this technique varies with the morphology and the nature of the nanoparticles, in particular giving lower yields with certain smaller and shaped nanoparticles. Other techniques, such as dialysis, ultra-filtration, and diafiltration, remain problematic as they rely on controlled-pore-size materials that are expensive and suffer from fouling. Additionally, these techniques, along with size-exclusion and high-performance liquid chromatography, have high solvent burdens and suffer from slow dynamics. Other interesting techniques involving microemulsions or particular ternary solvent systems may be difficult to generalize. Collectively, the purification techniques developed to date for nanoparticles in nonaqueous media have issues with cost, scale-up, applicability, and/or the lack of green processing. These issues are tremendous obstacles for efficient manufacturing and, by consequence, to the widespread development and implementation of nanoparticle-based technologies. Herein, we report an effective nanoparticle purification method based on reversible electrophoretic deposition, defined here as electropurification, which overcomes most the disadvantages of traditional techniques. Electrophoretic deposition is the deposition of colloidal particles through the application of an electric field, a widelyadopted industrial process used, for example, in automobile coatings. Electrophoretic deposition has also been applied in several embodiments to deposit nanoparticles such as CdSe to form permanent fixed coatings. Herein, however, we describe a method to reversibly deposit nanoparticles onto an electrode surface. We can use the reversible nature of this process, achieved through the addition of particular nonsolvents to the electrodeposition solution, to selectively collect and separate nanoparticles from unwanted impurities in solution. The collected nanoparticles are then redispersed into clean solvent. Figure 1 shows how a simple lab-scale setup can be used to separate and collect nearly 100% of oleic acid (OA)-capped CdTe nanoparticles from solution within a matter of minutes. The setup comprises two electrodes, an aluminum bar and a stainless steel mesh, placed in a glass beaker and connected by a glovebox feed-through to a DC power supply. The initial solution is the unpurified reaction media diluted with about 1.5 volume equivalents of acetone, a non-solvent. This reaction media contains the primary solvent (octadecene) and contaminants, such as excess OA, tributylphosphine (TBP), leftover precursors (cadmium oleate and Te-TBP), and reaction byproducts. Within minutes, an applied DC potential of 500 V causes the nanoparticles to collect on the aluminum anode, leaving a nearly colorless residual solution. The adsorbed nanoparticles are further washed with acetone while still on the electrode. Thus purified, the nanoparticles may be collected either as a solid or redispersed in a good nonpolar solvent, such as chloroform, hexane, or toluene. A real-time video demonstration, showing both the collection and redissolution of CdTe nanoparticles in less than two minutes, is available in the Supporting Information. Figure 2 shows the absorption and emission of electropurified CdTe nanoparticles as compared to those before purification and those conventionally purified by repeated precipitation–dissolution. The essentially identical first exciton and emission peak positions and spectral broadening indicate that there are no significant variations between the electropurified nanoparticles and those purified by conventional precipitation–dissolution. The H NMR spectrum of these electropurified nanoparticles (Figure 3) reveals a clean product. Broad resonances consistent with bound oleic acid are observed at d= 1.1, 1.44, and 2.3, and 5.56 ppm. There is a notable absence of resonances attributable to the cadmium [*] Dr. J. D. Bass, Dr. X. Ai, Dr. P. M. Rice, Dr. T. Topuria, Dr. J. C. Scott, Dr. H.-C. Kim, Dr. Q. Song, Dr. R. D. Miller IBM Almaden Research Center 650 Harry Road, San Jose, CA 95120 (USA) Fax: (+1)408-927-2073 E-mail: jbass@us.ibm.com qsong@us.ibm.com