An Efficient and Surface-Benign Purification Scheme for Colloidal Nanocrystals Based on Quantitative Assessment

Nano Research 1 NanoDOI 10.1007/s12274Res -015-0835-6 An efficient and surface-benign purification scheme for colloidal nanocrystals based on quantitative assessment Yu Yang, Jiongzhao Li, Long Lin, and Xiaogang Peng () Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0835-6 http://www.thenanoresearch.com on June 9, 2015 © Tsinghua University Press 2015 Just Accepted This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides “Just Accepted” as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication. An efficient and surface-benign purification scheme for colloidal nanocrystals based on quantitative assessment Yu Yang, Jiongzhao Li, Long Lin, and Xiaogang Peng* Center for Chemistry of Novel & High-Performance Materials, Department of Chemistry, Zhejiang University, Hangzhou, 310027, P. R. China All non-volatile impurities, including metal carboxylate precursors, non-volatile solvents, and non-nanocrystal side products, can be quantitatively removed from colloidal nanocrystal solutions without varying their surface coverage of ligands by a new purification scheme. An efficient and surface-benign purification scheme for colloidal nanocrystals based on quantitative assessment Yu Yang, Jiongzhao Li, Long Lin, and Xiaogang Peng () Received: day month year ABSTRACT Revised: day month year General practice of “greener methods” for synthesis of colloidal nanocrystals Accepted: day month year brings the field monodisperse nanocrystals with similar impurities, including (automatically inserted by metal carboxylate precursors, non-volatile solvents, free ligands, and the publisher) non-nanocrystal side products. These impurities seriously discounts the solution processibility and potential of applications of colloidal nanocrystals. A © Tsinghua University Press protocol was established for evaluating purification schemes. Results revealed and Springer-Verlag Berlin that commonly applied purification schemes and their variants were not of Heidelberg 2014 high-performance and could potentially damage their surface coverage of ligands. A new scheme, i.e., chloroform-acetonitrile precipitation, quantitatively KEYWORDS removed all impurities from colloidal solutions of CdSe and CdS nanocrystals colloidal nanocrystals, coated with a variety of carboxylate ligands. The new scheme was found to be purification, metal benign to the surface structure of the nanocrystal-ligands complexes, which carboxylate, ligands, resulted in nanocrystals with a close-packed monolayer of carboxylate ligands. cholorform/acetonitrile Address correspondence to Xiaogang Peng, [email protected] 2 Nano Res. 1 Introduction of the composition and band structure in the following growth steps [23]. Colloidal nanocrystals are single crystalline The so-called ‘greener methods’ for synthesis fragments of the corresponding bulk crystals of colloidal nanocrystals has promoted usage of with their sizes in nanometer regime and metal fatty acid salts as the most common metal processed in solution [1-3]. With rapid progress precursors in organic solutions [5]. As a result, in the past 20 years [4, 5], numerous types of metal fatty acid salts become a general impurity colloidal nanocrystals can be synthesized in in colloidal solutions of nanocrystals, including solution with great control of their size and shape, metal [24-27], oxide [28-32], semiconductor [5] especially for the mostly studied semiconductor ones. Structurally, metal fatty acid salts and nanocrystals. In the recent years, discoveries of colloidal nanocrystals coated with organic “focusing of size distribution” and “self-focusing ligands are quite similar, i.e., either an inorganic of size distribution” enabled formation of nearly ion or an inorganic crystal as the core and monodisperse colloidal nanocrystals in solution hydrocarbon chains as the periphery. This makes [4, 5]. This makes size-selective purification separation of metal fatty acid salts from the unnecessary in most cases. However, when corresponding nanocrystals be challenging. To studies extend beyond a certain limit, unreacted our knowledge, systematic and quantitative precursors, free ligands, non-volatile solvents, documentation on purification of colloidal and non-nanocrystal side products might become nanocrystals is scare. a serious issue. Studies related to surface The first aim of this work was to establish chemistry of colloidal nanocrystals [6-12] can be methods for quantitative study of available strongly interfered by some of these impurities. purification procedures using CdSe Removal of metal carboxylate salts—most nanocrystals—the most studied colloidal common precursors used today for synthesis of nanocrystals—as the model system. The second colloidal nanocrystals due to introduction of aim was to develop a new purification scheme “greener methods” [5]—is a prerequisite for that is highly efficient, simple, and benign to the fabrication of high performance surface ligands. Given the similarity between light-emitting-diodes based on quantum dots [13]. different types of colloidal nanocrystal systems At present, colloidal nanocrystals often require available today, this new scheme should be multiple synthetic steps to build complex generally applicable to many colloidal structures with multiple components, such as nanocrystals synthesized using metal fatty acid bandgap and composition engineering on single salts as the precursors and/or fatty acid as colloidal semiconductor nanocrystal [14]. In fact, ligands. nearly all colloidal semiconductor nanocrystals applied for optoelectronic devices [13, 15], 2 Results and discussion biomedical labeling [3, 16, 17] and lasers [18] are in the form of core-shell configuration, which are Characterization of the model system prior to products of bangdap and composition purification. The CdSe nanocrystals were engineering[19-23]. To build such complex synthesized using a method reported in literature with octadecene (ODE) as the solvent, cadmium nanocrystals, residuals of the precursors and stearate (Cd(St)2) as the cadmium precursor, and ligands in the solution of initial seed nanocrystals Se powder suspended in ODE as the Se precursor often cause serious problems for precise control | www.editorialmanager.com/nare/default.asp NanoRes. 3 [33]. For certain sizes, addition of fatty acid as known molar ratio (nHSt/Cd(St)2). For the FTIR ligands were needed [33]. It is well-known that, spectrum in Figure 1(c), ACOOH/COO- was calculated when the size (or diameter) of CdSe nanocrystals to be 1.03, which corresponded to nHSt/Cd(St)2 is in the quantum confinement regime, both being 1.91 for the raw reaction mixture prior to absorption and photoluminescence spectra can be applied for determining the size and size any purification. This means that, for a reaction distribution of a given sample [34, 35] which is starting with 0. 20 mmol of Cd(St)2 and 0.10 demonstrated by the sharp features in the mmol of Se, there was ~0.11 mmol of Cd(St)2 absorption spectrum and narrow PL band in leftover in the reaction solution and ~0.09 mmol Figure 1(a). The nearly monodisperse size of Cd(St)2 was converted to CdSe nanocrystals. distribution of the example in Figure 1(a) is Furthermore, these results imply that Se should further confirmed by transmission electron microscopy (TEM) in Figure 1(b). The size of the be consumed almost completely, which was nanocrystals in Figure 1(b) was 3.0 nm in found to be consistent with early reports [33, 36]. diameter with ~7% standard size deviation. CdSe nanocrystals with different sizes with similar Purification schemes. Two popular purification optical and structural properties synthesized schemes were reported in literature to remove using this protocol were documented in literature impurities from the solution of colloidal [33]. nanocrystals. Murray et. al. [34] demonstrated a The reaction mechanism for formation of purification scheme by precipitation of colloidal CdSe nanocrystals in the current protocol was nanocrystals from the solution by addition of a illustrated as activation of elemental Se by the non-solvent, and it was further applied to hydrocarbon solvent to form H2Se followed by separate nanocrystals with different sizes by the rapid reaction of H2Se with Cd(St)2 [33]. size-selective precipitation. The most common Along with formation of one molecular unit of pair of solvent and non-solvent has been toluene CdSe as the main product, two molecules of and methanol though other combinations, such stearic acid (HSt) should be generated. The as CHCl3 or hexanes as the solvent and methanol synthesis of CdSe

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