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

Just Accepted

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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

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[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 nanocrystals was generally or acetone as the non-solvent, could also be performed with Cd rich to avoid precipitation of found in literature [9, 10, 30, 37]. the nanocrystals. Specifically, for the 3.0 nm CdSe The other popular purification scheme is nanocrystals in Figures 1(a) and 1(b), the Cd extraction. Raw reaction mixture is dispersed in precursor to Se precursor molar ratio was 2 : 1 an organic solvent. Into this dispersion, a and no additional free fatty acid was added. non-solvent for the nanocrystals is added, which The FTIR spectrum of the reaction mixture in is immiscible with the first solvent and can Figure 1(c) reveals existence of both carboxylic extract the impurities from the solution of acid (~1710 cm-1 labeled as –COOH in the inset colloidal nanocrystals. Hexane-methanol plot) and carboxylate (~1540 cm-1 labeled as extraction system was introduced [35] and has –COO- in the inset plot). To measure the degree been the main pair of extraction solvent system of reaction using the absorbance ratio between [22, 23]. In this two-phase system, the polar phase the –COOH and –COO- vibration bands (methanol phase) should disperse polar (ACOOH/COO-), a calibration curve (Figure 1(d), and impurities but cannot disperse colloidal all original FTIR spectra in Figure S1) was nanocrystals terminated with hydrocarbon obtained by measuring the ACOOH/COO- values of a ligands. series of HSt and Cd(St)2 mixtures in ODE with

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4 Nano Res.

These two most popular schemes for replacement of the original surface ligands, and purification of colloidal nanocrystals were acetonitrile was one of few polar organic liquids studied first. Their brief procedures (I and IIa) are that did not affect the surface ligands of CdSe provided in Figure 2. Results in the next nanocrystals [11]. subsection shall reveal that both popular schemes did not work well for the current system. Evaluation of existing purification schemes and Careful observation of the cooling process of their variations. This subsection shall the reaction mixture revealed that the reaction quantitatively evaluate Scheme I, IIa, IIb, and IIc mixture was clear until the solution temperature outlined in Figure 2. As mentioned above, dropped to ~50-60 oC. This inspired the possible impurities in the current system and establishment of Scheme IIb, which changed the many common systems of colloidal nanocrystals extraction temperature from room temperature to should include metal fatty acid salts (Cd(St)2), 60 oC. reaction side products (HSt), and non-volatile It was noticed that the hexane-methanol solvent with high boiling point (ODE). The most extraction worked well for CdSe nanocrystals troublesome impurity in future study/application synthesized via a different procedure [38]. The and for removal is Cd(St)2 (see results below). main differences of that synthetic procedure from Based on this judgment, we defined a term— the current system were addition of fatty amine purification efficiency of metal salt (PE) as ligands and employment of the follows. removal of Cd(St) Se-tributylphospine precursor. This invited us to PE  2 (1) explore another variation of the hexane-methanol total of Cd(St)2 extraction, namely Scheme IIc. This variation was The “removal of Cd(St)2” and “total Cd(St)2” mostly similar to Scheme IIb except addition of a refer to the amount of metal salts (or any other certain amount of butylamine into the extraction form of metal ion impurities not in the form of system. It should be noted that, because of the nanocrystals) removed by a given purification low density of hexane and removal of ODE into procedure and the original amount of Cd(St)2 in the methanol phase by the amine, it was the reaction mixture, respectively. If a procedure necessary to replenish ODE for the second and includes multiple repeating steps, PE would subsequent cycles of purification for Scheme IIc. count the overall effect of the current and Results below shall demonstrate that Scheme precedent steps. For the current system, PE was I and different versions of Scheme II all could not obtained by atomic absorption spectra of completely remove cadmium stearate in the cadmium and confirmed by FTIR measurements. solution of colloidal nanocrystals. In addition, For the precipitation scheme, determination of fatty amines are known ligands to most types of the “removal of Cd(St)2” was based on atomic nanocrystals. This means that IIc might cause absorption measurements of cadmium in the complication of surface ligands. These facts colorless supernatant of each precipitation cycle. promoted us to develop a new purification For the extraction, it was based on the same scheme, i.e., Scheme III in Figure 2. This scheme measurements of the colorless methanol phase. In applied CHCl3 as the solvent and acetonitrile as this manner, interference of the cadmium ions in the precipitation reagent. Interestingly, a recent the form of nanocrystals was avoided. article reported that all protic solvents including Figure 3 illustrates the purification efficiency methanol and fatty amines might cause for Scheme I and three versions of Scheme II.

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Scheme I, namely toluene-methanol precipitation the barely soluble salt into soluble species in scheme, performed poorly for the current system. methanol. Though PE increased steadily upon multiple The temperature effect was thought to be precipitation cycles, the overall PE after five associated with the temperature dependent cycles of precipitation was still less than 20%. solubility of the metal salt. To confirm this

The original Scheme II (Scheme IIa in Figure hypothesis, a laser scattering apparatus (Figure 2) was even worse than Scheme I. This was quite 4(a)) was set up for determination of the surprising to us as the early work reported a dissolution/precipitation temperature of Cd(St)2 reasonable purification efficiency for CdS in ODE (Figure 4(b)). Results revealed that, in nanocrystals synthesized by a similar procedure pure ODE, Cd(St)2 precipitated from the solution [37]. The main difference between two synthetic at ~83 oC. Addition of hexane into this solution procedures was replacement of cadmium stearate lowered the precipitation temperature to ~68 oC. by cadmium oleate, and during purification, some CHCl3 was added into the two-phase Establishment of Scheme III, system. Experimental results (Figure S2) revealed chloroform-acetonitrile precipitation. Results that cadmium oleate had a decent solubility in above revealed that both popular purification methanol phase (still significantly lower than that schemes and variations did not offer ideal in ODE-hexane phase), but cadmium stearate was performance. Though Scheme practically insoluble in both phases at room IIc—hexane-methanol extraction with butylamine temperature. With the present of CHCl3, at 60 oC—performed reasonably well, addition of cadmium oleate was found to be quite soluble in butylamine might become an issue because this the methanol phase. This means that, in light of short fatty amine would partially bond to the purification, as a metal precursor, metal oleate surface of the nanocrystals [8]. In ideal case, would be a better choice than metal stearate. purification should remove all impurities with Unfortunately, high purity oleic acid would be little impact to the structure of the nanocrystals much more expensive than stearic acid with the including their ligands. Replenishing ODE in same purity, and unsaturated fatty acids with each cycle in Scheme IIc was not only tedious but different number of carbons could not be easily also cause concerns when non-volatile organic available. impurities were problematic, such as in the case

Scheme IIb worked significantly better than of fabrication of optoelectronic devices. either Scheme IIa or Scheme I. after five cycles of The sharp temperature dependence of extraction, PE could reach ~60%. Performance of dissolution/precipitation transitions of Cd(St)2 in Scheme IIc was further improved, which both ODE and ODE-hexane mixture (Figure 4(b)) promoted PE to ~90% after five cycles of provided hints to develop a new purification purification. scheme. What we needed should be a specific

Though both Scheme IIb and IIc did not offer solvent system—mixture of ODE and other satisfactory performance for purification, the organic liquids, which could possess high results implied two positive effects, i.e., solubility of Cd(St)2 at a relatively low temperature and amine. temperature and not disperse the nanocrystals FTIR measurements (Figure S3) confirmed at all. Light scattering results (Figure 4(b)) that fatty amine could form complexes with revealed that, in comparison with other apolar cadmium stearate, which presumably converted solvent, CHCl3 could significantly improve

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solubility of Cd(St)2 in ODE at ~60 oC. This result only marginal turbidity for the first and second should be reasonable, considering the polar cycles of purification. As pointed out above, headgroup and hydrocarbon tail of Cd(St)2. For Cd(St)2 was found to be barely soluble at room this reason, CHCl3 was selected as the solvent temperature in any solvent systems tested. These additive to enhance the solubility of Cd(St)2. pictures visually confirmed excellent purification Acetonitrile was selected as the solvent efficiency of Scheme III. additive to precipitate the nanocrystals, which Figure 5(b) shows FTIR spectra of the was found to be much more powerful than nanocrystal products at different stages of methanol that has been the commonly used purification by Scheme III. The IR peak at ~1740 precipitation reagent. Results indicated that cm-1 is the carbonyl vibration band of methyl another common precipitation reagent, i.e., stearate added to each sample as the calibration acetone was even less efficient than methanol [9]. standard. Results in Figure 5(b) show that ODE, a Results below shall further demonstrate that, in non-volatile organic liquid and harmful for comparison with methanol, acetonitrile shows optoelectronic devices, was completely removed minimum damage to the surface ligands of the after three cycles of precipitation, so was HSt. As nanocrystals. With the present of CHCl3, expected, Cd(St)2 was the most challenging acetonitrile and ODE were found to form a single impurity to eliminate. After five cycles of phase needed for a precipitation scheme. For the purification, the absorbance of the corresponding second and consequent cycles of purification, the FTIR band of Cd(St)2 reached the value expected precipitate of nanocrystals isolated from previous for the surface ligands. Quantitatively, ~3% of the cycle would be dispersed in a small volume of stearate remained after five cycles of purification pure CHCl3 and precipitated readily by addition through Scheme III, which was equivalent to one of a small amount of acetonitrile (Figure 2) to monolayer of closely packed ligands on the offer excellent purification efficiency. surface of each nanocrystal. Furthermore, Figure 5(a) illustrates the PE of this new inductively-coupled-plasma with scheme. With a reduced amount of atomic-emission-spectroscopy (ICP-AES) precipitation/extraction reagent (Figure 2), the measurements demonstrated that, after five overall PE after two cycles of purification by cycles of purification, the Cd to Se ratio in the Scheme III was found to be significantly better nanocrystal samples was about ~1.16 : 1, which than that of either of Scheme I and II with five was found to be consistent with the expected cycles of purification. The PE of this new scheme value for CdSe nanocrystals coated with stearate could reach near unity with 3-5 cycles of ligands [10]. purification, about ~99% for the third cycle and > It is worth to mention that, if a synthesis was 99% for the fourth and fifth cycles. The photos in either carried out with great excess of cadmium the inset of Figure 5(a) illustrate that, after stearate and stearic acid or stopped in very early cooling down to room temperature, the stage, the reaction mixture could contain supernatant of the first cycle of precipitation was excessive amounts of cadmium stearate. In this extremely turbid and the one of the fifth cycle case, a small amount of butylamine (~0.2% in was completely clear. In comparison, for Scheme volume in the precipitation solution) for the first

IIb—the hexane-methanol extraction at 60 oC, the cycle of purification would help to retain the high digital photos (Figure S4) of the methanol phase PE of Scheme III. This would generally result in after cooling down to room temperature showed an extremely high PE for the first cycle and no

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more butylamine would be needed for the It should be mentioned that, since metal fatty subsequent cycles. Measurements indicated that acid salts became the most employed precursors the tiny amount of butylamine added in the first for synthesis of high quality nanocrystals [5], cycle was efficiently removed in the following fatty alcohols [30, 39] and fatty amines [38, 40, 41] cycles (see elemental analysis results in next have become common reagents to tune the subsection). reactivity of the salts and be ligands to the Scheme III worked well when the CdSe nanocrystals. Fatty alcohols and amines are nanocrystals were synthesized with different similar to fatty acids, which can be removed types of fatty acid salts. Furthermore, Scheme III readily (see Figure 5 as an example). Such was confirmed to perform similarly for the CdS reactions, however, often generate problematic nanocrystals synthesized in ODE. Purification for side products, i.e., esters and amides. These esters all systems with saturated fatty acid salts was and amides are similar to metal fatty acid salts in quite similar to the case with cadmium stearate. terms of purification, which possess a polar When cadmium oleate and oleic acid were the group in the middle of a molecule and long impurities in the reaction mixture, the operation hydrocarbon chains as the periphery. As a results, temperature for Scheme III could be lowered these troublesome side products behaved significantly, such as < 50 oC, and no addition of similarly with the metal fatty acid salts against butylamine was needed for the entire purification different purification schemes as described procedure. above. To further demonstrate the generality of The yield of the nanocrystals of Scheme III,

Scheme III, magnetite (Fe3O4) nanocrystals were similar to Scheme I and II, was high. In fact, no synthesized by the reaction of iron stearate in measureable loss of nanocrystals was identified. ODE either without or with addition of fatty If any nanocrystals were lost into the supernatant, alcohol. Without alcohol [30], the purification was it would became non-white and colorful, given quite similar to that for the CdSe and CdS the extremely high extinction coefficient of the nanocrystals as the impurities were the same nanocrystals [35]. The representative digital (data not shown). With addition of octadecanol pictures in Figure 5 (left) demonstrate either pure [39], it was possible to better tune the reaction to white or colorless supernatant. obtain magnetite nanocrystals with relatively small sizes (Figure S5, Supporting Information). Properties of purified CdSe nanocrystals. In the latter case, the most troublesome impurity Scheme III was found to be quite gentle to the was the ester, i.e., stearyl stearate. Scheme III was surface carboxylate ligands of the nanocrystals. It proven to be extremely efficient to remove this was reported that both chloroform and impurity. As shown in Figure S5 (Supporting acetonitrile are benign to the CdSe nanocrystals Information), one cycle of purification by Scheme coated with carboxylate ligands [11, 12]. After III removed ~99% of stearyl stearate, and > 99% of purification, the surface coverage of stearate the ester was removed by the second and third ligands for the nanocrystals was determined by cycles. In comparison, the traditional elemental analysis (Table 1). The footprint of a toluene-methanol precipitation—Scheme I—only stearate ligand on CdSe nanocrystals was removed ~40% of stearyl stearate after five cycles identified to be 0.21 ± 0.03 nm2 for all samples in of purification. average. Such a value of footprint implies that the ligands at the nanocrystal-ligand interface were

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8 Nano Res. closely packed, similar to that in the CdSe nanocrystals and the carboxylate Langmuir-Blodgget films [42]. The size ligands. Photoluminescence of semiconductor independence of the footprints revealed equal nanocrystals in solution has been known to be quality of surface passivation at the sensitive to surface changes. In principle, the inorganic-ligand interface. In addition, Table 2 photoluminescence quantum yield of shows that similar surface coverage of different semiconductor nanocrystals should decrease types of carboxylate ligands was obtained by the noticeably when the nanocrystals lost surface same purification scheme for CdSe nanocrystals ligands. As demonstrated in the case of CdSe with different sizes. nanocrystals coated with fatty amine ligands, Elemental analysis of organic contents for when a concentrated solution of colloidal CdSe the nanocrystal samples after purification by nanocrystals was diluted into a large volume of Scheme III offered further evidences about the pure solvent, the photoluminescence decreased purity of the nanocrystal powders after gradually along with the detachment of their evaporation of the volatile solvents. In Table 3, surface ligands [8]. the carbon to hydrogen ratios for the nanocrystals Figure 6(a) shows that the with either different size or different types of photoluminescence of CdSe nanocrystals coated carboxylate ligands matched the expectation of with stearate ligands after purification using the surface ligands. Furthermore, the Scheme III was stable in a diluted solution for a contribution of nitrogen was always below the long period of time at room temperature in a detection limit though some butylamine was typical nonpolar solvent, such as toluene, hexane, added in the first precipitation cycle for all and chloroform. This indicates that, unlike the nanocrystals with their sizes greater than 4.5 nm fatty amine ligands [8], the carboxylate ligands in Table 3. were stably bonded onto the surface of the CdSe Results in Tables 1 and 2 also implied that nanocrystals. Addition of acetonitrile into this the stearate and other carboxylate ligands should stable solution, no change on the be sufficiently stable in typical non-polar organic photoluminescence (Figure 6(b)) was observed as solvents. At least, the nanocrystal-ligands long as the amount was not in excess to cause complex could go through the precipitation of the stable nanocrystals. chloroform-acetonitrile purification procedure Conversely, addition of the same amount of without losing surface ligands. In literature, this methanol into the nanocrystal solution would has been a controversial topic. Some groups induce a gradual decrease of the suggested that the bonding of carboxylate ligands photoluminescence (Figure 6(c)), which was to surface cadmium ions of the nanocrystals was found to be consistent with some literature very strong and the nanocrystal-ligands reports [11, 12]. These results further confirmed complexes could remain intact in typical organic that acetonitrile is an ideal precipitation reagent solvents [9, 10, 43-45]. Others suggested that in comparison to methanol. carboxylate ligands didn’t bond on the surface of nanocrystals very well and could detach from the 3 Conclusion surface of the nanocrystals into the solution [11, 12, 46]. In conclusion, a quantitative protocol was A method reported in literature [8] was introduced to evaluate purification schemes for applied to examine the bonding stability between colloidal nanocrystals. Both common schemes

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reported in literature were found to be limited in dihydrate (Cd(Ac)2·2H2O, 98.5%) was purchased purification efficiency, especially for removing from Shanghai Tingxin Reagents. Iron(III) metal fatty acid salts and esters. Simple variations stearate were purchased from TCI. All organic of the existing hexane-methanol extraction could solvents were purchased from Sinopharm only improve the purification efficiency to a Reagents. All chemicals were used directly certain degree. A new scheme based on without any further purification. chloroform-acetonitrile precipitation was identified as an ideal system. Results reveal that, by this new scheme, metal carboxylate precursors, 4.2 Synthesis of Cadmium Stearate (Cd(St)2). non-volatile solvents, and non-nanocrystal side HSt (20 mmol) was neutralized with equal-mole of products (fatty acids in the current system) could tetramethylammonium hydroxide in methanol (200 be efficiently eliminated from colloidal mL). Into this solution, Cd(Ac)2·2H2O (10 mmol) nanocrystal solutions. The new scheme was dispersed in 50 mL of methanol was added benign to the surface of the nanocrystals. The dropwise under vigorous stirring. White Cd(St)2 new scheme worked universally for CdSe immediately precipitated, and the mixture was nanocrystals coated with different fatty acid salts stirred for another 20 min after completion of and also for CdS and oxide (such Fe3O4) dropping. Subsequently, the white precipitate was nanocrystals coated with similar ligands. Results collected through filtration and dried in vacuum in this work suggested that there are two key overnight. parameters for selecting a purification scheme. The first one is differentiation between the 4.3 Synthesis of CdSe nanocrystals. nanocrystals and challenging impurities that A typical synthesis for 3.0 nm CdSe nanocrystals possess a polar center and long hydrocarbon was conducted by injecting 1.0 ml Se-octadecene chains as the periphery, such as metal fatty acid suspension (0.1 mol/L) [33] into hot (250 °C ) salts, esters, and amides. The second parameter is mixture of Cd(St)2 (0.1356 g, 0.2 mmol) and 2.0 g of that all solvents/non-solvents used in purification ODE in a 25 mL three-neck flask. Needle tip should be benign to the surface ligands. Colloidal aliquots were taken for UV−vis and PL spectra nanocrystals with high purity and consistent measurements to monitor the reactions. When the surface structure offer an excellent basis for targeted size of nanocrystals was achieved, the surface chemistry studies, epitaxial growth, and reaction mixture was allowed to cool down to room various applications of these novel materials. temperature for purification. The molar ratios of cadmium and selenium precursors and reaction temperature are adjusted for synthesis of CdSe 4 Experimental nanocrystals with other sizes. 4.1 Chemicals. Stearic Acid (HSt, 98+%), tetramethylammonium 4.4 Synthesis of iron oxide nanocrystals. hydroxide (98 %, 25 % w/w in methanol), Preparation of nearly monodisperse iron oxide cadmium oxide (CdO, 99.998%), selenium nanocrystals coated with stearate (~5 nm) with powder (Se, 200 mesh, 99.999%), 1-octadecene alcohol activation was carried out with (ODE, 90%), methyl stearate(99%) and modification of ref [30]. Typically, 0.3 mmol iron butylamine (98%), 1-octadecanol (97%) were (III) stearate, 0.6 mmol stearic acid, and 4.5 ml ODE purchased from Alfa-Aesar. Cadmium acetate were loaded into a 25 ml three-neck flask. The flask

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10 Nano Res. was heated to 270 °C . 2.0 mmol 1-octadecanol scattering light signal and the solution dissolved in 0.5 ml ODE was warmed slightly and temperature was recorded by a computer, which injected into the hot solution to aid the formation of was further correlated by computer program. the nanocrystals (alcoholysis, similar to ref [39]). After one hour, the reaction mixture was allowed to 4.7 Atomic Absorption Measurements cool down to room temperature for purification. Solutions containing cadmium stearate (or other types of cadmium carboxylate) were digested by 4.5 Optical properties and structural excess aqua regia. The digested samples were characterizations. transferred into a volumetric flask to make an Absorption spectra were taken on an Agilent aqueous solution with a given volume. The Technologies Cary 4000 UV-vis spectrophotometer. cadmium concentration after digestion was Photoluminescence spectra were measured using determined using a HITACHI 180-50 Atomic an Edinburgh Instruments FLS920 spectrometer. Absorption Spectrometer (AAS). FTIR spectra were recorded on a Nicolet 380 spectrophotometer. For FTIR measurements, solid 4.8 ICP-AES Measurements. samples were grounded with KBr and pressed into To determine Cd to Se ratio in a nanocrystal a pellet, and liquid samples were dropped on a sample, the CdSe nanocrystals coated with

CaF2 substrate for measurements after evaporation carboxylate ligands after purification were of the solvents. digested by excess aqua regia with addition of some hydrogen peroxide to assist digestion. The 4.6 Laser scattering measurements. digested samples were transferred into a A four-side clear cuvette with clear solution of the volumetric flask to make an aqueous solution nanocrystal-ligands complex to be studied was with a given volume for the ICP-AES placed into a cuvette holder with a water bath measurements. ICP-AES measurements were thermostat. A thermocouple was dipped into the conducted using a Prodigy XP of Leeman solution to in situ monitor the temperature of the Inductively Coupled Plasma-Atomic Emission sample. For the laser scattering experiments, laser Spectrometer. sources with wavelengths between 780-450 nm usually yielded the same precipitation temperature for a given sample, but in order to minimize the interference from absorption and Acknowledgements emission of the nanocrystal samples, a 780 nm This work was supported by the National continuous-wave laser was used as the excitation Natural Science Foundation of China (NSFC, No. light source for all samples. The collimated laser 21233005 and 91433204) and Fundamental was with a power of ~100 mW and a spot size of ~ Research Funds for the Central Universities 6 mm2. After a proper neutral attenuator, (2014FZA3006). scattering light signal was collected by a convex lens and recorded by a spectrometer placed at the perpendicular direction of the incident laser Electronic Supplementary Material: Supplementary material is available in the online beam on one side of the cuvette. As the version of this article at temperature of the solution was reduced in a given rate, the temporal evolution of the

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http://dx.doi.org/10.1007/s12274-***-****-* [11] Hassinen, A., et al. Short-chain alcohols strip x-type (automatically inserted by the publisher). ligands and quench the luminescence of pbse and cdse quantum dots, acetonitrile does not. J. Am. Chem. Soc. References 2012, 134, 20705-20712. [1] Brus, L. E. Electron electron and electron-hole [12] Anderson, N. C., Hendricks, M. P., Choi, J. J.; Owen, J. interactions in small semiconductor crystallites - the S. Ligand exchange and the stoichiometry of metal size dependence of the lowest excited electronic state. chalcogenide nanocrystals: Spectroscopic observation , J. Chem. Phys. 1984 80, 4403-4409. of facile metal-carboxylate displacement and binding. J. [2] Weller, H. Colloidal semiconductor q-particles - Am. Chem. Soc. 2013, 135, 18536-18548. chemistry in the transition region between solid-state [13] Dai, X., et al. Solution-processed, high-performance , and molecules. Angew. Chem. Int. Ed 1993 32, 41-53. light-emitting diodes based on quantum dots. Nature [3] Alivisatos, A. P. Semiconductor clusters, nanocrystals, 2014, 515, 96-99. , and quantum dots. Science 1996 271, 933-937. [14] Peng, X. G. Band gap and composition engineering [4] Murray, C. B., Kagan, C. R.; Bawendi, M. G. on a nanocrystal (bcen) in solution. Accounts Chem. Synthesis and characterization of monodisperse Res. 2010, 43, 1387-1395. nanocrystals and close-packed nanocrystal assemblies. [15] Coe, S., Woo, W. K., Bawendi, M.; Bulovic, V. , Annu Rev Mater Sci 2000 30, 545-610. Electroluminescence from single monolayers of [5] Peng, X. G. An essay on synthetic chemistry of nanocrystals in molecular organic devices. Nature 2002, , colloidal nanocrystals. Nano. Res. 2009 2, 425-447. 420, 800-803. [6] Bronstein, L. M., et al. Influence of iron oleate [16] Chan, W. C. W.;Nie, S. M. Quantum dot complex structure on iron oxide nanoparticle formation. bioconjugates for ultrasensitive nonisotopic detection. , Chem. Mater. 2007 19, 3624-3632. Science 1998, 281, 2016-2018. [7] Moreels, I., Fritzinger, B., Martins, J. C.; Hens, Z. [17] Michalet, X., et al. Quantum dots for live cells, in Surface chemistry of colloidal pbse nanocrystals. J. Am. vivo imaging, and diagnostics. Science 2005, 307, , Chem. Soc. 2008 130, 15081-15086. 538-544. [8] Ji, X. H., Copenhaver, D., Sichmeller, C.; Peng, X. G. [18] Klimov, V. I., et al. Optical gain and stimulated Ligand bonding and dynamics on colloidal emission in nanocrystal quantum dots. Science 2000, nanocrystals at room temperature: The case of 290, 314-317. alkylamines on cdse nanocrystals. J. Am. Chem. Soc. [19] Hines, M. A.;Guyot-Sionnest, P. Synthesis and , 2008 130, 5726-5735. characterization of strongly luminescing zns-capped [9] Morris-Cohen, A. J., Donakowski, M. D., Knowles, K. cdse nanocrystals. J. Phys. Chem-Us. 1996, 100, E.; Weiss, E. A. The effect of a common purification 468-471. procedure on the chemical composition of the surfaces [20] Peng, X. G., Schlamp, M. C., Kadavanich, A. V.; of cdse quantum dots synthesized with Alivisatos, A. P. Epitaxial growth of highly , trioctylphosphine oxide. J. Phys. Chem. C 2010 114, luminescent cdse/cds core/shell nanocrystals with 897-906. photostability and electronic accessibility. J. Am. Chem. [10] Fritzinger, B., Capek, R. K., Lambert, K., Martins, J. Soc. 1997, 119, 7019-7029. C.; Hens, Z. Utilizing self-exchange to address the [21] Heine, J. R., Rodriguez-Viejo, J., Bawendi, M. G.; binding of carboxylic acid ligands to cdse quantum Jensen, K. F. Synthesis of cdse quantum dot zns , dots. J. Am. Chem. Soc. 2010 132, 10195-10201. matrix thin films via electrospray organometallic

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monolayers in metastable stearic-acid langmuir-blodgett-films. J. Phys. Chem-Us. 1992, 96, 3170-3174. [43] Morris-Cohen, A. J., Frederick, M. T., Lilly, G. D., McArthur, E. A.; Weiss, E. A. Organic

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Figure 1. (a) UV–vis and PL spectra, (b) TEM image, (c) FT-IR spectra of 3.0 nm as-synthesized CdSe nanocrystals coated with stearate ligands. (d) Calibration curve for

determination of the molar ratio between HSt and Cd(St)2 in ODE using FTIR measurements.

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Figure 2. Schematic diagram of all purification schemes examined in this work. Details are provided in the text. For simplification, only the first cycle of purification is illustrated. After each cycle (including the first one), the dispersion to the right goes back to the procedure described on the left arrow and initiate the next cycle. Well purified products can be dispersed well in common organic solvents.

Figure 3. Purification efficiency (PE) of 3.0 nm CdSe nanocrystals coated with stearate ligands using Scheme I and three versions of Scheme II.

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Figure 4. (a) Schematic diagram of the measurement setup for temperature-dependent laser scattering. (b) A

series of laser scattering curves of 0.01 mol/L Cd(St)2 in

different solvents. The volume ratio of ODE/CHCl3 and ODE/hexane in mixed solvent was both 2/3.

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Figure 5. (a) Purification efficiency (PE) of Scheme III, i.e., chloroform-acetonitrile precipitation, for 3.0 nm CdSe nanocrystals coated with stearate ligands at 60℃. Inset, the digital photos of the supernatant at different purification cycles after cooling down to room temperature. (b) FTIR spectra of 3.0 nm CdSe nanocrystal coated with stearates after different purification cycles.

Figure 6. Temporal evolution at room temperature of PL spectra of 3.0 nm CdSe nanocrystals coated with stearate ligands purified using Scheme III. (a) Dispersed in pure toluene. (b) Dispersed in toluene with addition of acetonitrile with the molar ratio between the ligands and acetonitril as 1:2400. (c) Dispersed in toluene with addition of methanol with the molar ratio between the ligands and acetonitril as 1:2400.

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18 Nano Res.

Table 1: The footprint area per stearate ligand on the CdSe nanocrystals purified via Scheme III.

Size of core (nm) 2.6 3.0 3.8 4.6 5.2 7.8 Ligands per QD 94 117 200 287 366 1021 Ligands per nm2 4.4 4.1 4.4 4.3 4.3 5.3

Footprint (nm2) 0.23 0.24 0.23 0.23 0.23 0.19

Table 2: Examples of footprint area per ligand on the CdSe nanocrystals coated with other types of carboxylate ligands purified via Scheme III.

Ligands Myristate Docosanoate

Size of core (nm) 2.9 4.7 5.9 2.1 3.7 4.7

Ligands per QD 94 323 477 54 203 325

Ligands per nm2 3.5 4.7 4.3 3.7 4.7 4.7

Footprint (nm2) 0.29 0.21 0.23 0.27 0.21 0.21

Table 3: Carbon, hydrogen, and nitrogen contents of the CdSe nanocrystals with different ligands purified via Scheme III. Ligands Myristate Stearate Docosanoate

Size of core (nm) 2.9 4.7 5.9 2.9 4.7 5.2 2.1 3.7 4.7

Measured C [%] 34.17 20.97 16.51 32.15 26.02 22.06 39.39 33.34 28.88

Measured H [%] 5.50 3.36 2.67 5.26 4.22 3.58 6.51 5.47 4.74

Measured N [%] < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02

Measured C:H 7:13.5 7:13.5 7:13.6 9:17.7 9:17.5 9:17.5 11:21.8 11:21.7 11:21.7

Expected C:H 14:27 18: 35 22:43

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