Salts-Based Size-Selective Precipitation - American Chemical Society

Dec 22, 2009 - After decantation of the supernatant solution, the precipitates can be dispersed in water again. By means of adjusting the addition amo...
0 downloads 0 Views 2MB Size
Download PDF
pubs.acs.org/Langmuir © 2009 American Chemical Society

Salts-Based Size-Selective Precipitation: Toward Mass Precipitation of Aqueous Nanoparticles Chun-Lei Wang, Min Fang, Shu-Hong Xu, and Yi-Ping Cui* Advanced Photonics Center, School of Electronic Science and Engineering, Southeast University, Nanjing 210096, People’s Republic of China Received April 8, 2009. Revised Manuscript Received November 30, 2009 Purification is a necessary step before the application of nanocrystals (NCs), since the excess matter in nanoparticles solution usually causes a disadvantage to their subsequent coupling or assembling with other materials. In this work, a novel salts-based precipitation technique is originally developed for the precipitation and size-selective precipitation of aqueous NCs. Simply by addition of salts, NCs can be precipitated from the solution. After decantation of the supernatant solution, the precipitates can be dispersed in water again. By means of adjusting the addition amount of salt, size-selective precipitation of aqueous NCs can be achieved. Namely, the NCs with large size are precipitated preferentially, leaving small NCs in solution. Compared with the traditional nonsolvents-based precipitation technique, the current one is simpler and more rapid due to the avoidance of condensation and heating manipulations used in the traditional precipitation process. Moreover, the salts-based precipitation technique was generally available for the precipitation of aqueous nanoparticles, no matter if there were semiconductor NCs or metal nanoparticles. Simultaneously, the cost of the current method is also much lower than that of the traditional nonsolvents-based precipitation technique, making it applicable for mass purification of aqueous NCs.

Introduction Taking advantage of the broad excitation wavelength, the tunable emission, and the good photochemistry stability,1-5 semiconductor nanocrystals (NCs) have gradually replaced organic dyes in applications of light-emitting diodes, lasers, solar cells, bioimages, biomedical tags, etc.6-15 Synthesis of NCs with high quality is one of the preconditions before application. Among various physical and chemical routes of NC preparation, the colloidal chemical method has developed to one of the most *To whom correspondence should be addressed. E-mail: [email protected]. (1) Rogach, A. L.; Katsikas, L.; Kornowski, A.; Su, D.; Eychm€uller, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1772. (2) Cao, Y.; Banin, U. J. Am. Chem. Soc. 2000, 122, 9692. (3) Green, M.; O’Brien, P. Chem. Commun. 1999, 2235. (4) Wang, D.; He, J.; Rosenzweig, N.; Rosenzweig, Z. Nano Lett. 2004, 4, 409. (5) Zhong, X.; Feng, Y.; Knoll, W.; Han, M. Y. J. Am. Chem. Soc. 2003, 125, 13559. (6) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016. (7) Byrne, S. J.; Williams, Y.; Davies, A.; Corr, S. A.; Rakovich, A.; Gun’ko, Y. K.; Rakovich, Y. P.; Donegan, J. F.; Volkov, Y. Small 2007, 3, 1152. (8) Derfus, A. M.; Chan, W. C. W.; Bhatia, S. N. Nano Lett. 2004, 4, 11. (9) Lu, Z.; Li, C. M.; Bao, H.; Qiao, Y.; Toh, Y.; Yang, X. Langmuir 2008, 24, 5445. (10) Arachchige, I. U.; Brock, S. L. J. Am. Chem. Soc. 2007, 129, 1840. (11) Winiarz, J. G. J. Phys. Chem. C 2007, 111, 1904. (12) Gooding, A. K.; Gomerz, D. E.; Mulvaney, P. ACS Nano 2008, 2, 669. (13) Oertel, D. C.; Bawendi, M. G.; Arango, A. C.; Bulovic, V. Appl. Phys. Lett. 2005, 87, 213505. (14) Guldi, D. M.; Rahman, G. M. A.; Sgobba, V.; Kotov, N. A.; Bonifazi, D.; Prato, M. J. Am. Chem. Soc. 2006, 128, 2315. (15) Ma, J.; Chen, J. Y.; Idowu, M.; Nyokong, T. J. Phys. Chem. B 2008, 112, 4465. (16) Talapin, D. V.; Rogach, A. L.; Shevchenko, E. V.; Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 5782. (17) Yang, Y. A.; Wu, H.; Williams, K. R.; Cao, Y. C. Angew. Chem., Int. Ed. 2005, 44, 6712. (18) Chen, C. T.; Pawar, V. D.; Munot, Y. S.; Chen, C. C.; Hsu, C. J. Chem. Commun. 2005, 2483. (19) Lee, C. H.; Kim, M.; Kim, T.; Kim, A.; Paek, J.; Lee, J. W.; Choi, S. Y.; Kim, K.; Park, J. B.; Lee, K. J. Am. Chem. Soc. 2006, 128, 9326. (20) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature 2005, 437, 121. (21) Kumar, S.; Nann, T. Small 2006, 2, 316. (22) Malkmus, S.; Kudera, S.; Manna, L.; Parak, W. J.; Braun, M. J. Phys. Chem. B 2006, 110, 17334. (23) Kuno, M.; Ahmad, O.; Protasenko, V.; Bacinello, D.; Kosel, T. H. Chem. Mater. 2006, 18, 5722.

Langmuir 2010, 26(2), 633–638

popular method for preparing NCs with high quality.16-23 The colloidal method also makes it possible to synthesize NCs alternatively in aqueous solution and nonaqueous media.24-30 In the past 2 decades, great progresses have been made on nonaqueous synthesis. NCs with desirable sizes, shapes, and composition have been prepared, moreover, the underground growth mechanism of NCs has been also revealed.31-40 In parallel with nonaqueous synthesis routes, aqueous methods are also used to prepare II-VI semiconductor NCs, especially aqueous CdTe NCs.41-48 Thiol (24) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (25) Chin, P. T. K.; de Mello Donega, C.; van Bavel, S. S.; Meskers, S. C. J.; Sommerdijk, N. A. J. M.; Janssen, R. A. J. J. Am. Chem. Soc. 2007, 129, 14880. (26) Rajh, T.; Micic, O. I.; Nozik, A. J. J. Phys. Chem. 1993, 97, 11999. (27) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychm€uller, A.; Rakovich, Y. P.; Donegan, J. F. J. Phys. Chem. C 2007, 111, 14628. (28) Yu, W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854. (29) Pan, D.; Wang, Q.; Jiang, S.; Ji, X.; An, L. J. Phys. Chem. C 2007, 111, 5661. (30) Shieh, F.; Saunders, A. E.; Korgel, B. A. J. Phys. Chem. B 2005, 109, 8538. (31) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (32) Talapin, D. V.; Koeppe, R.; G€otzinger, S.; Kornowski, A.; Lupton, J. M.; Rogach, A. L.; Benson, O.; Feldmann, J.; Weller, H. Nano Lett. 2003, 3, 1677. (33) Xie, R.; Kolb, U.; Li, J.; Basche, T.; Mews, A. J. Am. Chem. Soc. 2005, 127, 7480. (34) Carbone, L.; Kudera, S.; Carlino, E.; Parak, W. J.; Giannini, C.; Cingolani, R.; Manna, L. J. Am. Chem. Soc. 2006, 128, 748. (35) Seo, H.; Kim, S. W. Chem. Mater. 2007, 19, 2715. (36) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343. (37) Dagtepe, P.; Chikan, V.; Jasinski, J.; Leppert, V. J. J. Phys. Chem. C 2007, 111, 14977. (38) Yu, W. W.; Wang, Y. A.; Peng, X. Chem. Mater. 2003, 15, 4300. (39) Breus, V. V.; Heyes, C. D.; Nienhaus, G. U. J. Phys. Chem. C 2007, 111, 18589. (40) Tsuruoka, T.; Takahashi, R.; Akamatsu, K.; Nawafune, H. Phys. Chem. Chem. Phys. 2008, 10, 2221. (41) Tang, Z.; Zhang, Z.; Wang, Y.; Glotzer, S. C.; Kotov, N. A. Science 2006, 314, 274. (42) Niu, H.; Gao, M. Angew. Chem., Int. Ed. 2006, 45, 6462. (43) Bao, H.; Wang, E.; Dong, S. Small 2006, 2, 476. (44) Gu, Z.; Zou, L.; Fang, Z.; Zhu, W.; Zhong, X. Nanotechnology 2008, 19, 135604. (45) Tang, B.; Yang, F.; Lin, Y.; Zhuo, L. H.; Ge, J.; Cao, L. Chem. Mater. 2007, 19, 1212. (46) Zhang, Q.; Gupta, S.; Emrick, T.; Russell, T. P. J. Am. Chem. Soc. 2006, 128, 3898.

Published on Web 12/22/2009

DOI: 10.1021/la903986v

633

Article

Wang et al.

molecules with diverse functional groups, such as thioglycolic acid, 1-thioglycerol, 2-mercaptoethylamine, etc., are usually used as the ligands for aqueous NCs. In recent years, aqueous methods have attracted a great deal of attentions due to their advantages of low cost, high reproducibility, and good biocompatibility.49-56 Moreover, the diverse surface functionalities of aqueous NCs facilitate their subsequent conjugation with other ions, molecules, or polymers in various applications.49-56 The NC solution prepared by using either aqueous routes or nonaqueous methods contains lots of excess ions, ligands, and compounds except the asprepared NCs.57-59 These excess matters are usually disadvantage to the subsequent coupling or assembling of NCs with other materials. For instance, in the bioimaging application, the excess ions in NCs solution will lead to the damage even the death of organisms.7,8 In order to remove them from NC solution, the purification process is necessary. The most common purification method is addition of nonsolvents into the NCs solution, viz. nonsolvents-based precipitation.24,60 Alcohols are the most common nonsolvents for both nonaqueous and aqueous NCs.24,60 Interestingly, in the precipitation process of aqueous NCs, some assisted manipulations (for instance condensation and heating) are usually necessary, especially in the case of low preparative concentration of NCs (such as 10-3 mol/L referring to the cation).16 Regrettably, such manipulations are extremely time-consuming and energy-consuming. In addition, the cost of the nonsolvents-based precipitation technique is high due to consume of a large amount of nonsolvents. These drawbacks also make it inconvenient to purify NCs with large production scale, for instance the future industrial production of NCs. In order to overcome these drawbacks, in this context, we developed a novel salts-based precipitation technique, which not only achieved the precipitation and size-selective precipitation of aqueous NCs but also avoided complicated manipulations of condensation and heating, and thus it was available for use in the purification of aqueous NCs on a large production scale.

Experimental Section Materials. All materials used in this work were analytical reagents. CdCl2, NaBH4, NaOH, NaNO3, MgCl2 6H2O, AgNO3, and 2-propanol were commercially available in China. Thioglycolic acid, 3-mercaptopropionic acid, and Te powder were purchased from Aldrich. NaHTe solution was prepared by using Te (47) Mandal, A.; Nakayama, J.; Tamai, N.; Biju, V.; Isikawa, M. J. Phys. Chem. B 2007, 111, 12765. (48) Qian, H.; Qiu, X.; Li, L.; Ren, J. J. Phys. Chem. B 2006, 110, 9034. (49) Zheng, Y.; Yang, Z.; Ying, J. Y. Adv. Mater. 2007, 19, 1475. (50) Green, M.; Harwood, H.; Barrowman, C.; Rahman, P.; Eggeman, A.; Festry, F.; Dobson, P.; Ng, T. J. Mater. Chem. 2007, 17, 1989. (51) Li, J.; Hong, X.; Liu, Y.; Li, D.; Wang, Y.; Li, J.; Bai, Y.; Li, T. Adv. Mater. 2005, 17, 163. (52) Shavel, A.; Gaponik, N.; Eychm€uller, A. ChemPhysChem 2005, 6, 449. (53) Gross, D.; Susha, A. S.; Klar, T. A.; Como, E. D.; Rogach, A. L.; Feldmann, J. Nano Lett. 2008, 8, 1482. (54) Danieli, T.; Gaponik, N.; Eychm€uller, A.; Mandler, D. J. Phys. Chem. C 2008, 112, 8881. (55) He, Y.; Lu, H. T.; Sai, L. M.; Lai, W. Y.; Fan, Q. L.; Wang, L. H.; Huang, W. J. Phys. Chem. B 2006, 110, 13370. (56) Wang, Q.; Kuo, Y.; Wang, Y.; Shin, G.; Ruengruglikit, C.; Huang, Q. J. Phys. Chem. B 2006, 110, 16860. (57) Wang, C.; Zhang, H.; Zhang, J.; Lv, N.; Li, M.; Sun, H.; Yang, B. J. Phys. Chem. C 2008, 112, 6330. (58) Zhang, H.; Liu, Y.; Wang, C.; Zhang, J.; Sun, H.; Li, M.; Yang, B. ChemPhysChem 2008, 9, 1309. (59) Zhang, H.; Liu, Y.; Zhang, J.; Wang, C.; Li, M.; Yang, B. J. Phys. Chem. C 2008, 112, 1885. (60) Chemseddine, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 636. (61) Wang, C.; Zhang, H.; Zhang, J.; Li, M.; Sun, H.; Yang, B. J. Phys. Chem. C 2007, 111, 2465. (62) Wang, C.; Zhang, H.; Xu, S.; Lv, N.; Liu, Y.; Li, M.; Sun, H.; Zhang, J.; Yang, B. J. Phys. Chem. C 2009, 113, 827.

634 DOI: 10.1021/la903986v

and NaBH4 according to the reference methods.57-59,61,62 Briefly, 0.34 g of NaBH4, 0.51 g of Te powder, and 6 mL of ultrapure water were transferred into a small flask. The reaction proceeded at 0 °C for 8 h, and then NaHTe formed in the supernatant solution.

Synthesis of 3-Mercaptopropionic Acid Capped CdTe NCs. 3-Mercaptopropionic acid capped CdTe NCs were pre-

pared according to previous methods.57-59 Namely, the mixture of CdCl2 and 3-mercaptopropionic acid was adjusted to pH 9.5 by using 1.0 mol/L NaOH solution, and then the mixture was aerated with N2 for 30 min. After injection of freshly prepared NaHTe solution into the mixture, CdTe crude solution was obtained. The total concentration of Cd in solution was 1.3  10-3 mol/L, and the initial molar ratio of CdCl2/3-mercaptopropionic acid/ NaHTe was 1.0/2.4/0.2. To obtain CdTe NCs with emissions from green to red, the crude solution was refluxed for specific time, and then cooled to room temperature in the open air. With a similar process, CdTe NCs with a high concentration (5.2  10-3 mol/L and 1.3  10-2 mol/L referring to the concentration of Cd) were also prepared. Synthesis of Thioglycolic Acid Capped Ag NCs. Typically, freshly prepared NaBH4 solution was dropped into the mixture of AgNO3 and thioglycolic acid under stirring, and then NaOH solution was employed to adjust the solution pH to 10.5. Before salts-based precipitation, the Ag solution was stirred for 24 h under dark. The total concentration of Ag in the solution was 2.8  10-2 mol/L, and the molar ratio of AgNO3/ thioglycolic acid/NaBH4 was 1.0/2.0/1.2.

Salts-Based Precipitation and Size-Selective Precipitation. In the process of salts-based precipitation, MgCl2 solution was used as the salt to be added into the freshly prepared CdTe solution under vigorous stirring. When the solution became slightly turbid, the mixture was centrifuged at 4000 r/min. The resultant precipitates were redispersed in water before spectra measurements. In our experiment, 25 μL of MgCl2 solution (1.76 mol/L) was enough to precipitate 5 mL of CdTe NCs from the solution. In the case of Ag nanoparticles, NaNO3 instead of MgCl2 was used as the salt in order to avoid the possible byproduct of AgCl. Typically, equal volume of NaNO3 solution (5 mol/L) was sufficient to precipitate Ag nanoparticles from the solution. In the process of size-selective precipitation, freshly prepared 3-mercaptopropionic acid capped CdTe NCs (5.2  10-3 mol/L referring to Cd) with green and red emissions were first mixed together with equal volume, and then MgCl2 solution was added until the red emitted CdTe NCs were completely precipitated from the solution. Usually, it needed about 10 μL MgCl2 solution (1.76 mol/L) to precipitate red emitted CdTe NCs from the mixture solution. After separation of red NC precipitates, 13 μL of MgCl2 solution (1.76 mol/L) was further added into the supernatant solution, and then the green emitted NCs in the supernatant solution would precipitate. Traditional Nonsolvents-Based Precipitation. Under vigorous stirring, 2-propanol was dropped into the NCs solution until the solution became slightly turbid, then the mixture was centrifuged at 4000 r/min. Typically, it needed about equal volume of 2-propanol before the thorough precipitation of CdTe NCs with relative high concentration (5.2  10-3 mol/L). At low preparative concentration (1.3  10-3 mol/L), most of the NCs could not be precipitated from the solution by addition of 2-propanol unless condensation manipulation was used. In order to simulate the condensation process of aqueous NCs, the NCs solution was condensed by using a rotor evaporator. It needed about 2.5 h for condensation of 150 mL solution to 20 mL at 50 °C. Characterization. UV-vis absorption spectra (UV) were recorded with a Shimadzu 3600 UV-vis near-infrared spectrophotometer. Fluorescence experiments were performed with an Edinburg FLS 920 spectrofluorimeter. X-ray powder diffraction Langmuir 2010, 26(2), 633–638

Wang et al.

Article

(XRD) investigation was carried out by using the D/max-2500/ PC diffractometer with Cu KR radiation (λ = 1.5418 A˚). Dynamic light scattering and ζ potential measurements were performed using a Zetasizer Nano-ZS (Malvern Instrument). The measurements were carried out immediately after the preparation of CdTe NCs (1.3  10-2 mol/L) in order to avoid the oxidation of nanocrystals. Because of the uncertainties of ζ potential measurements, each sample was measured for five times, and the average data were used.

Results and Discussion In the process of traditional nonsolvents-based precipitation, the manipulations of condensation and heating were necessary before the precipitation of aqueous NCs.16,60 In order to avoid these complicated manipulations, we developed a novel alternative salts-based precipitation technique in this context. Salts-Based Precipitation of Aqueous NCs. Simply by addition of salts, aqueous NCs could be precipitated from the solution. In order to contrast the precipitation effect of the traditional nonsolvents-based precipitation and the current salts-based one, 3-mercaptopropionic acid capped CdTe NCs were employed as the targets to be precipitated, and 2-propanol or MgCl2 was selected as the nonsolvent or salt. As shown in Figure 1, CdTe NCs prepared at high concentration (5.2  10-3 mol/L referring to Cd) were able to directly precipitate from the solution by using either of the two precipitation techniques without assisted manipulations (for instance the manipulations of condensation and heating). This was reflected by the colorless supernatant solution after precipitation. Moreover, compared with that of the original CdTe solution, the PL spectra of supernatant solutions showed no obvious PL signals (Figure 1), confirming the precipitation of NCs by using either of the two techniques. Noticeably, at the typical synthesis concentration of CdTe NCs (for instance 1.3  10-3 mol/L referring to Cd), the precipitation processes would be different by selection of different precipitation techniques. When the nonsolvents-based precipitation technique was used (Figure 2), the supernatant solution showed strong exciton absorption and luminescence in spectra. This indicated that most of NCs were not precipitated. In Figure S1 in the Supporting Information, we also compared the hydrodynamic diameters of NCs after addition of 2-propanol, and the result also confirmed that NCs were not aggregated in case of low concentration (1.3  10-3 mol/L). Interestingly, the PL peak position of the supernatant solution shifted to red compared to that of original NCs. This might be caused by the alteration of circumstance around NCs. According to the previous report,63 the emission of NCs is in direct proportion to the relative dielectric constant (ε) of NC surrounding media. Therefore, when we add 2-propanol (ε = 18.3) into aqueous solution (εwater= 80.1), the dramatically decreased ε of the mixture solution would lead to the red shift of NC emission. In addition, a similar red shift was also observed in case of CdTe NCs with high concentration as shown in Figure S2 in the Supporting Information. Herein, it should be emphasized that the supernatant solution always exhibited strong PL no mater how the volume ratio of alcohols was adjusted. Therefore, the manipulations of condensation and heating were necessary before the nonsolvents-based precipitation when NCs were prepared at low concentration.16 In comparison, when the salts-based precipitation technique was used, CdTe NCs were hardly found in both UV and PL spectra of the supernatant solution (Figure 2). Obviously, in the precipitation (63) Takagahara, T. Phys. Rev. B 1993, 47, 4569.

Langmuir 2010, 26(2), 633–638

Figure 1. PL spectra (λex = 400 nm) of CdTe NCs (a) and the supernatant solutions after precipitation by 2-propanol (b) or MgCl2 (c). The total concentration of Cd in the original CdTe NCs solution was 5.2  10-3 mol/L. The addition volumes of 2propanol and MgCl2 solution were 5 mL and 25 μL into 5 mL of CdTe NC solution.

Figure 2. PL spectra and UV-vis absorption (inset) of CdTe NCs (a), and the supernatant solutions after precipitation by 2-propanol (b) or MgCl2 (c). The total concentration of Cd in the original CdTe NCs solution was 1.3  10-3 mol/L. The addition volumes of 2-propanol and MgCl2 solution were 5 mL and 25 μL into 5 mL of CdTe NC solution.

process of aqueous NCs with low concentration, the current salts-based precipitation technique was superior to the traditional one due to the avoidance of additional assisted manipulations. The current salts-based precipitation technique was also available for other aqueous nanoparticles except semiconductor NCs. Herein, thioglycolic acid capped Ag nanoparticles were also employed as the targets to be precipitated. After addition of NaNO3 into Ag solution (Figure 3), the supernatant solution became colorless without notable plasmon absorption of Ag in UV spectrum, indicating the successful precipitation of Ag nanoparticles. Obviously, the current precipitation technique could be widely used for the purification and precipitation of aqueous nanoparticles regardless of their composition. Salts-Based Size-Selective Precipitation. Size-selective precipitation was an important method for separation of nanoparticles with different sizes on the basis of traditional nonsolvents-based precipitation.16,24,60 Herein, we also attempted the size-selective precipitation of CdTe NCs by using the current salts-based precipitation. 3-Mercaptopropionic acid capped CdTe NCs with green and red emissions were used in this experiment. The sizes of CdTe NCs with green emission and red emission were respectively 2.8 and 4.4 nm as calculated from the absorption spectra (Figure S3 in the Supporting Information) according to the reference method.28 After addition of green emitted CdTe NCs into equal volume of red emitted NC solution, DOI: 10.1021/la903986v

635

Article

Wang et al.

Figure 3. UV-vis absorption of Ag NCs (black solid line), and the supernatant solutions (red dash line) after precipitation by MgCl2. Inset picture: the corresponding images of Ag NCs (a), and the supernatant solution after precipitation (b). The total concentration of Ag in original solution was 2.8  10-2 mol/L.

Figure 4. PL spectra of the mixture solution with equal volume of green and red emitted CdTe NCs (black solid line), the supernatant solution after addition of 10 μL 1.76 mol/L MgCl2 (red dash line), and the dispersed solution of the CdTe precipitates (blue dash dot line). The total concentration of Cd in the original CdTe NCs solution was 5.2  10-3 mol/L. Inset: the images of the solutions under irradiation of a UV lamp (365 nm). The original mixture solution (a), the mixture solution after addition of MgCl2 and then centrifugation (b), the supernatant solution after decantation from the mixture solution (c), and the dispersed solution of the precipitates (d).

the mixture solution showed a vermeil emission under the irradiation of a UV lamp (Figure 4a). Absorption spectra in Figure S3 in the Supporting Information also exhibited the exciton peaks of both green and red emitted NCs, indicating the mixing of green and red emitted NCs. After addition of 10 μL of MgCl2 solution (1.76 mol/L) into the mixture solution, only the exciton peak of green emitted NCs could be observed in the absorption spectra of the supernatant solution (Figure S3 in the Supporting Information). This implied the red NCs had completely precipitated from the mixture. Under the irradiation of an UV lamp, the supernatant solution showed an obvious green emission (Figure 4b). Similar to the observations in the nonsolvents-based size-selective precipitation,16 in this work, NCs with large diameter (4.4 nm for red NCs) also tended to precipitate preferentially than the small ones (2.8 nm for green emission). Note that the visible red emission in Figure 4b mainly originated from the precipitates adsorbed on the wall of the centrifuge tube, while the observed green emission was produced by the supernatant solution. After decantation of the supernatant solution to another centrifuge tube, the green emitted supernatant solution (Figure 4c) and the red emitted precipitates could be perfectly 636 DOI: 10.1021/la903986v

Figure 5. XRD patterns of CdTe NCs after precipitation by alcohols (black line) and salts (red line). The standard XRD patterns of bulk cubic zinc blende CdTe (bottom) and bulk cubic zinc blende CdS (top) was also indicated. The PL peak position of original CdTe NCs used for XRD measurement was 680 nm.

separated (Figure 4d). It should be emphasized that NCs with green emission could be also precipitated if more MgCl2 solution is added (Figure S3 in the Supporting Information). In another viewpoint, NCs with green and red emission might precipitate simultaneously if the addition amount of salt is too much in the process of size-selective precipitation. Structure Alterations after Precipitation. In order to evaluate alterations of the crystals structure after precipitation, XRD measurement was employed. Herein, two shares of CdTe powders were respectively obtained by using the nonsolvents-based precipitation and the salts-based precipitation. As shown in Figure 5, both of the samples exhibited cubic zinc blende diffraction patterns. According to the Debye-Scherer formula, the diameter of CdTe NCs was calculated as 2.9 nm by using the diffraction peak at 25.1°. The diffraction peaks at 25.1°, 41.6°, and 49.0° were intermediate the bulk cubic CdTe and bulk cubic CdS. This was typical for aqueous CdTe NCs due to the incorporation of S atoms of ligands into the CdTe crystal lattice during refluxing in NC preparation proccess.27 In other words, such a shift in XRD patterns was caused by NC preparation process instead of the subsequent nonsolvents-based or salts-based precipitation process. Since the traditional nonsolvents-based precipitation would not alter NC crystal structure, the identical XRD pattern by using salts-base precipitation also indicated no serious damage of NC crystal structure by using the current salts-based precipitation technique. It should be emphasized that the selection of salts was very important in the current salts-based precipitation technique. Taking CdTe NCs for example, the salts containing Agþ, Cu2þ, or Hg2þ were unsuitable for the precipitation process since they could substitute the cations of CdTe, making the alteration of NCs structure. As a result, the XRD patterns of CdTe NCs showed obvious differences.64 In comparison, the current selection of Mg2þ and Naþ could avoid such issue since they were unable to substitute the cations in NCs. From XRD patterns, no obvious other diffraction peak was observed except the diffraction of CdTe. This also confirmed the current precipitation process would not seriously damage the crystal structure of NCs. Mechanism of Salts-Based Precipitation. As we known, NCs belonged to the category of colloids, and lots of the experimental observations of NCs could be comprehended according to the colloidal knowledge. For example, the recent works of Yang’s group indicated that the interparticle interactions (64) Xia, Y. S.; Cao, C.; Zhu, C. Q. J. Lumin. 2008, 128, 166.

Langmuir 2010, 26(2), 633–638

Wang et al.

between aqueous CdTe NCs were mainly the electrostatic repulsion, van der Waals attraction and dipolar attraction; moreover, the growth process of aqueous NCs was strongly affected by the interparticle electrostatic repulsion according to the colloidal model of Derjaguin-Landau-Verwey-Overbeek.57-59 Herein, we’d also like to give some simple discussion about the mechanism of salts-based precipitation on the basis of colloidal chemistry. It was well-known that the addition of electrolytes could dramatically decrease the surface potential of colloids.65 If the electrolytes were added enough, the interparticle repulsion (incarnated as the surface potential) would finally decrease smaller than the total attractions, and thus the macroscopic aggregation of NCs could be precipitated from the solution.57-59 Actually, the salts-induced decrease of electrostatic repulsion was very easy to comprehend on the basis of the explicit structure of aqueous NCs.57-59 According to the reference reports,57-59 the electrostatic repulsion between NCs was mainly come from the charges of ligand layer. The ions with opposite charges to the ligand layer could adsorb outside ligand layer, which could partly counteract the repulsion of ligand layer. As a result, the electrostatic repulsion exponentially decreased from the adsorb layer to the diffuse layer. In Figure S4 in the Supporting Information, we also showed the schematic illustration of the explicit structure of aqueous NCs. After addition of MgCl2 into NC solution, lots of Mg2þ cations adsorbed on the negatively charged ligand layer. As the result, the electrostatic repulsion decreased dramatically. In Figure 6, we also measured the ζ potential and hydrodynamic diameters of CdTe NCs after addition of MgCl2 solution into NC solution. Since ζ potential measurement could reflect the interparticle electrostatic repulsion,57-59 the decreased ζ potential with the addition of MgCl2 implied the decreased electrostatic repulsion between NCs. Moreover, the dramatically increased hydrodynamic diameters of NCs also confirmed the aggregation process of NCs with the addition of MgCl2 solution. It should be emphasized that the ζ potential and hydrodynamic diameter measurements of NCs during the precipitation process of NCs by addition of MgCl2 or 2-propanol were quite similar.66 This indicated the similar precipitation mechanism by addition of salts or alcohols. The detailed precipitation mechanism of alcohols-induced precipitation could be seen from another work,66 which could also reflected the mechanism of the current salts-based precipitation in detail. Overall, the salts-based precipitation was mainly induced by the alterations of interparticle interactions rather than the damage of NCs internal or surface structures (Figure 5), and hence the precipitates were able to keep strong luminescence after dispersion in water (Figure 4). In Figure S5 in the Supporting Information, the performance of different salts for precipitating NCs was also investigated. As could be seen, when monovalent salts (NaNO3) was used, the necessary amount of salts for precipitating NCs was much larger than that of bivalent salts (MgCl2). This phenomenon was also identical to the precipitation behavior of colloidal particles,65 which implied the validity of aforementioned precipitation mechanism on the basis of colloidal chemistry. Salts-Based Precipitation vs Nonsolvents-Based Precipitation. Both the traditional nonsolvents-based precipitation technique and the current salts-based precipitation technique were available for the precipitation and size-selective precipitation of aqueous NCs. Herein, we would like to compare the (65) Fu, X. C.; Shen, W. X.; Yao, T. Y. Physical Chemistry, 4th ed.; China Higher Education Press: Beijing, China, 1990. (66) Wang, C.; Fang, M.; Han, J.; Zhang, H.; Cui, Y.; Yang, B. J. Phys. Chem. C 2009, 113, 19445.

Langmuir 2010, 26(2), 633–638

Article

Figure 6. ζ potential (black solid line) and hydrodynamic diameter (red dash line) of CdTe NCs during their precipitation process by addition of MgCl2 (0.5 mol/L) into 5 mL CdTe solution.

advantages and disadvantages of them. The main disadvantage of the salts-based precipitation technique was its invalidation in the precipitation process of nonaqueous NCs due to the low solubility of electrolytes in organic solution. Moreover, the electrostatic repulsion between nonaqueous NCs was not as dominant as that between aqueous nanoparticles, and thus leading to the limited effect of salts in tuning interparticle interactions of nonaqueous NCs. Among advantages of the salts-based precipitation, its simplicity and rapidness should be mentioned first because of the avoidance of condensation and heating manipulations in the traditional method. Herein, we simulated the condensation manipulation by using a rotor evaporator at 50 °C. It almost demanded 2.5 h for condensing 150 mL of CdTe solution to 20 mL. At the typical evaporator temperature of 25-40 °C in ref16, the condensation time would be much longer. Second, the cost of salts-based precipitation was much lower than the traditional one since salts were usually much cheaper than alcohols. Moreover, the addition amounts of salt were also smaller than that of alcohols in precipitation process. In Figure S6 in the Supporting Information, we also simply evaluated the cost of precipitation, and the result indicated the cost of salts-based precipitation was about 0.02% of the cost of nonsolvents-based precipitation. Obviously, the simplicity, rapidness, and low cost of the salts-based precipitation technique made it suitable for purification of NCs with large production scale, for instance the industrial production process. Finally, it should be emphasized that the salts-based precipitation technique was compatible with the traditional nonsolvents-based precipitation technique. Our recent works also indicated that the precipitation effect would be superior to either of the aforementioned techniques when salts and alcohols were simultaneously added into the NCs solution.

Conclusions In this context, we developed a novel salts-based precipitation technique for the precipitation of aqueous nanoparticles. Simply by addition of salts, NCs could be precipitated from the solution. By adjusting the addition amount of salts, size-selective precipitation of aqueous nanoparticles could be achieved. Compared with the traditional nonsolvents-based precipitation, the current saltsbased precipitation was simpler and more rapid since the condensation and heating manipulations in the traditional precipitation method were avoided. Moreover, the cost of salts-based precipitation was much lower, making it possible to use it for purification of aqueous nanoparticles on a large production scale. DOI: 10.1021/la903986v

637

Article

We expect that the current salts-based precipitation technique could be widely used for the precipitation and purification processes of aqueous NCs henceforth. Acknowledgment. We thank to Prof. B. Yang and Prof. H. Zhang in the State Key Laboratory of Supramolecular Structure and Materials in Jilin University for the help of hydrodynamic diameter and ζ potential measurements. This work is supported by the National Natural Science Foundation of China (Grant No. 60877024), and the Research Start Project of Southeast University for Introducing Talents (Grant Nos. 4006001063 and 9206002481).

638 DOI: 10.1021/la903986v

Wang et al.

Supporting Information Available: Figures showing and text discussing hydrodynamic diameters of CdTe NCs after addition of 2-propanol, PL spectra of the supernatant solution before and after addition of salts, UV-vis spectra of green and red emitted CdTe NC mixture after addition of different amount of salts, schematic illustration of electric double-layer of aqueous CdTe NCs, UV-vis spectra of CdTe NCs after addition of different amounts of MgCl2 or NaNO3, and evaluation of the cost of salts-based precipitation and nonsolvents-based precipitation. This material is available free of charge via the Internet at http://pubs. acs.org.

Langmuir 2010, 26(2), 633–638