J. Phys. Chem. C 2010, 114, 11087–11091
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Aqueous Synthesis of ZnSe Nanocrystals by Using Glutathione As Ligand: The pH-Mediated Coordination of Zn2+ with Glutathione Jie Zhang, Jun Li, Jiexian Zhang, Renguo Xie, and Wensheng Yang* State Key Laboratory for Supramolecular Structures and Materials, College of Chemistry, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: March 21, 2010; ReVised Manuscript ReceiVed: May 23, 2010
A series of ZnSe nanocrystals was synthesized in the range of pH 6.5-11.5 in aqueous solution by using glutathione (γ-glutamyl-cysteinyl-glycine, GSH) as ligand. The photoluminescence quantum yields (PLQYs) of the ZnSe nanocrystals were dependent on the pH of the reaction solutions, which increased from 0.5 to 23% as the pH increased from 6.5 to 11.5. It is identified that Zn2+ primarily coordinates with thiol group of the cysteine residual at low pH (6.5-8.3); then, the deprotonated amino group of the glutamyl residue also contributed to the coordination at medium pH (8.3-10.3). With increasing pH to >10.3, N atom of the amide bond further took part in the coordination after its deprotonation. As a result, the growth rate of the ZnSe nanocrystals increased with increasing pH, indicating the improved activity of the Zn2+ precursors with increased pH. At pH 8%). It is likely that coordination of Zn2+ with the N atom of the amide bond not only affects the activity of the Zn2+ precursor but also promotes the release of sulfur from GSH, thus resulting in the formation of ZnSe/ZnS core/shell nanocrystals with higher PLQY. 1. Introduction Aqueous synthesis of semiconductor nanocrystals has been explored extensively during the past two decades because of its simplicity, environmental friendliness, and low cost.1-4 CdTe is one of the most successful semiconductor nanocrystals synthesized by aqueous approach.5-9 CdTe nanocrystals with photoluminescence quantum yield (PLQY) >30% can be readily prepared by using short chain thiols as ligands, which will be further increased to >60% by the formation of CdS shell after photooxidation of the thiols ligands.4 However, the use of conventional thiol ligands in syntheses of other semiconductor nanocrystals in aqueous solution is less successful. For example, PLQY of ZnSe nanocrystals synthesized by using short chain thiols as the ligands, such as 3-mercatopropionic acid (MPA), cysteine (Cys), or mercatoacetic acid (MA), is usually lower than 2%,10 which could increase to 17-30% after UV irradiation11 or microwave heating.12 Moreover, Fang et al. reported that PLQY of ZnSe nanocrystals increased to 60% after intentional coating of ZnSe with ZnS by using Zn(Ac)2 and thiourea as precursors.13 Recent works showed that ZnSe nanocrystals with PLQY of 22% could be obtained by using glutathione (GSH) as ligands in the aqueous approach.14 PLQY of CdTe nanocrystals can also be improved by using GSH instead of the short chain thiols15,16 because GSH is an important tripeptide in organisms that is known to be an effective chelating agent for a wide variety of metal ions.17,18 It is documented that the coordination of GSH with metal ions is pH-dependent because its functional groups will undergo deprotonation under different pH (the carboxyl group of residual, pK1 ) 2.13; carboxyl group of glycine residual, pK2 ) 3.51; the amino group of residual, pK3 ) 8.74; sulfhydryl group of cysteine residual, pK4 ) 9.66; NH group of the amide bond, pK5 ) 11.70).19-21 It is reasonable to deduce that the formation and PLQY of the * Corresponding author. E-mail:
[email protected].
semiconductor nanocrystals synthesized by using GSH as ligand is sensitive to pH because of its different coordination fashion with the metal ions. In this work, a series of GSH-capped ZnSe nanocrystals was synthesized under different pH to investigate the effect of coordination of GSH with Zn2+ ions on the formation and emission properties of the resulting ZnSe nanocrystals, sulfur from GSH, and the in situ formation of passivated ZnS shell on ZnSe nanocrystals. 2. Experiment Section a. Chemicals. Glutathione (Sangon, 99+%), sodium borohydride (Aldrich, 99+%), zinc acetate dehydrate (Zn(AC)2 · 2H2O) (Sigma-Aldrich, 98+%), quinine sulfate salt dehydrate (SCRC, 99%), selenium powder (Shanghai Meixin, 99.95%), L-cysteine (shanghai Huishi), and MPA (Aldrich, 99+%) were used as received. High-purity water (Pall Purelab Plus) with a resistivity of 18.2 ΩM · cm was used for the preparation of all aqueous solutions. b. Synthesis of ZnSe Nanocrystals. The synthesis of ZnSe nanocrystals was based on the reaction of zinc acetate with sodium hydroselenide (NaHSe). All reactions were carried out in oxygen-free water degassed by nitrogen. We prepared sodium hydroselenide by mixing sodium borohydride and selenium powder in water. After the complete reduction of selenium powder by sodium borohydride, 2 mL of freshly prepared NaHSe solution (0.2 M) was added to a flask containing 98 mL solution of Zn(AC)2 and ligand with a certain pH. The amounts of Zn, Se, and GSH ligands introduced were 1, 0.4, and 1.2 mM, respectively. The resulting mixture was heated to 100 °C. The as-prepared ZnSe nanocrystals were precipitated and washed several times with 2-propanol. The nanocrystals were dried at room temperature in vacuum overnight. In controlled experiments, the ZnSe nanocrystals were prepared under the same experimental conditions, except Cys or MPA was used instead of GSH.
10.1021/jp102540w 2010 American Chemical Society Published on Web 06/08/2010
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J. Phys. Chem. C, Vol. 114, No. 25, 2010
Zhang et al. the sample and the reference at the excitation wavelength, and QYs and QYr are the quantum yields of the sample and the reference, respectively. 3. Results and Discussion
Figure 1. (a) Emission spectra of the ZnSe nanocrystals prepared under pH of 6.5, 8.5, 10.5, and 11.5. (b) Variation of PLQY of the ZnSe nanocrystals prepared under different pH. Insert shows the photos of the samples (from left to right, pH 6.5, 8.5, 10.5, and 11.5) taken under a 365 nm UV lamp.
c. Characterization. UV-vis measurements were performed on a Shimadzu UV-2450 spectrometer. Fluorescence spectra were obtained with an FS900 steady-state fluorescence spectrometer with a 450 W xenon lamp as excitation source. Fourier transform infrared (FT-IR) spectra were measured by using a Perking-Elmer Spectrum One FT-IR spectrophotometer at a resolution of 4 cm-1. Powder X-ray diffraction (XRD) patterns were collected on a Rigaku D/max X-ray diffractometer with Cu KR radiation (λ ) 1.5405 Å). X-ray photoelectron spectra were taken on a VG ESCALAB MKII spectrometer with a Mg KR excitation (1253.6 eV). Binding energy calibration was based on C 1s at 284.6 eV. All measurements were carried out at room temperature (25 ( 2 °C). PLQY of the nanocrystals was determined from the integrated fluorescence intensity of the nanocrystals and the reference (quinine sulfate solution in 50 mM of H2SO4, PLQY ) 54.6% at 310 nm excitation).
QYs ) (Fs × Ar × QYr)/(Fr × As) where Fs and Fr are the integrated fluorescence emissions of the sample and the reference, As and Ar are the absorbance of
Figure 1a shows the emission spectra of the ZnSe nanocrystals prepared under different pH values (6.5, 8.5, 10.5, and 11.5) by using GSH as ligand when fixing the Zn/Se/GSH ratio at 1:0.4:1.2. The emission spectra of the samples prepared at pH 7.5 and 9.5 were not shown because of their similarity to the one prepared at pH 8.5. Two emission bands were observable from the spectra. The band in the UV-blue region was attributed to the bandgap emission, and the one in the longer wavelength region was related to the surface-related emission.10 The samples prepared at pH of 6.5, 8.5, 10.5, and 11.5 presented bandgap emissions at 344, 353, 368, and 377 nm, respectively, possibly because of the increased sizes of the nanocrystals prepared with increasing pH. At the same time, the surface-related emission was suppressed greatly with increased pH (Supporting Information, Figure S1), indicating improved photoluminescence properties of the nanocrystals prepared at high pH. Figure 1b gives the corresponding PLQYs for the bandgap emissions of the ZnSe nanocrystals. PLQYs of the samples prepared at pH of 6.5-9.5 were 10.3 in the presence of metal ions.29,30 On the basis of the above FT-IR results and previous literature,17,20,28,31,32 the Zn-GSH complexes formed under different pH are proposed as shown in Figure 5. Because of the strong affinity of the sulfhydryl group (pK4 ) 9.66) of the cysteine residual, the coordination between GSH and Zn2+ was dominated by the sulfhydryl group at pH 6.5-8.3 (complex I). At pH 8.3-10.3, the deprotonated amino group further contributed to the coordination (complex II). At pH 10.3-11.5, Zn2+ promoted the deprotonation of the amide group,20 and thus the negatively charged nitrogen atom can also participate in the coordination (complex III). It is speculated that the different growth rate of the ZnSe nanocrystals is related to the different complexes of GSH-Zn2+ formed under different pH. From the temporal evolution of the absorbance peaks of ZnSe nanocrystals under different pH, it is deduced that the activity of the Zn2+ precursors should be in the order of complex III > complex II > complex I, which is consistent with that reported in the literature.17 Therefore, the growth of ZnSe nanocrystals was promoted at higher pH because of formation of the Zn2+ precursors with improved activities with the increased pH.
Zhang et al. Another issue of this present work is the effect of pH on the PLQY of ZnSe nanocrystals. It is documented that the pH can influence the metal-ligands complexes on surface of CdTe nanocrystals, thus resulting in the difference in PLQY of the nanocrystals.33 Control experiments, in which pH of the ZnSe nanocrystals prepared at pH 8.5 was decreased to 6.5 or increased to 10.5 and 11.5 were carried out to illustrate the effect of pH. The ZnSe nanocrystals prepared at pH 8.5 showed no obvious change in emission intensity under different pH (Figure S3 in Supporting Information), implying that the difference in PLQY of the ZnSe nanocrystals was not induced by the pHdependent change in metal-ligand complexation on the surface of the ZnSe nanocrystals. From the temporal evolution of the emission spectra shown in Figure 2, it is seen that PLQY was improved greatly at high pH after the surface-related emission was suppressed. Therefore, it is deduced that the difference in PLQY is attributed to the difference in surface composition of the nanocrystals prepared under different pH. XPS analyses were employed to investigate the surface composition of the nanocrystals prepared under different pH. Three elements, Zn, Se, and S, were observable in all ZnSe samples investigated (Supporting Information, Figure S4). Contents of the elements were quantified on the basis of the integration areas of the peaks, as shown in Table 1. The Se/Zn ratio in the ZnSe samples decreased from 0.52 to 0.30 as the pH increased from 6.5 to 11.5. It is noted that the S/Se ratios were 0.89 and 0.84 for the ZnSe samples prepared at pH 6.5 and 8.5, which increased to 1.38 and 2.38 for the samples prepared at pH of 10.5 and 11.5. It is obvious that there is more S element on the surface of the ZnSe nanocrystals prepared at high pH. It is supposed that ZnS shell was formed on the surface of the ZnSe nanocrystals prepared under pH 10.5 and 11.5, which eliminated the surface defects and thus improved the PLQY of the ZnSe nanocrystals. XRD patterns were collected to understand further the structures of the ZnSe nanocrystals (Figure 6). All ZnSe nanocrystals prepared under different pH values presented the same zinc blende structure. The ZnSe nanocrystals prepared at pH 6.5 and 8.5 showed almost the identical crystal structure as bulk ZnSe (bottom). For the samples prepared at pH 10.5 and 11.5, the diffraction peaks shifted to wide angle region and gradually approached those of ZnS crystals (top). These results mean that the products prepared at pH 6.5 and 8.5 were ZnSe nanocrystals, and those prepared at pH 10.5 and 11.5 were ZnSe/ ZnS composite nanocrystals. The XRD patterns of the ZnSe nanocrystals prepared at pH 11.5 collected at reaction time of 1 and 9 h showed that the resulted nanocrystals were ZnSe and ZnSe/
Figure 7. TEM images of (a) the as-prepared ZnSe cores with an average size of 3.2 ( 0.3 nm collected at 1 h of the reaction and (b) the ZnSe/ZnS core/shell nanocrystals with an average size of 5.0 ( 0.5 nm collected at 9 h of the reaction.
Aqueous Synthesis of ZnSe Nanocrystals ZnS composite nanocrystals, respectively, suggesting the core/shell structure of the composite (Supporting Information, Figure S5). The diameters of the nanocrystals collected at 1 and 9 h of the reaction were calculated to be 3.3 and 5.5 nm, respectively, according to the Scherrer equation, which were consistent with the diameters of the ZnSe and ZnSe/ZnS core/shell nanocrystals obtained from TEM observations, as shown in Figure 7. Therefore, the thickness of the ZnS shell of the composite nanocrystals was determined to be ∼1.1 nm.34-36 The release of the sulfur should be attributed to the thiol groups of GSH. It is known that in the absence of metal ions, the decomposition of GSH is readily in acidic or basic solution.37 After the formation of Zn-GSH complexes, the thiol groups become more stable because of the strong interaction between the thiol group and Zn2+.38 The coordination of the amino group of glutamyl residue with Zn2+ cannot greatly weaken the interactions between Zn2+ and sulfhydryl group. Therefore, no sulfur was released at pH of 6.5 and 8.5, and the products were primarily ZnSe nanocrystals. However, at pH >10.3, after the deprotonation of the NH group of the amide bond, the negatively charged N atom will further participate in the coordination as a strong ligand, which weakens the interactions between Zn2+ and sulfhydryl group (thiolate). As a result, the sulfhydryl groups of GSH became freer and then decompose into sulfur to contribute the formation of ZnS.39,40 Deposition of ZnS shell on the surface removed the surface defects of ZnSe nanocrystals effectively, and thus PLQY of the ZnSe nanocrystals was improved greatly (Supporting Information, Figure S6). Control experiments in which MPA or Cys was used as ligand instead of GSH were carried out to prepare ZnSe nanocrystals at pH 11.5. The surface-related emission cannot be eliminated until the end of the reactions, and PLQY of the nanocrystals was
