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J. Phys. Chem. B 2006, 110, 19280-19284
Factors Governing the Quality of Aqueous CdTe Nanocrystals: Calculations and Experiment Alexey Shavel, Nikolai Gaponik, and Alexander Eychmu1 ller* Physical Chemistry, Technische UniVersitaet Dresden, Bergstrasse 66b, 01062 Dresden, Germany ReceiVed: May 31, 2006; In Final Form: July 7, 2006
The aqueous synthesis of thiol-stabilized semiconductor CdTe colloidal nanocrystals has been revisited. We found optimal conditions for the synthesis of high-quality CdTe NCs through a study of the influence of the initial conditions (structure and concentration of Cd-thiol complexes) on the quality of the CdTe nanocrystals. A numerical calculation shows a clear correlation between the concentration of CdL (where L is (SCH2COO)2-) in the initial solution and the photoluminescence quantum efficiency of the CdTe nanocrystals.
Introduction Colloidal quantum dots (QDs) have attracted considerable interest during the past decades. There is a wide range of very efficiently light emitting QDs which can be synthesized by chemical approaches. Worth mentioning are ZnSe,1,2 ZnSe/ZnS3 and CdS 4,5 (UV-blue spectral region), CdSe,6-8 InP,9,10 CdTe11-13 (visible region), PbSe,14,15 HgTe,16 and InAs17 (nearinfrared region). The majority of successful syntheses were done in organic media. However, the potential applications of such particles are restricted by their compatibility with water and/or air. This kind of compatibility is especially important in the field of bio-applications. Worth mentioning is that the phase transfer of NCs from organics to aqueous solution is a timeconsuming procedure involving capping-agent exchange,18 encapsulation in phospholipid micelles,19 silanization of the individual QDs,20 or the preparation of polymeric or dendrimer shells around the particles.21 Besides the complexity, the main drawbacks of these procedures are occasionally limited stability, low emission quantum yield, and large increase of the actual size (critical in the case of conjugation with single biomolecules or intended penetration as markers through biomembranes). At the same time the direct aqueous synthesis of NCs can be a good alternative to the organometallic approach, especially when possible applications demand stability in aqueous media. The simplicity and use of relatively soft conditions, as well as less dangerous materials in comparison with the organometallic approach, have made the aqueous synthesis attractive for researches during the last two decades. For example, IR-emitting HgTe16 and CdHgTe22 as well as blue-emitting CdSe23 are now available. Aqueous CdTe NCs have been successfully synthesized by using the traditional procedure,11,24 a hydrothermal approach (in the autoclave) as well as in a microwave irradiated vessel.25 Thus, possibilities to synthesize a wide range of aqueous II-VI semiconductor colloidal nanocrystals have been revealed. The “traditional” aqueous CdTe NCs synthesis reported previously allowed CdTe NCs to be produced with a photoluminescence (PL) quantum efficiency (QE) of about 5-10% for “as prepared” particles and up to 40% after post-preparative treatments.11 At the same time the organometallic synthesis of CdTe NCs leads to nanocrystals with a QE of up to 65%.12,13 Thus, an improvement of the synthesis of CdTe NCs in water appeared to be a task of special interest. * Address correspondence to this author.
Very recently it was shown by Murase et al.26 that solely the proper choice of the relative concentration of the precursors (cadmium salt and thioglycolic acid) makes it possible to gain strongly emitting (PL QE up to 60%) CdTe NCs without any changes in the preparative approach and avoiding post-preparative treatments. Guo et al.27 proposed that the formation of appropriate Cd-thiolate complexes might be responsible for the improvement observed. The importance of the metal complexes for a successful aqueous synthesis of the CdS NCs has recently been pointed out by Winter et al.28 Thus, in the present paper we report on a systematic experimental study of the CdTe NCs synthetic procedure supported by a calculation of the concentration of various Cdthiolate complexes during the synthesis as a function of pH, concentration of Cd-salt, and the concentration ratio of Cd to ligand. The aim is to reveal a correlation between the availability of a particular complex in the synthetic mixture and optimal luminescence properties of the final product. Experimental Section All chemicals used were of analytical grade or of the highest purity available. All solutions were prepared with Milli-Q water (Millipore) as the solvent. Al2Te3 lumps (Cerac Inc) were used as the source of H2Te. Thioglycolic acid (TGA, Fluka) was used as capping agent without additional purification. Briefly, CdTe nanocrystals were prepared as follows.11 In a typical synthesis 0.985 g (2.35 mmol) of Cd(ClO4)2‚6H2O (Alfa Aesar) was dissolved in 125 mL of water, and an appropriate amount of the thiol stabilizer was added under stirring, followed by adjusting the pH by dropwise addition of a 1 M solution of NaOH. The solution was placed in a three-necked flask fitted with a septum and valves and was deaerated by N2 bubbling for 1 h. Under stirring, H2Te gas was passed through the solution together with a slow nitrogen flow. CdTe precursors were formed at this stage. The further nucleation and growth of the nanocrystals proceeded upon refluxing at 100 °C under openair conditions with a condenser attached. PL measurements (λex ) 450 nm) were performed at room temperature with a FluoroLog-2 spectrofluorimeter (Instruments SA). To avoid the influence of the post-preparative treatment (i.e., the size-selective precipitation)11,29 on the PL QE the NCs were characterized as prepared immediately after the synthesis. The room temperature PL QE of the CdTe nanocrystals was estimated according to the procedure of ref 30 by comparison
10.1021/jp063351u CCC: $33.50 © 2006 American Chemical Society Published on Web 09/12/2006
Influences on the Quality of Aqueous CdTe Nanocrystals
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19281
Figure 1. Typical evolution of the absorption and luminescence spectra of a crude solution of CdTe NCs during the synthesis (left), TEM (right), and HRTEM (inset) of the NCs after 20 h of synthesis. Synthetic conditions used: pH 11.5, Cd/TGA ratio 1/1.3.
Figure 2. Emission quantum efficiency as a function of the TGA/Cd ratio in the initial reaction mixture. Synthetic conditions used: 20 h of synthesis, pH 11.5.
with Rhodamine 6G (laser grade, Lambda Physik) in ethanol (Uvasol, Merck) assuming its PL QE to be 95%. UV-vis absorption spectra were recorded with a Cary 50 spectrophotometer (Varian). Results and Discussion The improvement of the synthesis of the aqueous CdTe nanocrystals was based on the optimization of the well-known experimental procedure.11,31 A typical evolution of the absorption and luminescence spectra during the synthesis is shown in Figure 1. According to ref 11 the optimum pH value is strongly dependent on the nature of the stabilizer. The recommended value in the case of thioglycolic acid is 11.2-11.8 while the proposed value for the TGA/Cd ratio is 2.45. To investigate the influence of the initial conditions on the optical properties and quantum efficiency the initial conditions of the synthesis (pH and Cd/thiol ratio) were systematically changed. TGA/Cd ratios between 2.45 and 1.1 were used. The experimental data show that decreasing the TGA/Cd ratio leads to a drastic increase of the quantum yield of the CdTe NCs (Figure 2). In an attempt to understand this drastic influence on the properties of the NCs, the experimental data were compared with the results of a calculation of the solution composition.
Only a few reports exist on the stability constants of cadmium complexes with thiols: thioglycolic acid,32-34 3-mercaptopropionic acid,33,35 2-mercaptopropionic acid,36,37 and mercaptoand dimercaptosuccinic acid.38 The most complete set of data devoted to the study of the cadmium complexes of thioglycolic acid (TGA) over a wide range of pH values are provided by Turyan and Mandler.32 These data together with the earliest results33,34 and with information about the stability constants of the hydroxy complexes of cadmium39 provide us with the complete information needed to perform the calculation of the composition of the cadmium complexes with thioglycolic acid. The stability constants of cadmium complexes with thioglycolic acid are several orders of magnitude higher than the stability constants of hydroxy complexes and as a result hydroxy complexes play an important role only at very high pH (see the Supporting Information for details of the calculations). Briefly, the calculation of concentration and composition of the cadmium complexes was carried out as shown below: Taking into account the total concentration of the cadmium ions and TGA:
CTGA ) [L2-] + [HL-] + [H2L] + 3[CdL34-] + 2[CdL22-] + [CdL] + [CdH1L1+] + 2[CdH1L2-] + 3[CdH1L33-] + 2[CdH2L2] + 3[CdH2L32-] (1) CCd ) [Cd2+] + [CdL34-] + [CdL22-] + [CdL] + [CdH1L1+] + [CdH1L2-] + [CdH1L33-] + [CdH2L2] + [CdH2L32-] + [Cd(OH)1+] + [Cd(OH)2] + [Cd(OH)3-] + [Cd(OH)42-] (2) where L represents the TGA2- ions. Using the expression for the protonation constants of mercaptoacetate ions
βi )
[(HiL)i-2] [L2-][H+]i
and for the stability complexes
(3)
19282 J. Phys. Chem. B, Vol. 110, No. 39, 2006
β1ij )
[Cd(H)i(L)j2+i-2j]
Shavel et al.
(4)
[Cd2+][H+][L2-]j
the final equations for the equilibrium concentration of cadmium and TGA ions in solution depend only on pH and the total concentration of cadmium and TGA:
CCd ) [Cd2+](1 + B3[L2-]3 +B2[L2-]2 + B1[L2-] + K) (5) CTGA ) B0[L2-] + [Cd2+](3B3[L2-]3 + 2B2[L2-]2 + B1[L2-]) (6) where B0, B1, B2, B3, B4, and K are parameters which depend only on pH and stability constants:
B0 ) β2[H+]2 + β1[H+] +1 B1 ) β111[H+] + β101 B2 ) β122[H+]2 + β112[H+] + β102 B3 ) β123[H+]2 +β113[H+]+ β103 K ) β10[OH-] + β20[OH-]2 + β30[OH-]3 + β40[OH-]4 Numerically solving eqs 5 and 6 allows the determination of the equilibrium concentration of cadmium ions and TGA ions and finally the concentration of all complexes in solution. Several presumptions which can reduce the validity of our calculation have to be mentioned here. The most important source of deviation is the use of the concentration of the ions instead of their activity. The deviation in the first ionization constant of thioglycolic acid estimated by using the Davis equation is 3% (ionic strength 0.064 M (pH 12; TGA/Cd ratio 1.3)). The calculation for the second ionization constant shows a similar result. In fact the ionic strength is even smaller due to the slight dilution by 1 M NaOH solution used for the adjustment of the pH. A second source of deviation is the low solubility of the uncharged complexes of cadmium (for example CdL). The precipitation of the uncharged species leads to a shifting of the equilibrium and further production of these substances. Thus, precipitation leads to the underestimation of the overall amount of the insoluble complex. It is assumed here that the deviations induced by all of our presumptions are within 10% (see the Supporting Information for details). The calculation shows that only three complexes play an important role at high pH: CdL3,4- CdL22-, and CdL. Figure 3 depicts the calculated concentration of these important complexes in solution together with the QE of the CdTe NCs depending on the TGA/Cd ratio at pH 11.5. As is seen from the figure, changing of the TGA/Cd ratio between 5 and 0.85 is followed by a decrease in the concentration of the CdL34complex concentration and an increase in the concentration of the CdL complex. At the same time the concentration of the complex CdL22- goes through a maximum at a TGA/Cd ratio of approximately 2. Simultaneously, Figure 3 shows that the evolution of the QE of CdTe NCs is in good correlation with the concentration of the CdL complex at the initial stage of the synthesis. In other words the increase of the PL QE of CdTe NCs with the decrease in the TGA concentration can be attributed to the increase in the concentration of the CdL complex. This tendency
Figure 3. Distribution of cadmium complexes in solution and PL QE of CdTe NCs after 20 h of synthesis (black dots) depending on the TGA/Cd ratio in initial reaction mixture.
has a natural limit at very low values of the Cd/TGA ratio (near 1) that can be explained by an insufficient amount of stabilizer in the system. In fact the thioglycolic acid plays the role of the stabilizer, a source of sulfur, and as a surfactant in the system, and is also necessary for the formation of cadmium complexes in the solution. Actually, there is a competition of at least two different factors during the synthesis: (i) improving the quality of the nanocrystals with the increase in the concentration of CdL complex (by means of decreasing the concentration of TGA) and (ii) the necessity for a sufficient amount of stabilizer to provide stability and surface passivation of growing particles to be present in solution. PL QE data obtained for the syntheses at Cd:TGA ratios close and below unity are not shown here since under these conditions the lack of stabilizing agent leads to less reproducible and less stable colloidal solutions which makes the results incomparable to those obtained for precursor ratios higher than 1. It should be noted that the solution of the Cd precursors at low TGA/ratios is turbid. This fact is taken as an additional indirect evidence of the domination of the uncharged, less soluble CdL complex. An analysis of the white precipitate was performed by the Gao group using XPS. They deal with a very similar synthetic system and found that the white precipitate appearing as a byproduct of the photochemically assisted improvement of the CdTe NCs PL quantum yield is the quasimonothio cadmium compound with a Cd:S:C(COOH) ratio equal to 1:0.85:1.40 The analysis of the white precipitate that appeared in the mixture of the Cd ions and TGA at ratios close to unity and at basic pH (as in our case) was also performed in the same group yielding a ratio of Cd:S close to 1:1.41 The appearance of the CdL precipitate also means that the concentration of this complex most probably is underestimated as mentioned above. The turbidity of the solution does not disappear during reflux but can be easily removed from the final solution of NCs by filtration. The precipitate of CdL may play an additional role as a source of cadmium. Gradual dissolution of the CdL complex during the particle growth could provide a constant rate of transport of Cd ions to the particles. A slow flux of the cadmium precursor provides the possibility to grow the particles under diffusion control which, as has been predicted theoretically,42 is preferable for the narrowing of the size distribution. The pH of the solution is another important factor that may influence the PL of thiol-capped aqueous nanocrystals.24,43,44 For example, it was demonstrated post-preparatively that the pH-dependent formation of a thiolate shell around the NCs may play a role in the hole trapping and by this influencing their
Influences on the Quality of Aqueous CdTe Nanocrystals
Figure 4. Distribution of cadmium complexes in solution as a function of the pH in the initial reaction mixture.
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19283
Figure 6. Evolution of the PL QE of CdTe NCs during the synthesis at different pH (left) and PL spectra of CdTe NCs with the same position of the PL (right). The PL spectra were taken after 285 (pH 11.5), 171 (pH 12.0), and 42 min (pH 12.5) of synthesis.
Summary
Figure 5. Evolution of the positions of the 1s-1s transition deduced from the absorption spectra (left) and the Stokes shift (right) during the synthesis at pH 11.5 (0), 12.0 (4), and 12.5 (b).
PL.44 Thus, one could expect that the pH might influence the quality of the NCs also in the stage of their synthesis. As is seen from Figure 4 the changing of the pH does not really influence the distribution of the cadmium complexes in the solution in a pH range of between 10.5 and 12.5. Thus we cannot expect the improvement of the NCs quality to be due to the governing of the complex composition in the synthetic solution. At the same time an increase of the pH of the initial solution is followed by a considerable acceleration of the nanocrystal growth (Figure 5). Moreover, as is seen from the figure, the NCs synthesized at pH 12.0 exhibit the smallest Stokes shift (near 0.1 eV for an emission in the red spectral region) which, to the best of our knowledge, is the smallest reported ever for an aqueous synthesis of CdTe NCs. As can be revealed from Figure 6 a narrower size-distribution (i.e., a smaller full width at half-maximum (fwhm)) and a higher PL QE are also attributive to the synthesis at pH 12. Thus, pH 12 seems to be an optimum for the synthesis of aqueous TGA-stabilized CdTe nanocrystals when the molar ratio of Cd and TGA precursors is 1:1.3. These experimental findings can be explained by the existence of an optimum growth rate of the CdTe particles at the given complex composition. A relatively quick particle growth leads to an insufficient quality reflected in low crystallinity and a large number of defects and surface states. Oppositely, a comparatively slow growth rate leads to a high content of sulfur (as a product of the TGA decomposition45) in the particles and a higher probability of the NC oxidation.
To gain further insight into the factors governing an “optimal” preparation of CdTe NCs the distribution of cadmium-TGA complexes has been studied across a wide range of pH values and concentrations of the cadmium ions. Only the CdL34-, CdL22-, and CdL (where L is (SCH2COO)2-) complexes play an important role in the synthesis at all pH values between 9.0 and 12.5. A numerical calculation shows a clear correlation between the concentration of CdL in the initial solution and the PL QE of the resulting CdTe NCs. An improvement of the PL QE of the CdTe NCs up to ca. 50% was achieved through decreasing the Cd/TGA ratio to 1:1.3, which is advantageous for increasing the concentration of the CdL complex. Moreover, simultaneously maintaining the solution pH at 12 allows the reduction of the fwhm and the PL Stokes shift. Acknowledgment. This work was supported by the EU Project “STABILIGHT” and the EU NoE “PHOREMOST”. The authors are thankful to Andreas Kornowski (University of Hamburg) for taking the TEM images. Supporting Information Available: Details of the calculations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. B 1998, 102, 3655. (2) Shavel, A.; Gaponik, N.; Eychmu¨ller, A. J. Phys. Chem. B 2004, 108, 5905. (3) Lomascolo, M.; Creti, A.; Leo, G.; Vasanelli, L.; Manna, L. Appl. Phys. Lett. 2003, 82, 418. (4) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (5) Cao, Y. C.; Wang, J. J. Am. Chem. Soc. 2004, 126, 14336. (6) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (7) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (8) Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Nano Lett. 2001, 1, 207. (9) Micic, O. I.; Sprague, J.; Lu, Z.; Nozik, A. J. Appl. Phys. Lett. 1996, 68, 3150. (10) Talapin, D. V.; Gaponik, N.; Borchert, H.; Rogach, A. L.; Haase, M.; Weller, H. J. Phys. Chem. B 2002, 106, 12659. (11) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177. (12) Talapin, D. V.; Haubold, S.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. J. Phys. Chem. B 2001, 105, 2260.
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