ARTICLE pubs.acs.org/JPCC
The Effects of Particle Concentration and Surface Charge on the Oriented Attachment Growth Kinetics of CdTe Nanocrystals in H2O Shungao Yin,† Feng Huang,‡ Jing Zhang,*,† Jinsheng Zheng,§ and Zhang Lin*,† †
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China § Laboratory of Analytical Science of Xiamen University, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China
bS Supporting Information ABSTRACT: Crystal growth kinetics of thioglycolic acid (TGA)-capped CdTe nanoparticles in H2O was studied at different particle concentrations, with the aim to understand the effects of concentration and surface state of primary particles on the growth mechanism of nanocrystals. The growth process of CdTe nanoparticles was monitored by in situ UVvis absorbance spectra, which reveal the coexisting growth mechanisms of oriented attachment (OA) and Ostwald ripening (OR). Increasing the primary nanoparticle concentration facilitated the OA-based growth at the initial stage, while the OR growth which was mainly occurring at the latter stage was restrained. On the basis of the fitting activation energy and experimental zeta potential, the effect of surface charge was further discussed and proved to be the critical factor that affected the OA process by directing the particleparticle interaction.
1. INTRODUCTION Semiconductor nanocrystals (NCs), such as ZnS, CdS, CdSe, and CdTe, have attracted much attention in recent years, due to their unique physical properties and promising values in optoelectronic and biomedical applications.16 As a result of the quantum size effect, physical properties of semiconductor NCs can be tuned over a wide range by tailoring their microstructures such as size, shape, and surface state. An effective method to control the microstructures is via optimizing the synthetic parameters during experiments, in which the wet chemistry has been considered as one of the most efficient method in controlling microstructure of nanomaterials. Many efforts and great progress have been made in synthesis of high-quality NCs,711 whereas the related growth mechanism still lacks general understanding. The classic mechanism to describe crystal growth is based on Ostwald ripening (OR) theory, in which the growth kinetics obeys an exponential relationship.12 The dependence of ion concentration on curvature, known as GibbsThomson effect, is the driving force of OR that causes small particles dissolving and large ones growing. OR growth may undergo three processes: dissolution, diffusion, and precipitation. These growth characteristics determine that OR growth depends on properties of material, nature of solution, and state of particle surface. When particle sizes are reduced down to nanoscale, the growth mechanism named oriented attachment (OA)1315 may happen r 2011 American Chemical Society
in crystal growth process, where two crystallographically oriented nanoparticles combine together to form a larger one. Recently, more attention is being paid to the role of OA in the formation of specific microstructure or self-assembly of nanomaterials.1627 It has been reported that nanomaterials obtained via OA mechanism exhibit unique microstructures, such as chain-like NCs,2426 nanorods,20,21 ultrathin nanosheet,16 dendrite NCs,27 and 3D superlattice.22 Meanwhile, crystallography defects are easily generated during the OA growth and can be retained inside crystals.2833 Increasing interest has been devoted to the study of OA mechanism,34,35 because of its important role demonstrated in the preparation of nanomaterials. The recent progress in kinetic modeling of OA has been made by several excellent works,28,29,3639 and recent experimental development in observing the OA growth of NCs by in situ TEM has also been achieved.4042 It is assumed that the OA process may consist of three steps: diffusion of NCs in solution before collision, desorption of surface ligands, and coalescence by self-adjusting. Thus any factors affecting one of these steps, such as solution media, surface state of NCs, and particleparticle interaction, might have influences on the OA growth. For example, it
Received: December 22, 2010 Revised: April 20, 2011 Published: May 12, 2011 10357
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2. EXPERIMENTAL SECTION 2.1. Synthesis of TGA-Capped CdTe NCs. The CdTe NCs were synthesized using a precursor method (reported elsewhere44), by adding freshly prepared NaHTe solution into N2-saturated CdCl2 solution at pH 9.0 in the presence of thioglycolic acid (Aldrich, 99.8%). In a typical synthesis procedure, NaHTe solution was first prepared by the reduction of 0.127 g (1 mmol) of tellurium powder (Aladdin, 99.999%, 200 meshes) with NaBH4 in 2 mL of water for about 8 h. Then, 0.912 g (4 mmol) of CdCl2 3 2.5H2O was dissolved in 100 mL of deionized water in the presence of 0.291 mL (4 mmol) of TGA. The obtained Cd-TGA solution was bubbled with N2 for half an hour, and the pH was adjusted to 9.0 using 1 M NaOH. Afterward, NaHTe solution was injected into Cd-TGA solution under vigorous stirring to obtain CdTe precursor solution. The precursor solution was heated to the boiling point for about 20 min to obtain the TGA-capped CdTe colloidal solution, which was then cooled down and stored for the next-step use. 2.2. Growth Kinetics of TGA-Capped CdTe NCs in H2O. CdTe NCs were precipitated by adding ethanol into 27-mL aliquots of the as-prepared CdTe colloidal solution. The precipitate was separated by centrifugation and redispersed in 9 mL of N2 saturated, deionized water to obtain the sample with 30 mmol/L of CdTe. Then, the solution with 30 mmol/L of CdTe was diluted to 3 times its volume to obtain the sample with 10 mmol/L of CdTe, which was further diluted to 3 times its volume to obtain the sample with 3.3 mmol/L of CdTe. (Supposing that the primary CdTe NCs have an average particle size of 2 nm and an average atom number of 200 and all CdTe are transformed into NCs, the samples with 30 mmol/L, 10 mmol/L, and 3.3 mmol/L of CdTe will have the actual particle concentrations of about 0.3 mmol/L, 0.1 mmol/L, and 0.033 mmol/L, respectively.) Growth kinetics experiments were conducted in these three concentration groups at four different temperatures (353, 348, 343, and 338 K). In each series, the aqueous solution of TGA-capped CdTe particles at a certain concentration was put into the cuvette with an optical path length of 1.0 mm, which was sealed to prevent CdTe oxidization. UVvis absorbance spectra were used to record the in situ kinetic growth of CdTe NCs in aqueous solution. 2.3. Characterizations of TGA-Capped CdTe NCs. The UVvis absorbance spectra were recorded using a Shimadza UV-2550 double monochromator UV visible spectrophotometer (Shimadza, Japan)
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Figure 1. XRD patterns and HRTEM (insert) observation of the assynthesized TGA-capped CdTe NCs. Inserted lines present the standard XRD pattern from reference data ICSD-93942.
with halogen lamp and deuterium lamp as the light sources. Data acquisition was processed in spectrum mode, with bandwidth set at 0.2 nm and scanning range from 700 to 300 nm. The temperature of samples in the cuvette (optical length: 1.0 mm) was kept constant using the NTT2200 constant-temperature water circulator (Shimadza, Japan) with temperature precision of 0.1 K. The average particle sizes of the CdTe NCs were estimated from UVvis spectra using the absorption edge method,45 with the variation of bandgap (ΔEg) in CdTe NCs as a function of diameter (d),46 ΔEg = 1/(0.049d2 þ 0.146d þ 0.387). X-ray diffraction (XRD) was used to identify phase structure and average particle sizes of primary particles. CdTe samples for XRD experiment were separated by precipitation using ethanol and centrifugation at the speed of 8000 rpm. The XRD data were collected using a PANalytical X’Pert PRO diffractometer with Cu KR radiation (40 kV, 40 mA) in the continuous scanning mode. The 2θ scanning range was from 15 to 55 in steps of 0.03 with a collection time of 20 s per step. The average crystalline size was calculated from the peak broadening using the Scherrer formula. High-resolution transmission electron microscopy (HRTEM) was used to characterize particle size, shape, and crystal structure. Samples were prepared for HRTEM study by dispersing the CdTe NCs solution onto 200-mesh ultrathin-carbon-coated copper grids. HRTEM images were obtained on a JEOL JEM2010 at 200 kV. Zeta potential of TGA-capped CdTe NCs in solutions was carried out on the ZetaPALS zetameter (Brookhaven Instrument Corp., New Jersey, U.S.A.). For each sample, an appropriate amount of NCs solution without further dilution was placed in the cuvette, and three runs of measurements were taken to obtain an average zeta potential value. The solution media was set as water for all zeta potential measurements.
3. RESULTS The as-synthesized TGA-capped CdTe NCs are characterized by XRD and HRTEM (Figure 1). XRD pattern reveals that the CdTe NCs are zinc blende phase with the particle size of ∼2 nm, and HRTEM observation confirms the XRD results. Figure 2a shows the typical evolution of UVvis absorbance spectra of CdTe NCs during the growth process from 0 to 200 min. It shows that the absorption edge of UVvis absorption spectra has a significant red shift, which implies that the NCs are continuously growing. The average sizes of the CdTe NCs were calculated from the absorption edges of UVvis absorption spectra by using the absorption edge method,45 which 10358
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Figure 2. (a) The typical in situ UVvis absorbance spectra for CdTe NCs solution with particle concentration of 0.3 mmol/L from 0 to 200 min at 353 K. (bd) Experimental data (scattering data) and fitting results (solid line) of variation in the sizes of CdTe NCs as a function of time for three different primary concentrations of (b) 0.3 mmol/L, (c) 0.1 mmol/L, (d) 0.033 mmol/L. The solid lines in (b), (c), and (d) are obtained by fitting experimental dots with OA and OR mixing growth model. The dotted and dashed lines represent the fitting by OA kinetic model and OR theory, respectively.
Figure 3. Average diameters d of the coarsening TGA-capped CdTe NCs (a) at 20 min of the initial growth stage, and (b) at 200 min of the latter growth stage vs primary particle concentrations N0 at different temperatures (353, 348, 343, and 338 K).
gives the growth curves of particle size against coarsening time. As shown in Figure 2b-d, the growth curves (symbolized as points) of TGA-capped CdTe NCs were obtained for three samples with primary particle concentration of 0.3, 0.1, and 0.033 mmol/L, respectively, at four different temperatures (353, 348, 343, and 338 K). For each system, we can clearly see that increasing temperature facilitates the particle growth. In order to highlight the influence of primary particle concentration on the growth of CdTe NCs in H2O, average particle sizes against particle concentrations at varied growth time points were extracted and plotted in Figure 3. Interestingly, we found that at the initial growth stage (such as 20 min in Figure 3a), increasing particle concentration accelerated the growth rate, while in the latter stage (such as 200 min in Figure 3b), the particle growth became slower in the system with more primary particle concentration.
We tried to fit the growth curves in Figure 2bd with OR theory,12 by using the exponential expression: dn d0 n ¼ KOR t
ð1Þ
where d is the mean particle size at the time t, while d0 is primary particle size; KOR is a temperature-dependent growth rate constant; n is an exponent relevant to the coarsening mechanism, which is physically meaningful in the range of 15. As shown in Figure 2bd, the dashed lines are the fitting results by the OR equation with the exponent n = 3. We can see that the OR theory cannot give a fit to the whole growth curves, but a satisfactory fit was achieved for the growth at a latter stage after 50 min. (Mathematically fitting the growth curves will generate the exponent n larger than 10, which has no physical meaning for OR theory.) 10359
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Figure 4. HRTEM images of CdTe NCs in water with primary particle concentration of 0.3 mmol/L coarsened at 353 K with two representative time points at 20 min (ad) and 200 min (eh). The corresponding diagrams on the right illustrate the profiles of nanocrystals on the left. Scale bars for all images are 2 nm.
Author: Meanwhile, the OA kinetic equation, first developed by Huang et al.,28 was used to fit CdTe growth in this work, in which the collision and coalescence between two particles (A) to form a larger particle (B) may be interpreted as follows: kOA
A þ A sf B
ð2Þ
and the corresponding kinetic equation for the evolution of particle size was given as: d ¼ d0
21=3 N0 kOA t þ 1 N0 kOA t þ 1
ð3Þ
where kOA is the OA rate constant, N0 is the primary particle concentration. Equation 3 describes an asymptotic behavior of the OA growth, and satisfactory fits to the growth curves with
eq 3 (dotted lines) can be obtained but were limited at the early stage before 40 min (Figure 2bd). In order to get a better understanding of the growth mechanism, HRTEM was used to detect the microstructure of CdTe NCs. Figure 4 illustrates the HRTEM observations for the sample with particle concentration of 0.3 mmol/L coarsened at 353 K. We can see that, at time point of 20 min (Figure 4ad), most of the irregularly shaped NCs exhibit the characteristics of growth by the OA mechanism, such as the retained lattice defects (twins and stacking faults) and clear coalescence profile of small “building blocks”. This suggests that OA might be mostly occurring in the initial growth period, whereas at 200 min (Figure 3eh), the particles evolved into a relatively perfect lattice and a more regular shape with smooth edges, which means the OR mechanism might be the main control of the growth of particles at this stage. The same features for the particles at these two growth stages can also be observed in other systems. 10360
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Table 1. Kinetic Parameters Estimated by Fitting the Experimental Growth Curves in Figure 2 kOA (L 3 mol1 3 min1)
KOR (103 nmn 3 min1)
n
N0 (mmol/L)
353 K
348 K
343 K
338 K
353 K
348 K
343 K
338 K
353 K
348 K
343 K
338 K
0.3
926.6
520.0
291.7
155.3
8.61
5.51
3.50
2.30
2.90
2.90
2.90
2.98
0.1
932.1
570.1
346.0
210.2
14.7
9.80
6.27
3.90
2.93
2.91
2.95
2.97
0.033
1849.7
1214.9
800.3
503.1
17.9
11.7
7.68
4.70
2.93
2.94
2.98
2.99
Table 2. Parameters Obtained by Applying Arrhenius Plot to the Rate Constants in Table 1 Ea (kJ/mol) N0 (mmol/L) 0.3
ln(A0)
4. DISCUSSIONS
OA
OR
OA
OR
117.8 ( 4.9
87.4 ( 3.3
37.8 ( 1.3
25.0 ( 1.7
0.1
98.5 ( 4.2
86.6 ( 3.2
31.2 ( 1.2
25.3 ( 1.9
0.033
85.7 ( 3.1
87.3 ( 3.8
27.5 ( 1.8
25.7 ( 1.2
On the basis of the above analysis and HRTEM observations, we propose that the growth of TGA-capped CdTe NCs in H2O is controlled by the coexisting growth mechanisms of OA and OR. In a previous study,29 Huang et al. developed a kinetic model to describe the growth of ZnS nanoparticle aggregates via the hybrid OA and OR mechanism, where the strongly aggregated state of the nanoparticles facilitates the OA growth, while OR has little contribution at the early stage. In our work, CdTe NCs are highly dispersed in H2O, and the particle surface charge has a large influence on OA between particles (see details in the Discussion). Thus, we propose that OR is always happening on individual particles and contributes to the nanoparticle growth from the beginning. This means that the primary particle size for OA is related to the OR equation. Therefore, by substituting the primary particle size d0 in eq 3 with the evolution of particle size d from eq 1, the mixed OA and OR growth equation can be obtained: d ¼ ðd0 n þ KOR tÞ1=n
21=3 N0 kOA t þ 1 N0 kOA t þ 1
ð4Þ
By fitting all the growth curves with eq 4, a remarkable goodness of fit was obtained. The fitting curves (solid lines) and experimental points are given in Figure 2bd, and the parameters of fitting are listed in Table 1. For the OR growth, the scaling exponent, n, is approximately equal to 3 for all the growth curves, indicating a diffusion-controlled ripening. With particle concentration increasing, both the growth rate constants of OA and OR decrease (see Table 1). The dependence of growth rate constant k on temperature T can be given by the Arrhenius equation, of which the natural logarithm form is expressed as: ln k ¼
Ea þ ln A0 RT
85.7 to 117.8 kJ/mol with the primary particle concentration increasing from 0.033 mmol/L to 0.3 mmol/L.
ð5Þ
where Ea is activation energy, A0 is the pre-exponential factor, R is the gas constant, T is the absolute temperature. Accordingly, the activation energies (see Table 2) were estimated by applying Arrhenius plots (Figure 5ac). As listed in Table 2 and shown in Figure 5d, the activation energies for the OR growth are close to each other, which are around 87 kJ/mol, for all the samples. While for the OA growth, activation energies increased from
4.1. The Influence of Particle Surface State on the OA Growth. Generally, colloidal QDs in solution undergo collisions
and coalescences, and therefore OA growth is closely related to the concentration of primary nanoparticles as well as to the sizes of the particles. A higher primary particle concentration provides more chances for collisions between nanoparticles and leads to a rapid OA reaction. As a result, we see that a higher growth rate at the initial stage was achieved in the aqueous solution with a higher primary particle concentration (Figure 3a). This result demonstrates in reverse that nanoparticle growth at the initial stage is mainly controlled by the OA mechanism. From Figure 3 and Table 1, we can see that a higher particle concentration gives rise to a more rapid growth in the initial stage but a surprisingly smaller OA rate constant, kOA, and a correspondingly higher OA activation energy, Ea,OA. As we know, for a chemical/molecular reaction the reaction rate constant has nothing to do with the concentration of reactants. While for the nanoparticle growth behavior, with the capping agents on their surfaces the surface state of nanoparticles is a non-negligible factor, as nanoparticles have a much larger surface area than bulk materials. In our case, primary particles were obtained by redispersing the initial CdTe precipitate into pure water again. Since the original CdTe precipitates contain excess TGA, the surface-capping state of the particles was highly dependent on the dilution times. In other words, the differences among the three systems not only lay in the particle concentration but also the concentration of the surfactants. The sample with a higher particle concentration might contain a relatively higher concentration of surfactants on the particle surface, due to the fact that more surfactants are lost into solvents in the diluted sample. To verify this hypothesis, for the three systems we measured the zeta potential, which is directly correlated with the surface charge from capping TGA. As shown in Figure 6, zeta potential decreased as the sample was diluted, which implies that more highly concentrated particles bore more surface charges. Therefore, two factors might be considered to cause the OA activation energy to be higher for the sample with relatively high particle concentration. First, more surfactants on the nanoparticles may increase the desorption energy of capping agents during the combination of two primary particles. Second, higher surface charge density may increase the electrostatic particleparticle interaction energy, thus preventing NCs approaching each other for OA. Previous work shows that the rate-limiting step for OA growth is the diffusion of NCs in solution.36 Therefore, we propose that the second factor plays a more important role in limiting OA growth and makes a higher OA activation energy for the system with a higher particle concentration. 10361
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Figure 5. (ac) Arrhenius plot of the OA and OR kinetic rate constants for three samples with different primary particle concentrations N0: (a) 0.3 mmol/L, (b) 0.1 mmol/L, and (c) 0.033 mmol/L. (d) The corresponding activation energies of OA and OR growth.
with the van de Waals attractive interaction energy,49 ! A 2a2 2a2 r 2 4a2 V ðrÞ ¼ þ 2 þ ln 6 r 2 4a2 r r2
Figure 6. Experimental zeta potentials (negative) for TGA-capped CdTe samples at different primary particle concentrations, N0.
In order to get a quantitative understanding of particleparticle interaction energy between NCs, we herein quote the DerjaguinLandauVerweyOverbeek (DLVO) theory to describe interaction energy between particles in solution.48 In DLVO theory, the interaction energy U(r) between pair of well separated spherical particles, which consists of electrostatic repulsion energy and van de Waals attraction energy, can be expressed as follows: ka 2 UðrÞ e ekr V ðrÞ 2 ¼Z þ ð6Þ λB kB T kB T 1 þ ka r
ð7Þ
where r is the center-to-center distance between two spheres of radii a; λB is Bjerrum length, λB = 0.7 nm (room temperature) for water; k1 is DebyeH€uckel screening length; Hamaker constant A ≈ 24 kBT; effective surface charge Z is correlated with the zeta potential ζ, eζ a Z¼ ð1 þ akÞ ð8Þ kB T λB By substituting eq 8 into eq 6, we can see that the particleparticle interaction energy, U(r), is correlated with zeta potential ζ, UðrÞ eζ 2 a2 e2ka kr V ðrÞ ¼ þ ð9Þ kB T kB T λB kB T r According to eq 9, we can calculate the interaction energy U(r) from the measured zeta potential (Figure 6), with the assumption that the spherical radius a = 5 nm, and DebyeH€uckel screening length k-1 = 10 nm. As shown in Figure 7a, at T = 338 K, a higher primary particle concentration N0 produces a higher electrostatic particleparticle interaction energy U(r). The reason is that the 10362
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Figure 7. (a) Predicted electrostatic particleparticle interaction energy U(r) between CdTe NCs in solutions with different particle concentrations N0: 0.3 mmol/L (solid line), 0.1 mmol/L (dashed lines), and 0.033 mmol/L (dashdot lines) at T = 338 K. (b) The OA activation energy Ea,OA vs the U(r) max (electrostatic barrier energy).
variation of particle concentration could lead to a change of surface agent density on particles, and thus, changes to both surface charge and zeta potential ζ. Though in eq 9, particle concentration (N0) is not directly correlated with interaction energy U(r), zeta potential experiments revealed that the TGAcapped CdTe NCs solution with a higher particle concentration has a larger (negative) zeta potential, which finally gives rise to higher interaction energy U(r). Also, this result is consistent with the fact that a higher OA activation energy is obtained in the system with a higher particle concentration (Table 2). Furthermore, Figure 7b shows the plot of OA activation energies against the U(r) max (electrostatic barrier energy) in Figure 7a, and the OA activation energy without electrostatic interactions was evaluated to be 63 kJ/mol by linear extrapolation. As a result, we draw the conclusion that, when considering the OA activation energy, in addition to the particle diffusion barrier energy the particleparticle repulsion energy should be taken into account. Though the above results were obtained at T = 338 K, the temperature effect on U(r) can be roughly overlooked in a small temperature range, such as 338353 K in our system, because DebyeH€uckel screening length k1 and zeta potential ζ in eq 9 vary slightly with temperature. According to ref 50 the DebyeH€uckel screening length k1 is proportional to the square root of temperature T1/2: k1 T1/2, so that the variation of k1 with T in the range of 338353 K can be neglected, and the zeta potential would increase slowly with temperature. For example, the zeta potential of silica in water increases linearly with temperature at a rate of 0.0175 mV/K,51 so that the results in Figure 7 are representative for other temperatures. For fitting our systems, we only consider the OA reaction between primary particles. As the particle growth continues, OA might be also occurring between larger particles. In a previous work,36 it has been revealed that with the increase of particle sizes, the OA rate constant largely deceases due to the diffusion effect. In addition, with the growth of particle sizes, the electrostatic particleparticle repulsion will increase according to eq 6, and thus the OA rate constant decreases abruptly. By combining these two effects, we may draw the conclusion that the OA rate constant will greatly decrease due to the particle growth in the presence of strong electrostatic repulsions between particles. Therefore, it is reasonable only to consider the OA between primary particles in our TGA-capped CdTe systems. 4.2. The Effect of Particle Concentration and Capping Agent on OR Growth. By fitting the growth curve of TGAcapped CdTe NCs in aqueous solution, it is revealed that OR growth is controlled by volume diffusion of ions (n ≈ 3).
Moreover, the OR growth activation energy is kept constant at about 87 kJ/mol at different particle concentrations. However, when particle concentration increases, the OR growth rate constant KOR decreases (see Table 1). Recent studies revealed that OR growth tends to be inhibited in the system where nanoparticles were strongly surfacecapped,28,36 because when the surfaces of the nanoparticles were strongly wrapped by surfactant, the rate of dissolution and precipitation of the free ions could be slowed down or even inhibited. According to the LifshitzSlyozovWagner (LSW) law,12 the OR rate constant KOR in eq 1 in the case of n = 3 can be expressed as: KOR ¼ 8γDVm C¥ =9RT
ð10Þ
where γ is the surface energy, D is the diffusion constant at temperature T, R is the gas constant, Vm is the molar volume, and C¥ is the equilibrium concentration at a flat surface. In our systems, as discussed above, a higher particle concentration in the solution has a higher TGA species strongly adsorbing on the particle surface, which will lead to a smaller surface free energy γ of CdTe NCs.47 Therefore, the OR growth rate constant, KOR, decreases in the system with more primary particles. As a result, as shown in Figure 3b, the growth rate at a latter stage is mainly via the OR mechanism decreaseing with the increase in particle concentration, although there is no contribution of particle concentration to OR growth according to eq 1.
5. CONCLUSION In this work, we have demonstrated that the growth of TGAcapped CdTe NCs in aqueous solution is controlled simultaneously by the OA and OR mechanisms. Furthermore, increasing the primary particle concentration facilitates the OA growth, while a higher surface charge inhibits OA by preventing NCs approaching each other. Meanwhile, OR growth can also be slowed down by strong surface adsorbing, which is not obviously affected by particle concentration. Normally, NCs obtained via the OA mechanism are prone to have irregular shapes and microstructural defects, but NCs obtained via the OR mechanism have regular shapes and fewer crystal defects. As a result, physical properties of NCs may be strongly affected by the growth mechanisms, for example, the photoluminescence of semiconductor CdS nanocrystals was recently reported to be directly related to the growth mechanism of OA and OR. Thus, in order to synthesize high-quality NCs, we may control the 10363
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’ ASSOCIATED CONTENT
bS
Supporting Information. Relationship between band gap and the average size; TEM observation of the as-synthesized TGA-capped CdTe NCs; consideration of the growth model of TGA-capped CdTe NCs. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
[email protected];
[email protected].
’ ACKNOWLEDGMENT Financial supports for this study were provided by the National Basic Research Program of China (2010CB933501), National Science Foundation Grants (20803082, 20971123), Fujian Science Foundation Grants (2009J5017, 2010J0102 , 2010J0103), and Fund of Fujian Key Laboratory of Nanomaterials (2006L2005). We thank Ping Huang at Fujian Institute of Research on the Structure of Matter for helping with transmission electron microscope characterization. ’ REFERENCES (1) Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V. Chem. Rev. 2010, 110, 389–458. (2) Mueller, A. H.; Petruska, M. A.; Achermann, M.; Werder, D. J.; Akhadov, E. A.; Koleske, D. D.; Hoffbauer, M. A.; Klimov, V. I. Nano Lett. 2005, 5, 1039–1044. (3) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385–2393. (4) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737–18753. (5) Fu, A. H.; Gu, W. W.; Boussert, B.; Koski, K.; Gerion, D.; Manna, L.; Le Gros, M.; Larabell, C. A.; Alivisatos, A. P. Nano Lett. 2007, 7, 179–182. (6) Yong, K.-T.; Ding, H.; Roy, I.; Law, W.-C.; Bergey, E. J.; Maitra, A.; Prasad, P. N. ACS Nano 2009, 3, 502–510. (7) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature 2005, 437, 121–124. (8) Xu, S.; Kumar, S.; Nann, T. J. Am. Chem. Soc. 2006, 128, 1054–1055. (9) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.; Park, J.-H.; Hwang, N.-M.; Hyeon, T. Nat. Mater. 2004, 3, 891–895. (10) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59–61. (11) Cademartiri, L.; Ozin, G. A. Phil. Trans. R. Soc. A 2010, 368, 4229–4248. (12) Thomson, R. M. In Physical Metallurgy; Cahn, R. W., Haasen, P., Eds.; North-Holland: Amsterdam, 1996; Chapter 9. (13) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969–971. (14) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751–754. (15) Penn, R. L.; Banfield, J. F. Am. Mineral. 1998, 83, 1077–1082. (16) Schliehe, C.; Juarez, B. H.; Pelletier, M.; Jander, S.; Greshnykh, D.; Nagel, M.; Meyer, A.; Foerster, S.; Kornowski, A.; Klinke, C.; Weller, H. Science 2010, 329, 550–553. (17) Penn, R. L.; Oskam, G.; Strathmann, T. J.; Searson, P. C.; Stone, A. T.; Veblen, D. R. J. Phys. Chem. B 2001, 105, 2177–2182.
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