J. Phys. Chem. B 2007, 111, 1449-1454
1449
Oriented Attachment Kinetics for Ligand Capped Nanocrystals: Coarsening of Thiol-PbS Nanoparticles Jing Zhang,† Yonghao Wang,† Jinsheng Zheng,† Feng Huang,† Dagui Chen,† Youzhao Lan,† Guoqiang Ren,† Zhang Lin,*,† and Chen Wang‡ State Key Lab of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China, and National Center for NanoScience and Technology, Beijing 100080, People’s Republic of China ReceiVed: October 26, 2006; In Final Form: December 15, 2006
In this work, the growth kinetics of thiol-capped PbS nanoparticles was studied. Two-stage growth process was observed, which was controlled first by oriented attachment (OA) mechanism and then by the hybrid Ostwald ripening (OR) and OA mechanism. Different from the NaOH-ZnS system, where OA will occur between any two multilevel nanoparticles, an OA kinetic model only considering the attachment related to original particles was fitted well with the experimental results. Analysis reveals that this model may be a universal one to describe the OA crystal growth process of nanocrystals capped with easily destroyed ligands, such as thiol-ZnS in the previous report. The OA crystal growth characteristics determined by the surface agent were discussed and compared. We propose that with stronger surface capping, the OR growth of nanocrystals is hindered, which facilitates the size controlling via OA kinetics during nanosynthesis.
Introduction Nanometer-scale materials, especially the semiconductor crystallites such as ZnS, CdSe, and PbS, have been investigated extensively for their unique size-dependent properties of optics, electricity, and magnetism.1,2 The fundamental characteristics from phase stability to electronic structure, can be influenced by the size, process of growth, and surface energy of nanoparticles.3-5 A detailed investigation of quantum size effects requires the preparation of nanoscale materials with controllable size, structure, and morphology. The synthetic approaches are various, including chemical vapor deposition,6 sol-gel methods,7 mechanical ball milling,8 molecular beam epitaxy,9 solution-phase method10 and so on. In various synthetic schemes, narrowly size-distributed, even monodisperse products are desired to investigate the fundamental physical phenomena that tightly associated with size and shape.11-14 Successful examples were reported on the basis of solution-phase methods, in which surface active reagent are introduced to “arrest” growth and thus limit particle size.15-17 Currently, the surface capping approach is widely used in the synthesis, whereas the related growth mechanism still lacks general understanding. Studies on the crystal growth kinetics, the size and shape evolution, and relevant phase transformation in ligand capped nanoparticles can help for understanding the essential of nanosynthesis, and achieving the desired nanoscale materials consciously.18 Early studies on the kinetic model for coarsening of crystals were based on the classic Ostwald ripening mechanism,19-21 which involves the growth of larger particles at the expense of smaller ones. The driving force for this process is the decrease of the total surface free energy. According to the Gibbs-Thompson equation,22 the equilibrium solute concentration at the surface of larger particles is lower than that of * Corresponding author. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ National Center for NanoScience and Technology.
smaller ones. So the resulting concentration gradients lead to solute ions flowing from small particles to larger ones. The coarsening mechanism is often controlled by the diffusion, particle growth via addition of ions to the particle surface from solution. The growth kinetics in this way mainly depends on the structure of the materials, the properties of the solution, and the nature of the interface between the crystals and the surrounding solution. The Ostwald ripening mechanism has been used extensively to describe and explain the crystal growth of particles with approximately micrometer size in solution. However, in many circumstances, it is found that the crystal growth of nanocrystals cannot be described by the Ostwald ripening mechanism completely. For example, Peng et al. have observed a discontinuous two-stage growth kinetics during the synthesis of the CdSe nanocrystals.23 Moreover, irregular and anisotropic morphologies were often obtained in nanocrystal synthesis.24-28 The special growth kinetics, the size and shape evolution, and the abundant dislocations and planar defects in nanocrystals indicate that the crystal growth may be dominated by another mechanism: crystallographically specific oriented attachment (OA).29,30 Unlike OR growth mechanism, where the growth is based on the migration of atoms between particles, the OA growth is related to the direct self-organization of two particles into a single crystal by sharing a common crystallographic orientation, despite the presence of strong surface-bound ligands.31,32 After the first discovery of the OA mechanism by Penn and Banfield,29,30 it has been extensively observed in other systems.24-28 In fact, the OA mechanism is regarded to generally exist in the early stage of nanosynthesis and crystal growth.33,34 The first and simplest OA kinetics model was proposed by Huang and Banfield et al. on the basis of the hydrothermal coarsening results of thiol-capped ZnS nanoparticles, where the OA of two primary particles (A1 + A1) was considered.31 In fact, the attachment between two other particles such as a primary particle and a secondary one, even a multilevel one,
10.1021/jp067040v CCC: $37.00 © 2007 American Chemical Society Published on Web 01/20/2007
1450 J. Phys. Chem. B, Vol. 111, No. 6, 2007 also exists and can often be experimentally observed.31,35,36 Recently, we presented a multistep OA kinetic model based on the principle of collision reactions of molecules, to describe the growth process of ZnS nanocrystals in concentrated NaOH.32 In this kinetic model, OA-based growth reaction between two primary particles (A1 + A1), reaction between primary particle and above (A1 + Ai), and reaction between two multilevel particles (Ai + Aj) are considered. As we have noticed, the pure OA processes are usually observed at the early stage of crystal growth, whereas the lasting time for this period varies with the surface capping ligands and the solution conditions. For a deeper understanding of this difference, in this work, thiol-capped PbS nanoparticles were selected as the second example of ligand capped nanoparticles for the kinetic study. A new OA kinetic model was proposed; moreover, the roles of OA during nanosynthesis were discussed.
Zhang et al.
Figure 1. XRD pattern (left) and HRTEM image (right) of the assynthesized PbS nanoparticle.
Experimental Section First, the primary PbS nanoparticles capped with dodecanethiol (C12H25SH) were synthesized. Methanol was used as the solvent to dilute dodecanethiol and minimize the growth of primary particles in water during synthesis. Three stock solution were prepared first: (a) 5 mmol of sodium sulfide (Na2S‚9H2O) in 50 mL of H2O and 50 mL of methanol; (b) 2 mL of dodecanethiol in 100 mL of methanol; (c) 5 mmol of lead acetate (Pb(AC)2‚3H2O) in 50 mL of methanol. Solutions a and b were mixed together first, followed with solution c dropped into them with continuous stirring. The mixture was continuously stirred vigorously for 30 min. The precipitates were separated by centrifugation, and the impurities were removed by washing with pure ethanol and DI water until the pH was ∼7.0. The product was dried and ground into powder for the following kinetic experiments. All of the above experiments were processed at room temperature. As-synthesized nanocrystalline PbS (about 1 g) and 200 mL of deionized water were mixed in two necked flask of 500 mL capacity and kept in a water bath at 60, 80, and 100 °C, separately. Samples (about 0.02 g) were taken out periodically for analysis. X-ray diffraction (XRD) was used to identify the crystal structures and average particle sizes. Diffraction data were recorded using a PANalytical X’Pert PRO diffractometer with Cu KR radiation (45 kV, 40 mA) in the continuous scanning mode. The 2θ scanning range was from 15° to 85° in steps of 0.03° with a collection time of 20 s per step. The average crystallite size was calculated from the peak broadening using the Scherrer equation, after the instrumental contribution was subtracted. High-resolution transmission electron microscopy (HRTEM) was used to confirm the particle size and to determine the particle morphology and the growth form of nanocrystals. Samples for HRTEM study were prepared by dispersing the PbS powder onto 200-mesh carbon-coated copper grids. HRTEM analyses were performed using a JEOL JEM2010 HRTEM at 200 kV. Results The typical XRD pattern and HRTEM of as-synthesized PbS are shown in Figure 1. XRD data reveal that the nanoparticles are cubic, with calculated average size of ∼4.6 nm. HRTEM observations confirm the size, morphology, and the phase structure from XRD. Figure 2 shows the increase of average particle sizes versus time at 60, 80, and 100 °C. It reveals that the growth of PbS nanoparticles shares the same rule for three coarsening tem-
Figure 2. Experimental data and fitting results showing the mean size vs time at each temperature in the system of PbS nanoparticles by hydrothermal treatments.
peratures. In each temperature, crystal growth can be divided into two stages. In the first stage, the growth of PbS nanoparticles follows an asymptotic curve, suggesting the character of the OA mechanism.31,37 Nanoparticles grow from the primary size of about 4.6 nm to a limiting size of about 5.5-6.2 nm, and the ceasing time of this stage decreases as the temperature rises. The growth rate of nanoparticles increases with rising temperature (from 60 to 100 °C). In the second stage, an abrupt transition can be obviously observed. The tendency of crystal growth in the second stage fits well with the parabola curves, and the size of nanocrystals increases continuously with longer time. The growth kinetics in the second stage can be described and fitted by the OR theory:19-21
D hn - D h n0 ) k(t - t0)
(1)
where D h and D h 0 are the mean particle sizes at time t and t0. k is a temperature-dependent material constant. The fitted result shows the exponent n ≈ 3, indicating that the coarsening kinetics in the second stage is mainly controlled by the volume diffusion
Attachment Kinetics for Ligand Capped Nanocrystals
J. Phys. Chem. B, Vol. 111, No. 6, 2007 1451
TABLE 1: Estimated Values by Fitting the Experimental Data at Each Temperature for the first stage
for the second stage
T (°C)
R1 (nm)
K1 ()D1N1(0))
D h 0 (nm)
t0 (h)
n
k
60 80 100
2.3 2.3 2.3
55 1 × 103 1.5 × 104
5.6 6.1 6.2
108 14 1.2
2.89 2.94 2.86
0.1627 0.4817 1.1020
of ions in the solution.19-21 (For fitting results see Figure 2 and Table 1.) Actually, as reported,31-32 though the crystal growth data of the second stage can be simply fitted by OR, it does not represent that the crystal growth in the second stage is via pure OR mechanism. TEM observation reveals that OA and OR actually coexist in this stage. Figure 3 shows the state of the PbS nanoparticles after being hydrothermally treated at 100 °C for 0.5 h (at the first stage). It reveals that small particles attach together with a common crystallographic orientation, and most of the produced crystals have irregular shapes and abrupt edges. The grown particles produced by small “building blocks” display the microstructural features of defects. The sizes of the assembling units are mostly
Figure 4. Typical TEM images of PbS nanoparticles hydrothermally treated at 100 °C for 3 h. Crystal growth in this stage is mainly controlled by the OR mechanism and most of particles present round shape with smooth edges. Scale bar: 5 nm.
4-5 nm (equal to the size of primary particles). However, the crystal growth in the second stage obviously shows different features. Figure 4 shows the state of the PbS nanoparticles after being hydrothermally treated at 100 °C for 3 h. It shows that most of the particles have round shapes and smooth edges, consistent with the crystal growth characteristics of the OR mechanism in this time period. Previous investigations show that under hydrothermal treatment, the OA of nanoparticles has an action similar to the collision motion of molecules.29,38 The collision and coalescence between primary particles and secondary particles, even multilevel particles, probably occur, and growth in this way will continue. The OA growth of nanoparticles can be analogously described by the Smoluchowski equation, which was used universally to simulate the collision and reaction of molecules.39-43 Different from the NaOH-ZnS system, where OA will occur between any two multilevel nanoparticles, in the thiol-PbS system, we found the modified “addition model”41,42 of the Smoluchowski equation to be suitable for describing the growth kinetics by the OA mechanism and to fit the experimental results very well. The “addition model” is a limiting case of aggregation. It involves monomer-monomer reactions and monomermultimer reactions. The multimer-multimer reaction can be neglected. Here, the monomers for OA under hydrothermal conditions are the primary nanoparticles. When the primary particles are exhausted, the crystal growth goes into the second stage. The process of OA-based growth in this way can be illustrated in Figure 5. The growth via OA can be described as follows: k1
A1 + A1 98 A2 ki-1
A1 + Ai-1 98 Ai
Figure 3. Typical HRTEM images of samples hydrothermally treated at 100 °C for 0.5 h. Larger crystals are constructed by smaller attached nanocrystals with size equal to primary particles (4-5 nm). The white parallel lines highlight the misorientation between two regions of the assembled particle. Schematic outlines of each image illustrate the attachment scheme of OA. Scale bars: 5 nm.
where A1 is the primary nanoparticle, Ai is the particle containing i primary particles, and ki is reaction rate constant. The hypotheses in the growth are (a) the reaction between Ai and Aj (i g 2, j g 2) is neglected, for their larger mass, immovability, and low probability of interaction, and (b) the reaction is irreversible. When the primary particles are exhausted, the reactions stop. That is, the crystal growth in this way finishes, which occurs at the end of the asymptotic growth kinetic curve.
1452 J. Phys. Chem. B, Vol. 111, No. 6, 2007
Zhang et al.
Figure 5. Schematic graph illustrating the growth of nanocrystals by oriented attachment: two primary particles collide and coalesce in the same crystallographic orientation (A1 + A1). Then, a secondary particle comes into being. The attachments also occur between primary particle and multilevel particle, such as (A1 + A2), (A1 + A3), etc. But the attachment between Ai and Aj (i g 2, j g 2) is neglected.
So the time evolutions of the concentration of A1 and Ai are ∞
dN1/dt ) -2k1N12 - N1
kiNi ∑ i)2
dNi/dt ) ki-1N1Ni-1 - kiN1Ni (i g 2)
(2) (3)
where N1 and Nt are the concentration (number per unit volume) of primary particle and the concentration of particle Ai (i ) 2, 3, 4, ...) after growth via OA for a period of time, t. The reaction rate constant ki can be taken from the Smoluchowski theory.44 It is assumed that the diffusion coefficients for secondary particles, Di(ig2), are negligibly small compared with D1 of the primary particles. So
ki ) 4π(R1 + Ri)D1
(4)
where Ri is the radius of the particle containing i primary particles. According to the equivalent-volume relation, it can be given by
Ri ) i1/3R1
(5)
R1 is the radius of the primary particle. According to the matter conservation ∞
N1(0) ) N1 +
iNi ∑ i)2
(6)
which assumes that at time t ) 0, there are only primary particles and the number is N1(0). For eq 2-6, a numerical simulation is used to get the particle distribution at different times. Euler’s polygon method is introduced to the program and a first-order expression of Taylor’s formula is used as follows:45
N(i,t+∆t) ) N(i,t) +
∂N(i,t) ∆t ∂t
(7)
So the time step ∆t in the calculation should be small enough to ensure the accuracy of result. Usually, ∆t ) 1/(ND1) is selected,43 where N is the number of nanoparticles. By this solution, the size distribution of particles can be obtained at a certain time. According to the definition of the volume-weighted average particle size,46 the average particle size, deq, which is consistent with the average particle size determined by XRD line broadening, can be expressed as
deq )
∑Nkdk4/∑Nkdk3
(8)
Figure 6. Arrhenius plot of the fitting kinetic constants for two stages.
where dk is the size of particle containing kprimary particles. Thus, the increasing particle size with time revolution will be calculated. The crystal growth controlled by the OA mechanism in the first stage was fitted by the proposed model. The comparison between the experimental data and the fitting results checked the propriety of the developed model. The fitting curves are shown in Figure 2, in which the increases of the particle sizes with time revolution by numerical calculation agree well with those by experiments. In the fitting, the parameter (D1N1(0)) is the only adjustable parameter that controls the growth rate. Table 1 shows the fitted results. The kinetic constants varied with temperature can be described by the Arrhenius equation:
log K ) -
Ea + A0 RT
(9)
where Ea is the apparent activation energy, A0 is the preexponential factor, R is the universal gas constant, and T is the absolute temperature. Figure 6 shows the Arrhenius plot of apparent kinetic constants K1 (K1 ) N1(0)D1) for the growth via OA and k for OR. From the Arrhenius plot of various kinetic constants, we obtained the apparent activation energy of OA, Ea(K1) ) 65.5 ( 6.5 kJ/mol and the apparent activation energy of OR, Ea(k) ) 19.5 ( 2.2 kJ/mol. As N1(0) is fixed, the diffusion coefficients of nanoparticles, D1, against temperature have the same relationship as that of the kinetic constants that can be evaluated from eq 9 by fitting the experimental data. Discussion Surface Adsorption, Solution Situation, and Crystal Growth Characteristics. In the three two-stage growth kinetics that were well investigated, such as mercaptoethanol-capped nanocrystalline ZnS in water,31 nanocrystalline ZnS in concentrated NaOH,32 and here dodecanethiol-capped nanocrystalline PbS in water, a similar growth rule was found: in the first stage, the crystal growth is mainly controlled by the OA mechanism; in the second stage, the crystal growth is controlled by the (OR + OA) mechanism. In contrast, when the nanoparticles are free of strong capping,34 the coarsening process is consistently controlled by the (OR + OA) mechanism. The capping on the nanocrystal surfaces holds back the OR growth and gives prominence to the OA crystal growth mechanism in the initial period of growth, resulting in the two-stage growth characteristics. The OA growth characteristics of thoil-capped nanoparticles and NaOH adsorbed nanoparticles are different. The coarsening of thiol-capped nanoparticles usually holds a very short OA dominating stage, where only the reactions related to the primary particles (A1 + A1, or A1 + Ai) were existed. The multistep OA (Ai + Aj, i > 1, j > 1) growth can be observed during the
Attachment Kinetics for Ligand Capped Nanocrystals second stage (OR + OA). As to nanoparticles coarsened in concentrated NaOH, the multistep OA growth kinetics (Ai + Aj) was observed in the first stage exclusively. We proposed that it is the effect of surface adsorption, the aggregation state of original nanocrystalline samples, and the coarsening solution that play critical roles in determining the different OA growth characteristics. For the systems of thiolcoated nanoparticles (PbS and ZnS) in aqueous solution, they have similarities as follows: (1) The capping agents on the nanoparticles’ surface hinder the OR growth; thus primary particles basically grow via the OA mechanism. (2) The assynthetic, thiol-capped nanocrystals are easily aggregated into balls in water.31 As the aggregation is in favor of the collision and coalescence of nanoparticles, it brings an inherent superiority for the OA growth. (3) As an organic ligand, thiol is an unstable surface adsorption agent under hydrothermal treatment. During coarsening, it can be destroyed and desorbed into water. Especially for secondary particles and multilevel ones built by the OA of primary particles, the adsorbed species may be irreversibly removed from the interface during the coalescence. It decreases the aggregation degree of the particles, as observed in the ZnS-thiol and PbS-thiol systems. The quantitative analysis of the IR spectrum was used to detect the decreasing amount of surface adsorbed thiol and proved the desorption of capped ligands, as shown in the Supporting Information (part 1). (4) Moreover, with the desorption of surface thiol agent from the secondary and multilevel particles, the dissolution/precipitation process of particles will not be hindered as before. The tendency for secondary and multilevel particles grown by OR mechanism becomes prominent. In a word, the above ingredients lead to the low possibility of OA between multilevel particles and a relative short OA dominant period. When nanocrystalline ZnS coarsens in concentrated NaOH, some changes occur: (1) The surface adsorption of NaOH is so strong that it prevents the achievement of saturation equilibrium for the nanoparticles in NaOH solution. Thus the OR growth of primary particles is prohibited thermodynamically. The OA mechanism controls the crystal growth. (2) As an inorganic agent, the surface adsorption of NaOH is relatively stable during coarsening. More importantly, the coarsened solution is a high concentration NaOH, which provides strong surface adsorbent continuously. The newly formed coalescent multilevel nanoparticles are also immediately surrounded by the strong surface adsorbent. Thus the probability of OA between multilevel nanoparticles is also very high. (3) Furthermore, in this system, the concentration of NaOH is so high that the solubility of the ZnS drastically increases, whereas the surface adsorption is so strong that the dissolution speed of ZnS slows down. Putting these two together brings a very long time period for the system to reach the dissolution equilibrium. In this time period, the OR is forbidden thermodynamically. Thus the multistep OA growth kinetics was observed with a large size range. Applicability of the Kinetic Model. The first and simplest OA kinetics model was proposed by Huang et al. on the basis of the hydrothermal coarsening results of thiol-capped ZnS nanoparticles, where the OA of only two primary particles (A1 + A1) was considered. In fact, the attachment between two other particles such as a primary particle and a secondary one, even a multilevel one, should exist. Thus in this work, the built model considers not only the OA of (A1 + A1) but also the OA of (A1 + Ai). Such consideration may be a more proper and general description of the OA growth kinetics of nanocrystals capped with easily destroyed organic ligands. As shown in the Sup-
J. Phys. Chem. B, Vol. 111, No. 6, 2007 1453 porting Information (part 2), the model is also appropriate to describe hydrothermal coarsening results of thiol-capped ZnS nanoparticles in Huang’s work.31 Meanwhile, the model in this work can be viewed as the simplification of our previous multistep kinetic model based on molecular collision and reaction.32 It greatly simplified the calculation and we believe it is more universal to describe the growth kinetics of the nanoparticles with an easily destroyed surface coating. Activation Energy of the OA Growth. The activity energy is an important parameter determined by the process of stepcontrols. There is little systemic study on the activity energy in the published work because of the scarcity of experimental data and the complicated nature of the work. For the OA-based crystal growth, it undergoes two reaction steps: one is the diffusion step of the nanocrystals; the other is the coalesce and desorption of surface species.33,37 And the obtained Ea represents the activation energy for the slowest step. Currently, we have a series experimental data for comparison: (1) Ea ) 125.0 kJ/mol for as synthesized ZnS coarsened in water.34 (2) Ea) 136.8 kJ/mol for mercaptoethanol-capped ZnS coarsened in water.31 (3) Ea) 54.5 kJ/mol for ZnS coarsened in 4M NaOH.32 (4) Ea ) 65.5 kJ/mol for dodecanethiol-coated PbS coarsened in water (this work). In the previous work, it reveals that it is the diffusion of the nanoparticles in specific solution but not the attachment that determines the Ea.32,33 In this work, we found the Ea in the PbS system is similar to that of ZnS in a strong NaOH system. As we know, the desorption of NaOH from a nanoparticle surface needs more energy than an organic thiol ligand does. However, the Ea in the thiol-PbS system is higher than that in NaOHZnS system. This rules out the possibility that the desorption step of thiol from PbS is the slowest step. Combining with previous results32,33 and the above analysis, we confirm that the diffusion of the nanoparticles determines the Ea. Moreover, by comparing (4) with (2), it can be seen that under almost same conditions, the Ea in the PbS system is much less than that in ZnS system. We proposed that the diffusion of the dodecanethiol-coated PbS nanoparticles in water is much easier than mercaptoethanol-capped ZnS nanoparticles in water, possibly because the alkyl is hydrophobic and the hydroxyl is hydrophilic. Effect of OA in Controlling the Size Distribution. The synthesis of nanoparticles with a uniform size and morphology is one of the most significant challenges in nanotechnology. It is well-known that without surface active agents, a narrow size distribution is hardly achieved. So, in general chemical solution syntheses, capping ligands are introduced to “arrest” growth and thus limit particle size. But it is always found a finite size distribution cannot be avoided without sophisticated controls. The explorations of these kinetic controls are normally based on the experience in experiments. So the discovery of the rule for OA growth in surface capped nanoparticles helps to understand the process where the capping-ligand aids the synthesis of nanoparticles. When nanoparticles are free of capping ligands,34 the crystal growth is limited by the mixture mechanism of the OA and OR, which leads to an uncontrollable size distribution. In this and previous works, the two-stage growth character reveals that the capping situation can only hinder the OR crystal growth of nanoparticles for a certain period of time, whereas the OA based crystal growth is not prohibited at all. Moreover, the surface capped small primary nanoparticles are the most active reaction unit for the OA
1454 J. Phys. Chem. B, Vol. 111, No. 6, 2007 coalescence, which will multiple the sizes of nanoparticles very quickly. Thus factors related to OA growth actually become crucial for preparing the aimed nanoscale materials, for OA always occurs during the crystal growth, no matter what kind of surface active reagent were used for arresting growth. We further propose that a pure OA growth period of nanoparticles actually can help achieve the uniform size distribution. In the OA dominating growth stage, an asymptotic growth curve means that the smaller particles grow quickly (in the initial OA stage), and the larger ones grow slowly (near the balance stage). So when the growth approaches the equilibrium of the asymptotic curve, the smaller particles grow faster than the larger ones in the system, which facilitates a narrow size distribution. And prolonging the equilibrium period of OA will potentially achieve the monodispersity. It has been reported that a “focusing” of size distribution was found during the nanocrystal growth in the CdSe-TOPO system and the InAs-TOP system.23 The authors suggested that continuous monitoring and adjustment of the monomer concentration could reliably prepare larger amounts of uniform nanoparticles. Combining with our studies, we speculate that it is the monitoring of the OA process that helps to narrow the size distribution. In a word, because the OA of nanoparticles is the major influence factor that extends the size and determines the variety of shapes for nanosynthesis via surface capping, it makes us think that, during synthesis, the factors influencing the OA growth, such as temperature, the retaining time of nanoparticles in the solution, the species of capping ligands, the monomer concentration, and the solution situation (ligand concentration, solubility of nanoparticles), should be considered. Conclusions The growth process of thiol capped PbS nanocrystals controlled by OA was analyzed. A kinetic model only considering primary particles as active units for OA is built and fits well with the experimental data. This work provides a general understanding of the OA growth style for nanocrystals capped with easily destroyed ligands. We suggest that the OA growth is one crucial point for synthesizing nanoscale materials with narrow size distribution. Acknowledgment. We thank Feng Bao at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, for helping with the TEM. Financial support for this study was provided by the Foundation for Overseas Scholar Fellowship and the Foundation of Fujian Key Laboratory of Nanomaterials (2006L2005). F.H. thanks the Outstanding Youth Fund (50625205), One Hundred Talent Program in Chinese Academy of Sciences, and the National Natural Science Foundation of China (20501021). Supporting Information Available: 1. The quantitative analysis of IR spectrum to detect the amount of thiol on the surface of particles. 2. The developed kinetic model was extended to fit the OA growth of mercaptoethanol-capped nanocrystalline ZnS in aqueous solution performed by Huang et al.31 This material is available free of charge via the Internet at http://pubs.acs.org.
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