KINETICS OF COLLOIDAL CHEMICAL SYNTHESIS OF
6.4 KINETICS OF THE HOT INJECTION METHOD
The hot injection method is one of the earliest synthetic methods for monodisperse nanocrystals and has been widely used for the synthesis of uniform nanocrystals of various materials (4, 14–16). In this section, we will discuss the kinetics of the synthesis of monodisperse CdSe nanocrystals via the hot injection method.
6.4.1 The Synthetic Procedure
In 1993, the Bawendi group published the legendary paper in which the use of the hot injection method for the synthesis of uniform nanocrystals of cadmium chalcogenides was introduced (14). The size of the CdSe nanocrystals could be tuned from 1.2 to 12 nm by varying the experimental conditions. The CdSe nanocrystals synthesized exhibited the strong quantum confinement effect. This pioneering work has been extended to the synthesis of nanocrystals of various materials, including semiconduc-tors, metals, and metal oxides (14). Later various modified hot injection methods have been developed to synthesize high quality monodisperse CdSe nanocrystals (16).
CdSe is a compound II-VI semiconductor composed of Cd2þand Se22ions. In the original synthetic procedure described by the Bawendi group, dimethyl cadmium [(Me)2Cd] and tri-n-octylphosphine selenide (TOPSe) were used as the precursors for Cd and Se, respectively. Later, other cadmium compounds such as cadmium acet-ate [Cd(Ac)2] and CdO were used alternatively, because (Me)2Cd is extremely toxic and pyrophoric. Although the exact reaction pathway has not been clearly elucidated, it is thought that atoms of Cd and Se are released via the thermal decomposition of the precursors.
Surfactants are crucial in the colloidal chemical synthesis of nanocrystals. Popularly used surfactants include tri-n-octylphosphine (TOP), tri-n-octylphosphine oxide (TOPO; 6, 14), hexadecylamine (17), fatty acids, and phosphonic acids (18–20). In general, the polar head groups of the surfactants bind to the surface of the nanocrystals, as illustrated in Figure 6.10 (21). The surfactants play crucial roles in the nanocrystal synthesis (14–16). First, the surfactant capping prevents the agglomeration of the nanocrystals, endowing them with a good colloidal stability. Nanocrystals have a
Figure 6.10 A nanocrystal capped with TOPO molecules. The surface is magnified in the right-hand box. The honeycomb-like structure in the box represents the wurtzite structure of the CdSe crystal.
CHEMICAL SYNTHESIS OF MONODISPERSE SPHERICAL NANOCRYSTALS 142
strong tendency to aggregate to relieve their high surface free energy derived from the Gibbs – Thomson effect. The long alkyl chains in the tail groups of the surfactants prevent the agglomeration of the nanocrystals via so-called steric stabilization.
These hydrophobic alkyl chains of the surfactant make the nanocrystals hydrophobic, thus allowing them to be well dispersed in the organic solvent. Second, the surfactants control the nucleation and growth rates during the nanocrystal formation. For example, as the bulkiness of the surfactant tail decreases from TOP/TOPO to tri-n-butyl, -ethyl and -methylphosphine/-phosphine oxide, the crystal growth temperature can be low-ered from 2808C to 2308C, 1008C, and 508C (14). The dense surfactant layer can block the approach of the monomers in the solution to the nanocrystal surface, thus retarding the crystallization reaction. Lastly, the surfactant layer protects the nanocrystal surface against oxidation. Moreover, the coordination of the surfactant head groups onto the surface defects and dangling bonds can passivate the charge trap sites and improve the optical properties of the CdSe nanocrystals.
The synthetic reaction for CdSe nanocrystals is, in any case, basically the precipi-tation reaction of Cd and Se. The reaction temperature is usually higher than 3008C.
Although the precipitation reaction can occur at much lower temperatures, the syn-thesis is generally performed at such a high temperature because both the nucleation and crystallization rates need to be extremely fast to obtain monodisperse nanocrystals, as discussed in Section 6.2. In addition, the high temperature leads to the in situ annealing of the nanocrystals, improving their crystallinity (21). Coordinating solvents such as a mixture of TOPO and TOP are usually used as the precipitation medium (6, 14, 17). Alternately, instead of a coordinating solvent, long-chain hydrocarbon solvents containing strongly binding surfactants, for example, oleic acid dissolved in 1-octadecene, are used (20). To initiate the reaction, a stock solution containing the precursor is injected rapidly into the hot surfactant solution. The injection leads directly to the precipitation of CdSe nanocrystals. Between the injection and the pre-cipitation, there is little or no induction time. This immediacy is characteristic of the hot injection method and is essential for the formation of monodisperse nanocrystals, as is discussed below.
6.4.2 Size Distribution Control
In the hot injection process, the high reaction temperature, the reactive precursors, and the rapid injection cooperatively render the reaction system extremely highly supersa-turated at the start. Soon after the injection, the reaction kinetics afterwards runs on a steep downhill slope of the chemical potential, along with a rapid decrease in the supersaturation level. Then, how does the nanocrystal size distribution evolve as the reaction proceeds?
To characterize the size distribution control process in nanocrystal synthesis, we need to trace the temporal change of the following variables: the number concentration of the nanocrystals, N, the mean value of their size distribution,,r. and its relative standard deviation, sr(r). The experimental results are shown in Figure 6.11. The left-hand plot shows that the nucleation rate is extremely high at the start of the reaction.
After the burst increase, the number concentration of nanocrystals soon reached the
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maximum point, which can be regarded as the end of the nucleation period. The decrease in the number of nanocrystals after the maximum point is due to the dissol-ution of the smaller nanocrystals via the Ostwald ripening process, which indicates the lowering of the supersaturation level in the solution. In the right-hand plot, we can see that the high growth rate is correlated with the narrowing of the size distri-bution in the early stage of the growth process. After this rapid narrowing, the size distribution slowly broadens simultaneously with the increase in the mean size, which is another characteristic of Ostwald ripening. The crucial role of the high super-saturation level in the formation of uniformly sized nanocrystals is confirmed by the additional precursor injection. As shown in the figure, this injection reversed the Ostwald ripening process for a while by increasing the supersaturation level.
Consequently, the experimental data prove that burst nucleation and size focusing actually take effect in the hot injection process.
A numerical simulation using the theoretical model introduced in Section 6.3 can successfully reproduce the hot injection process (the left of Fig. 6.12). Using the simu-lation result, we can observe the main events of the size distribution control process in detail (the right of Fig. 6.12; Reference 4). The reaction process is divided into two periods, the nucleation and growth periods, which are similar to those in Figure 6.3.
In the nucleation period, the nucleation reaction proceeds while retaining a high sr(r). Because the activation energy for the nucleation reaction is much higher than that for the precipitation reaction, the nucleation stops at a supersaturation level that is still high enough for the precipitation reaction to occur. Consequently, the system evolves spontaneously from the nucleation period to the growth period. In the growth period, there is no nucleation and the supersaturation level is high, which Figure 6.11 The temporal change of the number concentration of CdSe nanocrystals for var-ious surfactant concentrations (left). The surfactant was bis-(2,2,4-trimethylpentyl) phosphinic acid (TMPPA). The time evolution of the mean size and the relative standard deviation are shown together (right). The arrows indicate the additional precursor injection time. In all plots, the injection time is set as zero (t¼ 0). Reprinted with permission from reference 22 (left) and reference 6 (right). Copyright 2005 and 1998, American Chemical Society.
CHEMICAL SYNTHESIS OF MONODISPERSE SPHERICAL NANOCRYSTALS 144
are exactly the conditions required for size focusing. As a result, the focusing effect takes place and the value of sr(r) decreases with increasing mean size of the nanocrys-tals. To summarize, in the hot injection process, the high initial supersaturation level induces both burst nucleation and subsequent size focusing with the consumption of the monomers.