On-Surface Reactions in the Growth of High-Quality CdSe

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On-surface Reactions in Growth of HighQuality CdSe Nanocrystals in Nonpolar Solutions Runchen Lai, Chaodan Pu, and Xiaogang Peng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04743 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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Journal of the American Chemical Society

On-surface Reactions in Growth of High-Quality CdSe Nanocrystals in Nonpolar Solutions Runchen Lai, Chaodan Pu and Xiaogang Peng* Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, China

Abstract On-surface reaction mechanisms during growth of high-quality CdSe nanocrystals are studied quantitatively and systematically by introducing a cyclic growth scheme. Prior to the repeating growth cycles, pre-synthesized CdSe QD seeds from a conventional scheme are reacted with an activated Se precursor, which is found to include three elementary steps and generate Se-terminated CdSe QDs. The cyclic growth in amine-octadecene solution includes two repeating half-reactions. The first half-reaction is between cadmium carboxylates in the bulk solution and the Se-terminated QDs, and the other is between the Se precursor in the bulk solution and the Cd-terminated QDs generated by the first halfreaction. While two elementary steps in the Se-surface half-reaction can be quantitatively treated as parallel kinetics, two elementary steps for the Cd-surface half-reaction must be treated as consecutive steps. These elementary steps are found to possess substantially different reaction rates as well as activation energies. Results indicate that, in growth of compound semiconductor nanocrystals with metal carboxylates as cationic precursor (or ligands), the elementary step between activated anionic precursors in the bulk solution and the cationic sites on the surface of nanocrystals would be the ratelimiting step. This rate-limiting step should be the one that causes nucleation (or formation of small clusters by solution reactions) to be substantially faster than the corresponding growth through onsurface reactions.

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Introduction Semiconductor nanocrystals with their sizes in quantum confinement regime (quantum dots, QDs) have gradually advanced as important materials in various applications, such as light-emitting-diodes, 1-4 solar cells,

5-7

bio-medical labeling,

8, 9

lasing,

10-12

and single-photon sources.

13, 14

For development of

synthetic chemistry of high-quality colloidal QDs, studies on their formation mechanisms have played a central role. Though most of the mechanistic studies are borrowing concepts and models from conventional theories on crystallization and colloidal science, the results sometimes may reveal unexpected insights. 15-24 For instance, though nucleation has long been believed to be more difficult than growth of the existing nuclei/crystals, 15, 16, 18, 25 recent evidences suggest this might not be the case, at least for formation of high-quality II-VI

26

and III-V QDs.

22, 27-30

This is so because nucleation and

growth kinetics might both be dominated by the kinetics of chemical reactions involved.

30-37

It is

becoming more and more clear that understanding formation of high–quality QDs—probably understanding crystallization in general—must take chemical mechanisms into account, besides mechanisms of nucleation and growth. Though chemical reactions involved in formation of colloidal nanocrystals should also occur in bulk solution, those occurred on nanocrystal surfaces (on-surface reactions) are unique to crystallization as well as synthesis of high quality colloidal nanocrystals. To our knowledge, such reactions remain largely unexplored in the field.

Growth of II-VI and III-V QDs by conversion of relatively stable precursors are likely the most studied nanocrystal systems at present, especially CdSe ones. For such binary-compound nanocrystals, conversion reactions of the precursors should include at least two sets, namely one for cationic precursors and the other for anionic precursors. Taking a different viewpoint, one would find that conversion reactions must occur in solution for nucleation—or formation of initial embryos—and on the surface of


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nanocrystals for growth of existing nuclei and/or nanocrystals. Though on-surface reactions are barely studied in literature, various schemes based on their existence have been proposed for epitaxial growth of core/shell QDs, such as successive-ion-layer-adsorption-and-reaction 38-41 and colloidal atomic-layerepitaxy. 42-44

Taking growth of CdSe QDs as the model system, this work aims to develop methods for studying chemical reactions on surface of nanocrystals. A cyclic growth scheme is explored for growth of highquality CdSe QDs, which separates on-surface reactions with anionic surface sites from those with cationic surface sites. Results quantitatively demonstrate that multiple elementary steps with drastically different activation energies are involved for on-surface reactions of either type of the surface sites.

Results and Discussion Cyclic growth of high-quality CdSe QDs. For growth of CdSe or other types of binary-compound nanocrystals with finite sizes, there are hundreds to thousands of surface reaction sites that can be classified to two categories, namely cationic (Cd) and anionic (Se) sites. For studying on-surface reactions, it would be convenient if one could control only one type of surface sites to participate reactions at a given moment. Furthermore, to make quantitative study possible, it would be ideal to eliminate competitive reactions in the bulk solution. These two concerns point to a special type of synthetic scheme shown in Figure 1a.

We start the scheme with pre-synthesized and purified zinc-blende CdSe QDs coated with cadmium carboxylate ligands. Such nearly monodisperse CdSe seeds are synthesized in octadecene with coexistence of cadmium (cadmium stearate) and selenium (Se suspension in octadecene) precursors. 45 To


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ensure efficient purification and reproducibility, the QDs are treated with a relatively small fatty acid (octanoic acid) to form entropic ligands,

46

which makes the QDs sufficiently soluble in non-polar

solvents at room temperature. After purification using the surface-benign purification scheme, seeds

are

coated

the QD

with

carboxylate ligands on their surface

and

somewhat

faceted (Figure 1a, left). Given carboxylates being negatively charged, these seeds are with excess Cd ions on their surface, instead of overall Cd:Se ratio being 1 :1.

47

Such seeds should

thus be suited for studying on-surface reactions with Se precursors.

Figure 1. (a) Scheme for cyclic growth on CdSe QD seeds. (b) UV-vis spectra for CdSe QDs at different growth stages. (c) FWHM of the first absorption peak, (d) ΔE12, and (e) particle size of CdSe QDs in the cyclic growth. (f) TEM images for CdSe QDs by different growth methods. The bottom two samples have the same absorption peak wavelength. (g) XRD patterns of CdSe QDs synthesized by the cyclic growth (red) and the conventional growth (black) with the same size.

Se powder in octadecene with/without fatty amine would generate reactive Se-H species (such as H2Se or Se-HR) under elevated temperatures.

45, 48-50

Such activated Se precursors are proven to be reactive

towards the purified CdSe seeds, which should convert the Cd-terminated QDs to Se-terminated ones.51 The latter would precipitate from the solution if no additional ligands are added. We adopt primary fatty amines—one type of most commonly applied neutral ligands for CdSe and other II-VI QDs—as appropriate ligands for the Se-terminated QDs (Figure 1a, middle). It should be pointed out that, H2Se


4

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solution made by the reaction among thiol, Se powder and oleylamine at room temperature

52

works

similarly (Figure S1, Supporting Information), indicating the on-surface reactions being quite universal.

After purification, the Se-terminated QDs (Figure 1a, middle) are injected into a solution with cadmium carboxylates to convert them to Cd-terminated QDs (Figure 1a, right) at a given reaction temperature. It should be noted that this type of Cd-terminated CdSe QDs are different from the initial seeds in two ways, i.e., being spherical and grown with amine in the solution (see below). Thus, their reactions with the activated Se precursors could be quite different.

Overall, the scheme in Figure 1a can be considered as a synthetic scheme with three half-reactions, with the last two half-reactions forming repeating cycles. Here, a half-reaction refers to on-surface reaction(s) between a given type of surface sites—either cationic or anionic ones—and the corresponding precursor in solution. In this definition, three half-reactions in Figure 1a are the initial half-reaction, the Se-surface half-reaction, and the Cd-surface half-reaction. Results in Figures 1b-g confirm that, without counter precursors in the solution, each half-reaction promotes growth of CdSe QDs in a well-controlled and reproducible fashion.

Figure 1b shows that, upon the cyclic growth, UV-vis spectra of QDs shift to red steadily and are always with sharp spectral features, which are expected for growth of nearly monodisperse QDs. Quantitatively, the full width at half-maximum (FWHM) of the first absorption peak determined by Gaussian fitting becomes narrower upon the growth (Figure 1c). Interestingly, evolution of the peak widths of the Seterminated and Cd-terminated QDs does not follow the same trend, especially for the samples after purification (Figure S2, Supporting Information). Furthermore, all Cd-terminated QDs are with decent


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band-edge photoluminescence but the Se-terminated ones don’t emit. All these results seem to be consistent with the surface-modulated optical properties of QDs reported in literature. 51, 53-55 The energy difference between the first and second absorption peaks (ΔE12) in UV-vis spectra (Figure 1d) was reported to be a signature for surface coordination of CdSe QDs 56-58 but it does not seem to be applicable as an effective indicator for this cyclic synthesis scheme.

Figure 1e illustrates that the size, determined by the sizing curve of the absorption peak wavelength 59 and confirmed by transmission electron microscope (TEM) for typical samples, is linearly dependent on the number of growth cycles. As pointed out above, CdSe QDs with different sizes synthesized using the conventional scheme

45

—the seeds—are somewhat faceted (Figure 1f, two left pictures) but the QDs

generated by the cyclic scheme are spherical (Figure 1f, two right pictures). Despite of their differences on growth routes, morphology, and ligands, both types of CdSe QDs with similar size give almost the same x-ray powder diffraction patterns, i.e., with the same crystalline structure and domain size (Figure 1g). It should be noted that, the QDs in Figure 1g from the cyclic scheme are 4.6 nm in diameter grown from 3.0-nm seeds, which represents 260% volume increase through the cyclic growth. This large volume increase results in noticeable narrowing of diffraction peaks in the corresponding x-ray powder diffraction patterns (Figure S3, Supporting Information).

On-surface reactions with the CdSe seeds. This is the initial half-reaction for the cyclic scheme and does not participate in the following reaction cycles. In order to observe the morphology change in this specific half-reaction (see Figure 1f), seeds with a relatively large size (4.8 nm in diameter, the first UVvis peak at 604–605 nm) are selected as the typical starting materials though seeds with other sizes give similar results. The established scheme 45 allows us to obtain zinc-blende CdSe QD seeds with their first


6

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UV-vis peak being controlled with 1-nm accuracy.

Both types of activated Se precursors, namely elemental Se reacted at elevated temperatures with either octadecene or amine/octadecene mixture, are tested, which gives nearly identical results for the final products after the initial half-reaction (Figure S4 (left panel), Supporting Information). However, the selenourea derivatives in the amine-based Se precursor solution, 50 which is difficult to be removed by purification from the Se-terminated QDs, become reactive in the following Se-surface half-reaction (Figure S4 (right panel), Supporting Information). For this reason, the amine-based activated Se precursor could be applied as the Se precursor for studying both initial and Cd-surface half-reactions but not for either preparation of the Se-terminated CdSe QDs for studying the Se-surface half-reaction or any growth steps involving further reactions with cadmium precursors. For the latter cases, any residual organoselenium (such as selenourea) might cause additional reactions with the cadmium precursors added into the reaction solution (Figure S4 (right panel), Supporting Information). Given fatty amines are necessary for stabilizing the Se-terminated QDs, the Se precursor is made through two steps for preparation of the Se-terminated QDs for continuing growth, namely activation of elemental Se in pure octadecene at moderate temperatures (200–210 °C) for ~10 minutes and mixing this activated solution with amine at a reduced temperature (below 120 °C).

UV-vis absorption spectroscopy reveals that, upon injection of the purified seeds into the Se precursor solution at a given temperature, no shift of the first absorption peak is observable in the first few minutes of reaction (Figure 2a). The duration of this inductive stage increases by decreasing the reaction temperature. Careful inspection of the spectra in Figure 2a reveals that, during the inductive stage, the second absorption peak decreases its relative intensity and shifts blue slightly, consistent with surface


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changes observed in literature. 53, 56, 57

The samples shown in Figure 2a are purified for FTIR measurements with a calibration standard (methyl stearate, IR peak at 1745 cm-1). Opposite to Figure 2a, an immediate drop of IR absorbance for both asymmetric (~1533 cm-1) and symmetric (~1439 cm-1) vibrational bands of carboxylates is evidenced (Figure 2b). When red-shift of UV-vis spectra (starting at ~7 minutes in Figures 2a and 2c) becomes observable, intensity of the asymmetric vibration of carboxylate groups is already decreased to ~25% of the original value (Figure 2d), indicating majority of the

on-surface

reaction

between the active Se-H species in the bulk solution and

surface

cadmium

carboxylates

being

completed.

Figures Figure 2. Typical temporal evolution of (a) UV-vis and (b) IR spectra during the initial half-reaction. (c) Red-shift of the first absorption peak and (d) decrease of the carboxylate IR absorbance area of CdSe QDs during the first 20 minutes of the initial half-reaction. (e) The first absorption peak wavelengths and (f) morphology change during the initial half-reaction. The vertical dotted lines indicates end of “inductive stages” in each case. Inset in (f): illustration of calculation of the aspect ratio of a CdSe QD, with a and b being respectively as the largest crossline in TEM image of a QD and its bisector. (g) Representative TEM pictures of CdSe QDs during the initial half-reaction. The y-axis error bars in (f) are provided, and the error bars in (c-e) and the other plots to be described below are within the data symbols. The red dashed line in Figure 2d is the mono-exponential fitting of the experimental data.

2c

quantitatively

and

2d

compare

temporal evolution of the UV-vis peak shift and FTIR intensity decease for the initial half-reaction at 120 °C. In the typical range


8

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of the size variation encountered in studying any of three half-reactions, the TEM-determined size and the first absorption peak position of UV-vis can be well approximated as a linear function (see Figure S5, Supporting Information).

47, 59

Given the small size range, Figure S5 (Supporting Information) also

indicate that the particle volume and UV-vis peak wavelength can be also fitted as a linear function. Thus, the UV-vis peak position can be applied as a quantitative indicator for growth of CdSe crystalline domain. Because the symmetric band is mixed with –CH2– vibration bands in the FTIR spectra, absorbance area of the asymmetric vibration (from 1515 cm-1 to 1550 cm-1) is applied for calculation of remaining carboxylate groups on the surface of QDs during the reaction.

Results in Figures 2c and 2d confirm that there are multiple elementary steps involved in the initial halfreaction. Upon mixing, reaction between the active Se species in the bulk solution and the surface cadmium carboxylates on the Cd-terminated QD seeds starts immediately and completes within ~20 minutes for the reaction at 120 °C, indicated by the FTIR measurements. The related results can be well fitted into a mono-exponential function (the dashed line in Figure 2d). Such fittings are obtained for this specific elementary step at different reaction temperatures (Figure S6, Supporting Information), which gives activation energy as 86 kJ/mol.

Interestingly, shifts of UV-vis spectra could not be fitted with either mono-exponential or doubleexponential function (Figure S7, Supporting Information). By examining the reaction for an extended duration by both UV-vis and TEM measurements (Figures 2e-g), one would suspect that shape change of QDs might have played a role in determining the red-shift of UV-vis spectra during the initial halfreaction. Red-shift of UV-vis spectra by shape variation from faceted CdSe QDs to spherical ones has been noticed in literature recently. 60 Figure 2g illustrates three representative TEM pictures during the


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entire reaction period, indicating shape evolution of the QDs from faceted ones to spherical ones. Figure 2f plots percentage of spherical QDs—defined as b/a ratio (Figure 2f, inset) between 0.8 and 1.0—at different reaction time. Evidently, if we would regard the shifts of UV-vis spectra being composed of two elementary steps, the significantly large inductive time in Figure 2f relative to that in Figure 2e implies that the shape change is the slowest step among all three elementary steps.

Results above suggest that the initial half-reaction probably proceeds in three consecutive steps. In the first elementary step, the active Se-H species in the bulk solution reacts with the surface cadmium carboxylates. In addition to release fatty acids into the bulk solution as reported in literature, 33, 37, 45 this step would likely result in either Cd–Se–H species—the H atom further binding to amine ligands—or Cd–Se–R species (R as alkyl group) on the surface of QDs.

50, 61

These new molecular species on the

surface of QDs would further react through an intra-particle reaction to fully integrate the surface Se atoms into the lattice and shifts the UV-vis spectra, which is the second elementary step. The seeds are locked in thermodynamically unstable faceted shape—with significantly large surface area in comparison with spherical ones with the same volume—by the strong surface ligands, i.e, cadmium carboxylates. 60 Gradual removal of the surface carboxylate ligands would activate the shape evolution

62

, which is the

third elementary step in the initial half-reaction and should introduce additional red-shift of the UV-vis spectra.

In the mechanism proposed above, the second elementary step can be initiated after a few Cd–Se–H(R) species are formed on the surface of the seeds but the shape evolution needs a significant surface area of a QD to be Se-terminated. Therefore, the third elementary step should possess a comparatively longer inductive stage than the second one does as shown in Figures 2e and 2f.


10

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For three proposed elementary steps, the first one can be well determined by the FTIR measurements (see Figures 2d and S6 (Supporting Information)). If we assume no morphology change during its inductive stage, the shape evolution (see Figure 2f for example) can reasonably define kinetics of the third elementary step (Figure S8, Supporting Information). Results on the shape evolution at different reaction temperatures yield an activation energy as ~90 kJ/mol. Given the first and third elementary steps being determined, the second elementary step should presumably be resolved by mathematic deconvolution of the temporal evolution of the first UV-vis peak during a reaction. However, such deconvolution seems to be quite complicated within our preliminary efforts.

It should be noted that the elementary step of shape change is a type of intra-particle ripening.

19

In

principle, after intra-particle ripening, inter-particle ripening (or conventional Ostwald ripening) would follow. 19 Indeed, if one observes the initial half-reaction after removal of strong cadmium carboxylate ligands for a very long period of time, the size distribution of QDs would gradually change from nearly monodisperse (see the bottom picture in Figure 2g for example) to polydisperse (data not shown).

In literature, fatty amines are reported to etch the surface cadmium carboxylate ligands under certain reaction conditions, such as oleylamine at relatively high temperatures (>240 °C), 57, 60 or amines with shorter chain lengths at sufficiently high concentration.

63

With 20% volume fraction of oleylamine in

the temperature range studied here, results (Figure S9, Supporting Information) reveal that, without addition of Se precursors, surface carboxylate groups can retain on the surface of QDs upon heating in the amine/octadecene solution. Results on the Se-surface half-reaction to be discussed below also support this conclusion.


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On-surface reactions with the Se-terminated CdSe QDs. This sub-section deals with on-surface reactions involved in the Se-surface half-reaction, i.e., reactions between cadmium carboxylates in the bulk solution and the purified Se-terminated QDs generated by either the initial half-reaction or the Cdsurface

half-reaction. To

keep consistency, the same concentration of fatty amine is applied for all halfreactions. In addition to cadmium carboxylates and amines, free fatty acids are found to be essential to prevent this amine-based reaction

system

from

forming

CdO

under

Figure 3. Typical temporal evolution of (a) UV-vis and (b) IR spectra of CdSe QDs during a Se-surface half-reaction. (c)Evolutions of UV-vis peak position and IR absorbance area of CdSe QDs during a Se-surface half-reaction. (d) Red-shifts of the UV-vis peak at various temperatures. Dashed lines in (c) are mono-exponential fitting for the IR absorbance and double-exponential fittings for the UV peak positions. (e) TEM pictures of CdSe QDs before (top) and after (bottom) a typical Se-surface half-reaction.

extended reaction duration, especially at relative high temperatures (Figure S10, Supporting Information). This observation provides one reason why excessive fatty acids have been in place for synthesis of II-VI QDs even with a large excess of fatty amines as the main ligands. 38, 51, 64

Temporal evolution of UV-vis spectra during this half-reaction is drastically different from that for the initial half-reaction discussed above. Instead of existence of an inductive stage, instantaneous red-shift of the first absorption peak is observed at any reaction temperature tested (Figures 3a and 3b). The


12

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instantaneous red-shift is observable for reactions with cadmium carboxylates with different hydrocarbon-chain lengths, different concentrations of cadmium carboxylates, and with/without free acid (Figure S11, Supporting Information). All these results suggest that instantaneous red-shift of the first UV-Vis peak is quite universal.

FTIR measurements of the same samples in Figure 3a reveal a different evolution pattern for the Sesurface half-reaction as well (Figure 3c), in comparison to that of the initial half-reaction. FTIR results in Figures 3b and 3c as well as those for other reaction temperatures can be fitted in mono-exponential function with an almost identical time constant (see Figure 3b for example), though the time constant is much shorter than that for the initial half-reaction (see Figure 2d for example).

It should be noted that FTIR only detects the surface cadmium carboxylates that can overcome competition with the largely abundant amines in the reaction solution. Thus, results in Figures 3b and 3c suggest that cadmium carboxylates can well compete with fatty amines in the current reaction system, which is consistent with the conclusion mentioned at the end of the last sub-section.

Though photoluminescence of QDs is found to be difficult for quantitative kinetic studies, it can offer some supporting evidences for understanding the kinetics. Measurements of relative photoluminescence quantum yield reveal that the photoluminescence goes through a burst upon mixing of cadmium carboxylates in the bulk solution and the Se-terminated QDs. Furthermore, photoluminescence of the QDs remains quite stable in the diluted solution. Specifically, ~5% drop upon dilution is observed for the immediate aliquot after mixing and no change upon dilution is observable for the last aliquot (Figure S12, Supporting Information). It should be noted that the Se-terminated QDs before the Se-surface half-


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reaction show little luminescence, indicating the efficient photoluminescence observed for the samples in the Se-surface half-reaction should not be a result of the amine ligands, at least not solely due to the amines added in the solution. In addition, photoluminescence of the amine-coated CdSe QDs is known to decrease steadily and sharply upon similar degree of dilution. 65 Thus, all photoluminescence results are consistent with the FTIR measurements, that cadmium carboxylates react to the Se-terminated surface rapidly and bond to the surface quite tightly.

Early reports found that CdSe QDs grown with a significant excess of cadmium carboxylate ligands would become faceted. 45, 60 However, the resulting Cd-terminated CdSe QDs from the Se-surface halfreaction remain spherical (Figure 3e). This is true even if cadmium fatty acid salts are in a large excess. If there is a noticeable amount of the Se precursors—including the less reactive selenourea—left by incomplete purification of the Se-terminated QDs from the previous half-reaction, shape of the QDs may become faceted (Figure S4 (right panel), Supporting Information). Noticeably, epitaxial growth of CdSe/CdS core/shell

QDs through successive-ion-layer-adsorption-and-reaction with similar

solvent/ligand system often elongated the QDs or made them faceted. 38, 66, 67 Presumably, co-existence of both cationic and anionic precursors should be in place in the epitaxial growth of those core/shell QDs though two types of precursors were added successively during the core/shell growth.

Results described above suggest the Se-surface half-reaction—on-surface reactions between the cadmium precursor in the bulk solution and the Se-terminated QDs—should involve two elementary steps. After two reactants are mixed at a given reaction temperature by a rapid injection, cadmium carboxylates would instantaneously react with the Se-terminated QDs, which is the first elementary step and fully defined by the FTIR measurements. However, different from the other half-reactions, this FTIR-


14

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detectable elementary step is also reflected in photoluminescence (qualitatively) and UV-vis measurements. Figures 3b and 3d show the double-exponential fitting of the red-shifts of the first UVvis peak (dashed lines), with a rapid elementary step corresponding to the fitting function for the related FTIR results and a slow step (see more discussions in the last sub-section).

On-surface reactions with the Cd-terminated CdSe QDs. Though the Se precursor in the bulk solution remains the same as the initial half-reaction, the QDs in the Cd-surface half-reaction are different, which are those resulted from the Se-surface half-reaction. Specifically, these Cd-terminated CdSe QDs are spherical in shape (Figure 3e) and possibly with some ligands as fatty amines from the solution. Results below shall show that these structural variations might contribute to different behaviors in on-surface reaction kinetics (Figure 4).

Temporal evolutions of both UV-vis and FTIR spectra (Figures 4a and 4b) of the Cd-surface half-reaction at a given temperature reveal a gradual red-shift of the spectra and gradual decrease of the intensity of carboxylate vibration bands, respectively. However, quantitative analysis reveals a critical difference between two sets of data. Specifically, there is an inductive stage observed by the UV-vis measurements (Figure 4c) but no induction for the FTIR results (Figure 4d).

Similar to the initial half-reaction, FTIR results can be well fitted as mono-exponential kinetics (see Figure 4d for example), which defines the first elementary step between the active Se-H species in the bulk solution and the Cd-terminated QDs. Similar to that in the initial half-reaction, this elementary step is not reflected by shifts of the UV-vis peak through formation of the Cd-Se-H (or Cd-Se-R) species on the surface of QDs. As the reaction proceeds, the surface Se atoms in Cd-Se-H (or Cd-Se-R) species


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Page 16 of 36

would gradually incorporate into the lattice, which shifts the UV-vis spectra to red. The overall effect of two consecutive steps results in a sigmoid curve for the temporal evolution of the UV-vis peak shifts (Figure 4c).

Evidently, without interference of the QD shape evolution from faceted to spherical ones, the UV-vis results can be fitted with two exponential functions with the first one identical to that determined by the corresponding FTIR measurements. However, existence of the inductive stage requires the kinetics to be two consecutive exponentials (Figure 4c), instead of double-exponential for the Se-surface half-reaction (Figure 3d). Figure 4c (inset) shows the experimental and fitting results for the volume increase of the QDs determined by the sizing curve, 59 which is nearly the same as those directly using the UV-vis peak position.

Quantitative

kinetic

analysis of a growth cycle. The

initial

half-reaction

only initiates the synthetic scheme shown in Figure 1a, and the cyclic growth of QDs is completed by the Sesurface and Cd-surface halfreactions. According to the results illustrated above, kinetics

of

the

growth

Figure 4. Temporal evolution of (a) UV-vis, (b) IR, (c) the first UV-vis peak (inset, volume), and (d) IR absorbance area of the CdSe QDs during a typical Cd-surface half-reaction.


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cycles can be summarized below. While either half-reaction in a growth cycle can be treated in two elementary steps, the quantitative model is quite different.

For the Se-surface half-reaction, or on-surface reactions between the cadmium carboxylates in the bulk solution and the Se-terminated QDs, the elementary steps can be written as below (Equations (1) and (2)).

QD  Se

kSe1 NH3 R n  nCd  COOR '2  QD*  CdCOOR 'n  n R ' COOH

kSe 2 QD*  CdCOOR 'n  QD  CdCOOR 'n

(1)

(2)

The Se-terminated QDs—the final product of the initial (or Cd-surface) half-reaction—are written as QD  Se

NH 3 R n . We used dotted line to represent the quasi-hydrogen-bonding between surface Se-H

species and amines (NH2R) suggested in several reports.

50, 61, 68

The reaction products between the

cadmium carboxylates (Cd(COOR’)2) in the bulk solution and the Se-terminated QDs in the first elementary step are metastable QD*  CdCOOR 'n and the corresponding fatty acids (Equation (1)). In the second step, the metastable QD*  CdCOOR 'n is supposed to optimize in surface configuration, which yields the stable Cd-terminated QDs ( QD  CdCOOR 'n ). Assuming Equations (1) and (2) being parallel as suggested by the results in Figure 3c, one could readily formulate the mathematic forms for fitting the kinetic results for FTIR and UV-vis measurements. AbsIR  A(1  e kSe1t )

(3)

UV  A1 (1  e kSe1t )  A2 (1  e kSe 2t )

(4)

Here, A, A1, and A2 are constants related to initial reaction conditions. ∆λUV is the amount of red-shift of the first UV-vis peak at a given moment with the initial peak position as the reference. Equation (4)


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represents kinetics with a set of two parallel elementary steps, with the first one defined by Equation (3). The ratio between A1 and A2 contains contribution factors of two steps. Examples of fitting of the experimental data using the above equations are shown in Figure 3c. The related van’t Hoff plots are shown in Figure 5a. As expected, activation energy of the first elementary step (Ea,Se1) is practically zero while the second elementary step possesses a relatively large activation energy ((Ea,Se2).

For the on-surface reactions between the activated Se precursor (represented as SeH2 in Equation (5)) in the bulk solution and the Cd-terminated QDs, the elementary steps are written as follows. If the active Se species is Se-HR, instead of SeH2, it does not affect the quantitative analysis of the experimental data. kCd 1 QD(CdCOOR ')n  n SeH 2  n NH 2 R    QD(Cd  Se

QD(Cd  Se

kCd 2 NH3 R)n   QD(Se

NH3 R)n  n R ' COOH (5)

NH3 R)m  (n  m) SeH 2  (n  m) NH 2 R

The QD product of the first elementary step is written as QD(Cd  Se

(6)

NH3 R)n to reflect the Se atoms

not incorporated into the lattice. In the second elementary step (Equation (6)), we suggest elimination of some SeH2 along with the incorporation of the rest of Se atoms into the lattice. This suggestion seems to be necessary if one believes the surface Se atoms in the molecular Cd-Se-H (or Cd-Se-R) species are incorporated into the

lattice by

forming additional bonds with the surface Cd atoms. We have tried to confirm this hypothesis by reacting the Se-terminated QDs from the first elementary step (Equation (5)) with

Figure 5. The van’t Hoff plots for two elementary reaction steps in (a) Se-surface half-reaction and (b) Cd-surface half-reaction.

cadmium carboxylates in the bulk


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solution. In comparison to the standard half-reaction with the Se-terminated CdSe QDs, the reaction using the Se-terminated QDs from the early stage of the initial (or Cd-surface) half-reaction indeed results in ~95% more of the instantaneous red-shift of the UV-vis spectra (Figure S13, Supporting Information).

As pointed out above, the overall kinetics of the Cd-surface half-reaction need to be fitted with a consecutive two-step scheme as follows, instead of a set of two parallel elementary steps for the Sesurface half-reaction. AbsIR  B(1  e kCd 1t )

UV  B '(1 

kCd 2e kCd 1t  kCd 1e kCd 2t ) kCd 2  kCd 1

(7) (8)

Here, B and B’ are constants related to the initial conditions. Though Equations (3) and (7) are practically the same, Equations (4) and (8) differ from each other substantially. The consecutive kinetics with the first elementary step determined by the FTIR measurements can well reflect the sigmoid nature of the experimental results for either red-shift of the UV-vis spectra or volume increase of the QDs in the entire reaction time range (Figure 4c). The van’t Hoff plots using kinetic constants at different reaction temperatures for both elementary steps involved in the Cd-surface half-reaction are shown in Figure 5b. Evidently, the resulting activation energy for the elementary step between the active Se precursors in the bulk solution and Cd-terminated QDs (Ea,Cd1, 139 kJ/mol) is much larger than that of the second elementary step (Ea,Cd2, 60 kJ/mol).

Comparing four activation energies in Figure 5, one would find the following order. Ea,Cd1  2.3Ea,Cd 2 ~ Ea ,Se 2  Ea,Se1

(9)

The similarity between Ea,Cd2 and Ea,Se2 is not surprising, given two corresponding steps being both on-


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surface optimization in an intra-particle manner. While both Ea,Cd1 and Ea,Se1 are related to the reactions between one type of reaction species in the bulk solution and the other type of the reaction sites on the surface of QDs, their activation energies differ from each other substantially ( ~0 versus ~139 kJ/mol). This should be caused by the nature of surface ligands. While fatty amines are known to bond on the surface of CdSe QDs in a dynamic mode, 65 carboxylates are tightly coordinated to the surface cadmium ions.

47, 63

This means that the Se-terminated QDs are often “naked” to the attack from cadmium

carboxylates in the bulk solution, but the Se-H species in the bulk solution must go through the steric barriers provided by the tightly bonded ligands and find a suited orientation on the surface of QDs to initiate reaction.

Given reaction rates are related to the corresponding activation energy exponentially, it is reasonable to conclude that, among four elementary steps involved in a growth cycle in Figure 1a, the rate-determining step should be the first step in the Cd-surface half-reaction, namely, that between the active Se precursors in the bulk solution and the Cd-terminated QDs. Its extremely large activation energy gives one reason why growth of II-VI and III-V QDs is often more difficult than the corresponding nucleation.

It is interesting to notice that the huge difference of their reaction constants and activation energies makes it possible to deduce Equation (4) from consecutive kinetics (Note 1, Supporting Information). This means that, though the experimental results in Figure 3 suggest parallel kinetics at the first glance, we cannot completely rule out consecutive kinetics with the limited time resolution of the experiments. However, even if consecutive kinetics would be involved for both half-reactions, their chemistry nature is still different from each other in certain ways (Note 1, Supporting Information).


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Conclusions and Perspectives In conclusion, growth of high-quality CdSe QDs can be realized using a cyclic scheme, which includes an initial half-reaction between the pre-synthesized CdSe seeds and activated Se precursors and two halfreactions in cycle. Such a scheme enables quantitative and reproducible studies on chemical kinetics between the reactive Se (Cd) precursors in the bulk solution and the Cd-terminated (Se-terminated) QDs using UV-vis, FTIR, TEM, and photoluminescence techniques. Both half-reactions in a cycle are found to be consisted of two elementary steps while the initial half-reaction likely involves three elementary steps due to involvement of shape change in this specific half-reaction. Given all crystallization starts from nanometer-size objects, the rich chemistry revealed here should not only offer critical information for designing and understanding synthetic chemistry of high-quality nanocrystals but also provide insights for understanding crystallization in general.

Experimental Section Chemicals. Cadmium oxide (CdO, ≥99.99% trace metals basis), selenium powder (Se, 100 mesh, ≥99.5% trace metals basis), dodecanoic acid (≥99%), oleylamine (primary amine ≥98%), and methyl stearate (99%) were purchased from Sigma-Aldrich. Stearic acid (98+%), tributylphosphine (TBP), 1-octadecene (ODE, 90%), octane (99+%), dodecane (99+%), octylamine (98%) and 1-tetradecanethiol (94%) were purchased from Alfa Aesar. Solvents such as toluene, hexane, acetone, chloroform and acetonitrile were purchased from Sinopharm Reagents. If not specified, all reagents were used without further purification.

Synthesis of zinc blende CdSe seeds were mainly performed according to literatures with modifications.


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In a typical synthesis of zinc-blende CdSe seeds with 4.8 nm in diameter (the first UV-vis peak at 604-

605 nm), 0.384 g CdO (0.3 mmol), 0.344 g steric acid (1.2 mmol), and 4 mL ODE were loaded into a 25-mL three-neck flask, bubbled with Ar for 10 minutes and heated to 250 °C to obtain a clear and colorless solution. Se-ODE suspension (0.7 mL of 0.1 M) was injected rapidly into the flask and the growth temperature was set at 235 °C. After growth of about 8 minutes, the reaction temperature was raised to 240 °C, and 0.06 mL of 0.1 M Se-ODE suspension was added dropwise into the flask with time interval of 15 seconds. Aliquots were taken out to monitor the reaction progress. Addition of the Se-ODE suspension was repeated every 6 minutes until the desired size of CdSe QDs was reached. For smaller CdSe QDs, the amounts of CdO and steric acid were reduced, and the reaction time was shortened.

Purification of zinc blende CdSe seeds with purely cadmium carboxylate ligands was adopted from our previous work. 46 The final reaction mixture from the above procedure was cooled down to 100 °C, and 0.15 mL of octanoic acid was injected into the flask. The reaction mixture was stirred and kept at 100 °C for 10 minutes. The mixture was cooled down further to 60 °C and 30 uL of octylamine was added. CdSe QDs precipitates were obtained by chloroform-acetonitrile precipitation at ~60 °C. The precipitation was repeated four times, first three times with and the last time without octylamine. Purified CdSe QDs were used as the seeds.

Se precursor activated with amine. In a 25-mL three-neck flask, 2.3 mL ODE, and 0.7 mL oleylamine were stirred and bubbled by argon and heated to 205 °C. 0.5 mL of 0.3 M Se-ODE suspension was injected and the reaction temperature was kept at 200 – 210 °C. After 10-minutes reaction, the mixture was allowed to cool down to stop the reaction under protection of Ar atmosphere.


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Se precursor activated without amine. In a 25-mL three-neck flask, 2.3 mL ODE was bubbled by argon and heated to 205 °C. 0.5 mL of 0.3 M Se-ODE suspension was injected and the reaction temperature was kept at 200 – 210 °C. After 10-minutes reaction, the mixture was allowed to cool down below 100 °C, and 0.7 mL oleylamine was injected into the solution.

Se precursor made by thiol, Se powder, and oleylamine. Se powder (0.02 g, 0.25 mmol), oleylamine (2 mL), and 8 mL ODE were loaded in a 10-mL flask and degassed for 10 minutes. Then Ar flow was stopped and 150 μL 1-tetradecanethiol was added into the flask. Under continuous stirring, solution with red-to-brown color formed. The solution was kept sealed for further use.

Cd precursor solutions. In a 25-mL three-neck flask, 0.0288 g CdO (0.225 mmol), 0.135 g lauric acid (0.675 mmol), and 2.8 mL ODE were mixed and Ar bubbled for 10 minutes. The mixture was heated to and kept at 260 °C with Ar bubbling. After light-yellow transparent solution formed, the solution was cooled below 100 °C, and 0.7 mL oleylamine was added. Ar bubbling was further remained for 10 minutes before the solution was heated for Se-surface half-reaction.

Initial half-reaction between the activated Se precursor and pre-synthesized CdSe seeds. The Se precursor activated with amine was chosen and heated to designated temperature, and 7.5×10-8 mol of purifed CdSe seeds was injected into the mixture. The reaction temperature was kept and the reaction progress was monitored by taking out needle-tip aliquots for UV-vis and PL measurements. For IR measurements, ~0.8 ml of the reaction solution was taken out and precipitated by hexane/acetone for four times in order to have the purified CdSe QDs.


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Se-surface half-reaction on the Se-terminated CdSe QDs was performed by injecting purified Seterminated CdSe QDs into the Cd precursor solution at designated temperature. Selenium precursor activated with amine was used here to obtain the Se-terminated CdSe QDs from the seeds. The Seterminated CdSe QDs was purified with TBP at 80 °C and precipitated four times by hexane/acetone. The typical first UV-vis peak for the purified Se-terminated CdSe QDs is usually at 607-608 nm. The Cd precursor solution was heated to designated temperature, into which the purified Se-terminated CdSe QDs were injected. The temperature was kept during the whole reaction. Sampling and monitoring of the reaction were the same as those described above for the initial half-reaction.

Cd-terminated QDs for studying the Cd-surface half-reaction. For studying kinetics of this halfreaction, CdSe seeds with the first UV-vis peak at 590 nm were selected as the starting materials, which were designed to react with the Se precursor activated without amine. Temperatures for the initial and Se-surface half-reactions were set at 120 °C and 190 °C, respectively. After the specific Se-surface halfreaction, the Cd-terminated CdSe QDs were precipitated by oleylamine/hexane/acetone for twice and washed by hexane/acetone for another twice. The first UV-vis peak of the purified Cd-terminated CdSe QDs is at ~604 nm, which were applied for studies described below.

Cd-surface half reaction on the Cd-terminated CdSe QDs was executed similarly to that in the initial half-reaction. Typically, the Se precursor activated with amine was chosen, and 7.5×10-8 mol of the Cdterminated CdSe QDs were injected into the Se precursor solution at a designated temperature. Sampling and monitoring of the reaction were the same as those described above for the initial half-reaction.

FT-IR spectra were collected using a Thermo Scientific Nicolet 380 spectrometer. Generally, each


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purified CdSe QD sample was dissolved in 0.5 mL octane. 10 uL octane-solution was taken out and further diluted into 2.5 mL toluene for UV-vis measurements to determine the QD concentration. Methyl stearate with a known amount was added into the QD solution as internal standard for the FTIR measurements of cadmium carboxylates on the QD surface. The QD solution with the internal standard was dropped onto a CaF2 (or ZnSe) substrate for FTIR measurement after the solvent was evaporated.

Other measurements. UV-vis spectra were collected using a Cary 4000 spectrometer and photoluminescence spectra were measured using a Cary Eclipse fluorescence spectrometer. For both measurements, samples were diluted in toluene and loaded in a liquid cell. For relative PL QY measurements in Fig. S11, samples were extensively diluted into dehydrated toluene (toluene to reaction mixture being 50 to 1 in volume). PL spectra were scanned with the same instrument parameters every minute. Each scan was completed in 10 s. TEM images were taken by a Hitachi 7700 transmission electron microscope operating at 100 kV, and the nanocrystals were deposited onto ultra-thin carbon film supported by a copper grid. XRD patterns of purified QD powder were obtained by a Rigaku Ultimate IV X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å, tube operating at 40 kV and 40 mA).

Associated Content Supporting Information Additional UV-vis and FTIR spectra, XRD patterns, relative PL intensities, experimental data and fitting parameters for the first and the third elementary steps in initial half-reaction, and mathematical deduction of parallel kinetics from consecutive reaction model.


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Author Information Corresponding author *[email protected] ORCID Runchen Lai: 0000-0002-5827-8991 Chaodan Pu: 0000-0001-5028-5516 Xiaogang Peng: 0000-0002-5606-8472 Notes The authors declare no competing financial interest.

Acknowledgements This work is supported by the National Key Research and Development Program of China (2016YFB0401600), Joint NSFC-ISF Research (Grant 21761142009), Science and Technology Planning Project of Guangdong Province, China (Grant 2015B090913001) and China Postdoctoral Science Foundation (2016M601930).

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Rosenthal, S. J., J. Chem. Phys. 2008, 128, 084713.

TOC Figure


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Figure 1. (a) Scheme for cyclic growth on CdSe QD seeds. (b) UV-vis spectra for CdSe QDs at different growth stages. (c) FWHM of the first absorption peak, (d) ∆E12, and (e) particle size of CdSe QDs in the cyclic growth. (f) TEM images for CdSe QDs by different growth methods. The bottom two samples have the same absorption peak wavelength. (g) XRD patterns of CdSe QDs synthesized by the cyclic growth (red) and the conventional growth (black) with the same size. 225x190mm (300 x 300 DPI)


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Figure 2. Typical temporal evolution of (a) UV-vis and (b) IR spectra during the initial half-reaction. (c) Redshift of the first absorption peak and (d) decrease of the carboxylate IR absorbance area of CdSe QDs during the first 20 minutes of the initial half-reaction. (e) The first absorption peak wavelengths and (f) morphology change during the initial half-reaction. The vertical dotted lines indicates end of “inductive stages” in each case. Inset in (f): illustration of calculation of the aspect ratio of a CdSe QD, with a and b being respectively as the largest crossline in TEM image of a QD and its bisector. (g) Representative TEM pictures of CdSe QDs during the initial half-reaction. The y-axis error bars in (f) are provided, and the error bars in (c-e) and the other plots to be described below are within the data symbols. The red dashed line in Figure 2d is the monoexponential fitting of the experimental data. 227x178mm (300 x 300 DPI)



Figure 3. Typical temporal evolution of (a) UV-vis and (b) IR spectra of CdSe QDs during a Se-surface halfreaction. (c)Evolutions of UV-vis peak position and IR absorbance area of CdSe QDs during a Se-surface half-reaction. (d) Red-shifts of the UV-vis peak at various temperatures. Dashed lines in (c) are monoexponential fitting for the IR absorbance and double-exponential fittings for the UV peak positions. (e) TEM pictures of CdSe QDs before (top) and after (bottom) a typical Se-surface half-reaction. 219x128mm (300 x 300 DPI)


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Figure 4. Temporal evolution of (a) UV-vis, (b) IR, (c) the first UV-vis peak (inset, volume), and (d) IR absorbance area of the CdSe QDs during a typical Cd-surface half-reaction. 203x139mm (300 x 300 DPI)



Figure 5. The van’t Hoff plots for two elementary reaction steps in (a) Se-surface half-reaction and (b) Cdsurface half-reaction. 189x85mm (300 x 300 DPI)


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On-Surface Reactions in the Growth of High-Quality CdSe

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