Shape‐Controlled Synthesis of CdSe Tetrapods Using Cationic Surfactant Ligands (2007)

Keeping in shape: Quaternary alkylammonium compounds promote the formation of faceted quantum dots. Their use in hot-injection synthesis chemistry provides a new means to form uniform CdSe tetrapods without a selective precipitation step (see picture; TOPSe: trioctylphosphine selenide), and introduces the prospect of nanoparticle shape control through ligand–nanoparticle charge interactions. The hot-injection route is a well-established approach to generating colloidal, monodisperse quantum dots (QDs)1, 2 and other nanoparticles (NPs) in an organic solvent.3–5 Shape control represents the next stage in NP synthesis chemistry,6, 7 as it has already been demonstrated for a number of QD compositions.4, 8–17 However, it remains quite challenging to synthesize NPs that are uniform in both shape and dimension, as in the case of CdSe tetrapods. Improved synthesis methods would increase the yield of desired nanoshapes and eliminate the need for selective precipitation. Here, we report that CdSe tetrapods can be synthesized with surprisingly high shape and dimensional uniformity through the novel use of quaternary ammonium surfactant ligands. Tetrapod-shaped nanocrystals have been synthesized for a variety of semiconductor compositions, such as ZnO,18, 19 MnS,20 CdS,20–22 CdTe,11, 12 and CdSe.10, 11, 15, 23 Uniquely, CdTe tetrapods can be synthesized with high shape selectivity (up to 70 % of QDs were tetrapods) and high size uniformity (arm lengths were “within a few percent”) directly without selective precipitation.12 In contrast, CdSe tetrapods are more difficult to synthesize with high shape selectivity (limited to <40 %) and high size uniformity.10, 11 Mohamed et al. reported on the synthesis of “tetrapod-like” CdSe NPs with a selectivity of 90 %, with many of the observed NPs not strictly having the tetrapod shape (i.e., with four arms).23 Pang et al. reported the formation of CdSe tetrapods by incorporating acidic aqueous solutions into a variant of the hot-injection method; tetrapod selectivities as high as 80 % were claimed.15 We note that nearly all hot-injection syntheses of CdSe nanoshapes, including tetrapods, involve alkylphosphonates (e.g., methyl-,24 hexyl-,8 tetradecyl-,10, 12, 25 and octadecylphosphonic acids13). These phosphonate ligands bind to certain particle facets preferentially to reduce growth rates of those facets, a property not shown by trioctylphosphine oxide (TOPO), amine, or carboxylate ligands.10, 12, 25, 26 Precursor concentration and injection sequence provide other synthesis parameters for shape control.10, 11, 25 The interesting use of metallic NPs as seeds in the colloidal synthesis of CdSe tetrapods and other nanoshapes was recently shown.27–29 The use of quaternary ammonium salt compounds (“quats”) in the hot-injection synthesis of QDs (or NPs in general) has not been previously reported. Quats differ substantially from phosphonate and the other electron-rich ligands in that the head group has a cationic NR4 structure containing an electron-poor nitrogen center and four alkyl groups. Depending on the alkyl substituents, quats are variously used as wetting agents,30 emulsifiers, and phase-transfer agents.31 In materials chemistry, they are used as structure-directing agents in zeolite32 and mesoporous materials synthesis,33 in aqueous-phase metal nanorod synthesis,34 and in phase-transfer synthesis of metal NPs.35 The possibility that quats can bind electrostatically to surface-exposed anionic sites motivated us to study their possible role in particle shape control. We modified the synthesis method for spherical CdSe QDs,36, 37 in which trioctylphosphine selenide (TOPSe; Se precursor) is injected into an octadecene (ODE) solution of CdO and oleic acid (OA). The quat was introduced into the synthesis by combining it with the Se solution prior to injection at 300 °C. Toluene was used to solubilize the quat, and so care was needed to avoid uncontrolled boiling of the reaction volume. The particles were grown at 280 °C, and recovered and washed with acetone. Selective precipitation was not carried out. The CdSe QDs were unexpectedly found to be tetrapod shaped when the synthesis was carried out in the presence of cetyltrimethylammonium bromide (CTAB; Figure 1 a). Tetrapods accounted for 90 % of the 500 particles analyzed, and the balance comprised single rods, bipods, and tripods. Defined as the percentage of all observed NPs identified to be tetrapods, the selectivity was 90 %. No spherical particles were detected. The mean average arm length and width were 10.9 nm (relative standard deviation 14.0 %) and 3.4 nm (15.0 %), respectively (Figure 1 b and c). The dimensions of the fourth arm (pointing out of the images and observed as the higher-contrast center) could not be quantified. Representative transmission electron microscopy (TEM) images of CdSe tetrapods synthesized with a) CTAB and d) DDAB, and g) spherical CdSe QDs synthesized without any quat ligand. The particles were recovered 1 min after injection. Insets: higher magnification of individual tetrapod particles. Histograms of tetrapod arm width and length distributions for tetrapods synthesized with b, c) CTAB and e, f) DDAB. h) Particle-diameter histogram for spherical CdSe QDs. i) TEM image of CdSe tetrapods synthesized with CTAB through the slow-decomposition method, and recovered after 1 h at 300 °C. All scale bars: 20 nm. On replacing CTAB with didodecyldimethylammonium bromide (DDAB), we found that the resultant QDs were also tetrapodal in shape, with a selectivity of 92 % (Figure 1 d). With assumed Gaussian size distributions, these particles had a mean average arm length and width of 7.4 nm (13.5 %) and 3.2 nm (12.0 %), respectively (Figure 1 e and f). Without having to perform selective precipitation, we found that these as-synthesized CdSe tetrapods were also of higher quality (i.e., higher selectivity towards the tetrapod shape and more uniform dimensions) than those described in the early papers of Manna et al.14 and Peng and Peng10 and in more recent reports.15, 23, 28 For comparison, we carried out the QD synthesis without using any quats but kept the toluene in the Se precursor solution. Spherical QDs with a mean average diameter of 5.6 nm (10.8 %) resulted, which indicates that toluene was not responsible for the tetrapod shape (Figure 1 g and h). The monodispersity of these particles could be improved by replacing the toluene co-solvent with a higher-boiling-point solvent that can solubilize the quat ligands (thereby allowing for a more rapid injection). Both tetrapod materials contained the CdSe wurtzite phase as the spherical QDs, according to X-ray diffraction (XRD) analysis (Figure 2). The more intense and narrow diffraction peak at 2θ≈25° relative to the peaks at 2θ≈24 and ≈27° was consistent with tetrapod arms lengthened in the c-axis direction of the wurtzite phase, as others have found for II–VI tetrapods.13, 15, 22, 24 Scherrer analysis of the (002) wurtzite peak indicated grain sizes of ≈7 and ≈8 nm, respectively, for the CTAB- and DDAB-derived samples. XRD grain sizes and average TEM particle sizes are volume-weighted and number-weighted averages, respectively, and so a direct comparison of these values to determine the presence of internal defects (i.e., stacking faults and twinning boundaries) cannot be done. For the spherical QDs (Figure 1 g), analysis of the peaks of 2θ≈24, ≈25, and ≈27° indicated (100), (200), and (101) grain sizes of 7.0, 7.7, and 7.9 nm, respectively. XRD patterns of CdSe tetrapods synthesized with a) CTAB and b) DDAB, and of c) spherical CdSe QDs synthesized without any quat ligand. Diffraction peaks for the wurtzite and zinc blende crystal structures of CdSe are shown as thin (gray) and thick (black) lines, respectively. Through the “slow-decomposition” synthesis approach38 to NPs, we found that the Se/quat precursor can be combined in one pot with the Cd precursor solution and carefully heated to generate the tetrapods. By heating to 300 °C at a ramp rate of 10 °C min−1 and holding at 300 °C for 1 h, CdSe tetrapods with longer arms (≈10–40 nm) were synthesized (Figure 1 i). The tetrapods were less uniform in size and shape however, and spherical QDs were also formed. The selectivity was estimated to be 8 %, and the arm width and arm length were in the ranges of 2.1–3.8 and 16–26 nm, respectively. A sudden color change in the reaction medium was observed during the heat ramp at ≈140 °C, which indicates initiation of particle nucleation. Nucleation most likely continued during the heat ramp, thus leading to the nonuniform tetrapods and spherical QDs. Alivisatos and co-workers theorized that phosphonate-assisted QD tetrapod formation is the result of nucleation of zinc blende particles followed by surface-initiated growth of wurtzite arms.11, 12 High tetrapod selectivities result from the nucleation of zinc blende-only NPs followed by growth of wurtzite-only arms, as in the case of CdTe tetrapods. The cohesive energy difference between the two polymorphs of CdTe is sufficiently large (≈10 meV)12 for zinc blende (and not wurtzite) particles to nucleate, and small enough to allow the wurtzite phase to grow off the zinc blende NP surfaces. By extension, low selectivities result from the nucleation of both zinc blende and wurtzite phases (Figure 3 a). The theory thus suggests that it is inherently difficult to synthesize CdSe tetrapods with high selectivities, since CdSe has a smaller energy difference between its two phases (<1 meV). Possible mechanisms for the a) nonselective and b) selective formation of CdSe tetrapods with CTAB or DDAB ligands. The high selectivity to CdSe tetrapods with CTAB or DDAB ligands suggested the nucleation of zinc blende-only CdSe NPs as one possible mode of formation (Figure 3 b). To gather evidence for this, we attempted to recover the NP products at very short synthesis times but were unsuccessful. We then sought to separate nucleation from arm growth by significantly reducing the amount of injected Se/quat precursor. The resultant particles had the zinc blende phase, consistent with CdSe NPs nucleating in the zinc blende phase (Figure S1, Supporting Information). The observation that ligands can modify the crystal structure of nuclei is consistent with previous reports.24, 39 To gain insight into the structure-directing role of the quats, we explored the effects of quat concentration and quat molecular structure. Increasing CTAB concentrations led to decreased tetrapod selectivity (down to ≈30 %) and increased faceting of all formed particles (Figure 4), which indicates that there is some optimum amount of quat that maximizes nucleation of the zinc blende phase and therefore CdSe tetrapod selectivity. XRD analysis showed that these particles were of the wurtzite phase. Representative TEM image of CdSe QDs synthesized with ten times the amount of CTAB used for the tetrapod sample of Figure 1 a. The particles were recovered 1 min after injection. Inset: higher magnification of an individual tetrapod particle. Scale bar: 50 nm. With increased amounts of quat, the tetrapod arms were shaped like petals instead of rods, indicative of the ligand modifying the growth of the arms. It is presumed that the mode of ligand–particle interaction is electrostatic, because of the cationic nature of the quat head group. We explored the synthesis of QDs of other compositions using DDAB, and found that the ligand caused a definite change in particle morphology (Figure 5). CTAB induced similar changes in particle morphology (data not shown). Although the tetrapod shape was not formed, the ZnSe, PbSe, and CdS particles were more faceted relative to particles synthesized without the quat. The faceted shapes were reasonably uniform, which suggests that quats can be optimized for NP shape control for various compositions. TEM images of ZnSe QDs synthesized a) with and b) without DDAB; PbSe QDs synthesized c) with and d) without DDAB, and CdS QDs synthesized e) with and f) without DDAB. The particles were recovered 1 min after injection. All scale bars: 10 nm. There was also a significant change in particle morphology when symmetric quats (with four equivalent alkyl groups, R=methyl, butyl, octyl, or decyl ) were tested as ligands. These also led to CdSe tetrapods, but the selectivities decreased significantly. Irregularly shaped QDs (not classifiable as spheres, rods, bipods, tripods, or tetrapods) made up about 50 % of the particle population (Figure S2 of the Supporting Information). These quats modified the particle formation pathway to yield nonspherical, nonuniform CdSe QDs, which confirmed the shape-inducing role of the head group and the importance of quat asymmetry (found in CTAB and DDAB) for CdSe tetrapod shape control. The molecular details of how the quats participate in CdSe zinc blende nucleation and wurtzite arm growth are not yet known and are the subject of ongoing studies. One possibility is that quats are not active for shape control but that quat degradation products are. Unlike other ligands, quats are known to be susceptible to thermal decomposition via a reverse Menschutkin reaction and Hofmann elimination, to yield trialkylamines, HCl, alkenes, and alkyl halides as possible decomposition products.40 To determine if such compounds play a role in CdSe tetrapod formation, we carried out QD syntheses using trioctylamine, HCl, and a trioctylamine/HCl mixture. Trioctylamine led to spherical CdSe QDs and the other two syntheses led to irregularly shaped particles, which indicates that quat decomposition products can modify particle shape but do not lead to CdSe tetrapod formation. The electronic structure of tetrapods is more complex than that of their spherical counterpart, which makes interpretation of their optical spectra difficult. The CTAB-derived tetrapods had an absorbance spectrum shape similar to those of the reported spectra of CdSe15 and CdTe tetrapods.12, 14 The first absorption peak was located at 597 nm, and the second peak was at ≈497 nm. Unusually, the photoluminescence (PL) peak was centered at 595 nm (Figure S3, curve i). This may be possible if the shorter-armed fraction of the tetrapod distribution fluoresced and the longer-armed tetrapods did not (while both contributed to the absorbance spectrum). Such particle-size-dependent PL is known for CdSe QDs.41 The measured quantum yields of the tetrapods (0.30 and 0.12 % for CTAB- and DDAB-derived samples, respectively) were lower than that of the spherical QDs (15.2 %), consistent with previous reports. The absorbance of DDAB-derived tetrapods was different from that of CTAB-derived tetrapods, in which the first and second absorbance peaks were located close to one another (610 and 574 nm, Figure S3, curve ii), thus resembling the spectrum reported by Mohamed et al.23 These authors attributed the higher-wavelength peak to the zinc blende core diameter and the other peak to arm length. For CdTe tetrapods, Alivisatos and co-workers reported that optical absorbance was dependent on arm width and not arm length.12 Further studies are being carried out. In summary, QD shape control through the use of quats is demonstrated for the first time. The surfactant ligands of CTAB and DDAB lead to the formation of CdSe tetrapods with unprecedented high shape selectivity and size uniformity, as established through rigorous TEM analysis. Quats with other structures lead to nonspherical particles, with faceting the probable result of charge interactions between the quat head group and particle surface. A deeper understanding of these charge interactions and the quat molecular-structure effect would lead to greater shape control for a variety of compositions. The use of quats eliminates both selective precipitation as a purification step and the more costly alkylphosphonate ligands for inducing nonspherical shapes. This new synthesis method offers the important advantages of greener chemistry and scalability, which could further the development of tetrapod-based photovoltaic and electronic devices.42, 43 Synthesis methods: The cadmium precursor solution was prepared by heating CdO (0.103 g, 0.8 mmol, 99.99 %, Sigma–Aldrich) in ODE (20 mL, 90 %, Sigma–Aldrich) containing OA (2.825 g, 10 mmol, 90 %, Sigma–Aldrich) to ≈140 °C under an argon atmosphere, at which the mixture turned clear and colorless. The Cd solution was then cooled to room temperature. The selenium precursor solution was prepared by combining Se metal (0.032 g, 0.4 mmol, 99.999 %, Sigma–Aldrich) with liquid trioctylphosphine (TOP; 1.5 mL, 90 %, Sigma–Aldrich), and then adding a solution of the quaternary ammonium compound (0.05 mmol) in toluene (2 mL). The quats studied were CTAB (98 %, Sigma–Aldrich), DDAB (98 %, Sigma–Aldrich), tetramethylammonium bromide (99 %, Fluka Chemika), tetrabutylammonium bromide (99 %, Fluka Chemika), tetraoctylammonium bromide (Sigma–Aldrich), and tetradecylammonium bromide (99 %, Fluka Chemika). The Cd solution was heated at a rate of 10 °C min−1 to 300 °C, at which point the Se/quat solution was injected within 6 s. Caution: injecting too rapidly leads to violent boiling of the reaction volume. In the slow-decomposition method, the Se/CTAB solution was combined with a cooled Cd precursor solution and heated slowly to 300 °C at a rate of 10 °C min−1. The reaction flask was kept at 300 °C for 1 h once that temperature was reached. In the synthesis of zinc blende CdSe QDs, one twelfth of the Se/CTAB precursor solution was used in otherwise identical hot-injection synthesis conditions. Withdrawn aliquots were immediately cooled in a room-temperature water bath. The resultant NPs were recovered by precipitation with acetone, centrifuged, and washed with acetone at least three times, and then redispersed in toluene. In the control experiment using trioctylamine, the ligand (0.05 mmol) was combined with toluene (2 mL) and TOPSe solution (0.032 g of Se metal in 1.5 mL of TOP). In the experiment using HCl, an aqueous solution of HCl (1 mL, 1 n) was added to the Cd precursor solution (0.8 mmol of CdO, 20 mL of ODE, and 2.825 g of OA) in a three-neck flask. This mixture was carefully heated to 300 °C, after which the TOPSe solution (0.032 g of Se metal in 1.5 mL of TOP) was injected. For the HCl/trioctylamine experiment, the ligand was injected along with the TOPSe into the HCl/Cd precursor mixture. Growth was maintained at 280 °C. Caution: heating of the flask containing HCl leads to violent boiling of the reaction volume. In the synthesis of ZnSe QDs, the zinc precursor solution was prepared by heating zinc(II) oxide (0.0813 g, 1 mmol, 99.7 %, Strem Chemicals) in ODE (20 mL) containing OA (2.825 g) to ≈250 °C under an argon atmosphere, at which the mixture turned clear and colorless. In the synthesis of PbSe QDs, the lead precursor solution was prepared by heating lead(II) oxide g, 1 mmol, 99.99 %, Sigma–Aldrich) in ODE (20 mL) containing OA (2.825 g, 10 mmol) to °C under an argon atmosphere, at which the mixture turned clear and colorless. The selenium precursor solution was prepared by combining Se g, mmol) with and then with a solution of DDAB (0.05 mmol) in toluene (2 mL). The solution was heated further to 300 °C at a rate of 10 °C after which the precursor solution was injected. In the synthesis of CdS QDs, the cadmium precursor solution was prepared by heating CdO g, 1 mmol) in ODE (20 mL) containing OA (2.825 g) to ≈140 °C under an argon atmosphere, the mixture turned clear and colorless. The precursor solution was prepared by combining g, mmol, %, Sigma–Aldrich) with and then with a solution of DDAB (0.05 mmol) in toluene (2 mL). The solution was heated further to 300 °C at a rate of 10 °C after which the precursor solution was injected. Withdrawn aliquots were cooled and with acetone, and washed at least three times with acetone. The NPs were then redispersed in toluene. The size and shape of the as-synthesized NPs were using a transmission electron at with a of nm. absorbance and (or were at room temperature using a and a respectively. The PL spectra were between and nm using an of nm. yield analysis was by using in water as the standard %) and an of nm. described by were analysis of TEM images was carried out with on particles Histograms were by using sizes of nm, such that NPs in the of nm were to have a size of nm, for For XRD the recovered particles were in on a and to A using with was Supporting for is on the under or from the The is not responsible for the or of any by the than be to the for the