Shell Heterostructured Nanocrystals

Jan 23, 2015 - *Tel., Fax: +86 1088 875298. E-mail address: [email protected]., *Tel., Fax: +86 1088 875298. E-mail address: [email protected]...
1 downloads 0 Views 7MB Size
Download PDF
Article pubs.acs.org/crystal

Shape-Controlled CdS/ZnS Core/Shell Heterostructured Nanocrystals: Synthesis, Characterization, and Periodic DFT Calculations Xiaoman Zhai, Rubo Zhang,* Jialun Lin, Yunqian Gong, Yafen Tian, Wen Yang, and Xiaoling Zhang* Key Laboratory of Cluster Science of Ministry of Education, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of Technology, 5 Zhongguancun Road, Beijing 100081, P. R. China S Supporting Information *

ABSTRACT: CdS/ZnS core/shell heterostructured nanocrystals (NCs) with six monolayers (MLs) of ZnS shell on a zinc-blende CdS core were synthesized via successive ionic layer adsorption and reaction. By adjusting the growth temperature of the ZnS shell from 220 to 280 °C, the shape of CdS/ZnS NCs can be tuned to tetrapods, tetrahedra, and dots. Shell growth was confirmed to be uniform by X-ray diffraction, transmission electron microscopy, UV−vis absorption, and photoluminescence spectroscopy. Periodic density functional theory calculations were used to further study the growth mechanism of the differently shaped CdS/ZnS core/shell NCs. Our calculations revealed that the binding energy of model CH3CH2NH2 molecules on the (110), (111), and (001) facets of the CdS core can determine which crystallographic facets are favored during the growth of the ZnS shell. The calculations provided insights into the effect of the interaction between the organic ligand and the facets of the CdS core on the shape engineering of CdS/ZnS core/shell NCs.

1. INTRODUCTION Heterostructures have generated great interest in both fundamental studies and technical applications because of their unique and superior optical and electronic properties.1−13 Since its inception, nanocrystal (NC) preparation has become more sophisticated, and currently the tailoring of NC structures (size, shape, and interconnectivity) is a major research subject in the field of materials chemistry. Engineering the shapeevolution of heterostructures allows scientists to further study the properties of NCs14 and will trigger a burst of research toward applications in biolabeling, photocatalysis, photonics, and optoelectronics.15−18 Colloidal synthesis is of critical importance to the production of core/shell heterostructured NCs of high quality.19,20 The preparation of complex shapes of core/shell heterostructured NCs will facilitate the systematic study of shape-dependent phenomena.21 However, compared with the plain core NCs, the quality of core/shell NCs is still limited in terms of shape, size, and size distribution control. Eliminating the nucleation of the shell materials and controlling the homogeneous monolayer growth of the shell are the two tough issues for maintaining a good size distribution of core/shell NCs. Moreover, balancing all these processes introduces difficulties such as a strong sensitivity to small variations in reaction conditions. One way to solve these problems is “successive ionic layer adsorption and reaction (SILAR)” that allows for the precise tuning of shell thickness. In the case of SILAR, each aliquot of cationic precursor added to the reaction is assumed to add onto the core surface before the subsequent addition of anion precursor takes © XXXX American Chemical Society

place. Therefore, the SILAR approach can afford exclusively heterogeneous nucleation and growth.22 In order to achieve optimal morphological and optical properties, many efforts were devoted to optimize the synthetic parameters of the SILAR procedure, with CdSe/CdS core/shell NCs being frequently studied systems.23,24 The Liu group reported the structural evolution of CdSe/ZnS NCs but with poor shapes and a broad size distribution.25 Basically, to form “ideal” core/ shell NCs both from a crystallographic and an electronic point of view, two factors should be taken into consideration. First, the core and shell materials should have a small lattice mismatch such that the shell growth favors epitaxial growth.5 Second, the shell material should possess a much higher band gap than the core to suppress tunneling of the charge carriers from the core to the newly formed surface atoms of the shell.26 For CdSe/CdS, the core and shell materials have similar lattice parameters but small band offsets, while for CdSe/ZnS, the lattice mismatch between the core and shell materials is too large.27 However, two factors can be satisfied and investigated simultaneously in the case of CdS/ZnS: synthesis of differently shaped CdS/ZnS core/shell NCs and elucidation of the mechanism of the shape control process, although for spherical core/shell NCs the SILAR synthesis techniques are indisputable. Received: December 1, 2014 Revised: January 22, 2015

A

DOI: 10.1021/cg501747e Cryst. Growth Des. XXXX, XXX, XXX−XXX

Article

Crystal Growth & Design

by acetone. The above centrifugation and isolation procedure was then repeated three times for purification of CdS NCs. The final precipitates were redispersed in hexane as a high-concentration solution for further use. 2.4. One-Pot Synthesis of CdS/ZnS through SILAR. The growth of the ZnS shell was carried out with the SILAR method. In a typical experiment, a hexane solution of the CdS core (75 nmol) was added to a mixture of ODE and OAm (6.0 mL, ODE/OAm = 1:1), and then hexane was removed under vacuum and the residue was flushed with argon. The flask was heated to 220 °C to grow the first two monolayers (MLs) of ZnS shell. The Zn precursor solution (0.63 mL) was first added via syringe to the flask. After 6 min, the same amount of S precursor solution was added, waiting 10 min to allow the growth of the first ZnS ML. The temperature was kept at 220 °C, and 0.87 mL of the Zn and S precursor solutions was injected alternately as before using the same time intervals. Then the ZnS shell growth temperature was raised to the desired temperature (240, 250, 260, 270, or 280 °C) to grow the third, fourth, fifth, and sixth MLs. The calculated amounts of Zn and S precursor solutions such as 1.14 mL (third), 1.44 mL (fourth), 1.77 mL (fifth), and 2.26 mL (sixth) of the precursor solutions were injected to the reaction flask, respectively. 2.5. Measurements. Transmission electron microscopy (TEM), high -resolution TEM (HRTEM), and selected area electron diffraction (SAED) were performed on a FEI TECNAI F20 transmission electron microscope operated at 200 kV. The TEM samples were prepared by dropping diluted solutions of NCs on copper grids covered with amorphous carbon film. X-ray diffraction (XRD) patterns were recorded by a Bruker-AXS Microdiff-ractometer (D8 ADVANCE) from 20° to 70°. Absorption spectra were collected at room temperature on a PE Lambda 35 UV−vis spectrometer with 4 mm × 10 mm quartz cuvettes. Room temperature photoluminescence (PL) measurement was carried out on a Hitachi F-7000 fluorescence spectrophotometer. 2.6. Calculation Details. Periodic DFT calculations were implemented by DMol3.36 The Generalized Gradient Approximation functional in the Perdew, Burke, and Ernzerhof (PBE)37 form, together with an all-electron double numerical basis set (DNP)38 with polarization functions, was used to optimize the geometries of the adsorbate, bare surface slabs, and adsorbate−substrate systems without any constraint. Throughout the spin-unrestricted DFT calculations, 0.002 hartree Fermi smearing was employed, and the real-space global orbital cutoff radius was 4.5 Å. The tolerances of the energy, force, and displacement convergences were 1 × 10−5 hartree, 2 × 10−3 hartree Å−1, and 5 × 10−3 Å, respectively. Based on the Monkhorst−Pack scheme, the Brillouin zone was sampled by 2 × 2 × 1 k points, which were employed to optimize the (110), (111), and (001) facets of ZB CdS, respectively. The additives were adsorbed only on one side of the three-dimensional (3-D) periodic facet slabs with 20 Å vacuum regions in the Z direction. In this way, the 3-D periodicity inherent in the model was transformed into an actual 2-D periodicity, thus simulating an infinitely extended facet. All the slabs contained 48 Cd and 48 S atoms, which constituted the supercell slabs with thickness of at least 3−4 layers. The oleylamine was replaced by CH3CH2NH2 as an additive in our present calculations. These models should be sufficient for investigations of similar adsorbate−ZB CdS systems. Furthermore, calculations under aqueous conditions, with dielectric constant 78.3 for the bulk solvent, were carried out for comparison to the results obtained in vacuum. The implicit COSMO (conductor-like screening model)39 provided a considerable simplification of the continuum solvation model approach without significant loss of accuracy.40 Both physical and chemical adsorption energies refer to the energy change of the additive−ZB CdS system induced by the additive molecule adsorbed on the CdS facets and can be calculated via eq 141

The organic ligands, as the capping ligands, play an important role in the growth of NCs. A general approach to tune NC shapes is to adjust the interaction between the organic ligands and the facets of NCs.28 Temperature is a direct factor that influences the interaction between the organic ligands and the facets of NCs and determines which crystallographic facet is favored during growth. However, from the experimental perspective, it is difficult to explain the mechanism to grow core/shell NCs with different shapes. Periodic density functional theory (DFT) provides a reliable tool to investigate the interaction between the organic ligand and the facets of NCs. DFT-based calculations have been successfully used to study the binding energy of the surfactants to the facets of model CdSe NCs and nanoclusters.29−31 For bulk and nanocrystalline CdS, calculations were carried out to study the binding of two model organic molecules (CH3CH2)3P and CH3CH2COOH on the different facets.32 In this work, the SILAR concept was applied to the growth of core/shell heterostructured NCs with different morphologies. CdS/ZnS NCs were chosen as our model system. By tuning the growth temperature of the ZnS shell, CdS/ZnS NCs with sophisticated heterostructures such as tetrapods, tetrahedra, and dots were successfully synthesized. A systematic analysis of the influence of shell growth temperature on the core/shell heterostructures was conducted, and optical properties were discussed in this work. Moreover, periodic DFT-based calculations were carried out to study the binding energy between the model organic ligand (CH3CH2NH2) and different facets of the CdS core and to explain the growth mechanism of differently shaped core/shell NCs. The present perspectives from our experimental and theoretical studies contribute to the general understanding of the kinetics of shape evolution of core/shell NCs and expand the knowledge of ligands and NC facets manipulation.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Sulfur powder (S, 99.999%), 1-octadecence (ODE, tech. 90%), and oleylamine (OAm, tech. 70%) were purchased from Sigma-Aldrich. Zinc stearate (Zn(St)2, count as ZnO% ≈ 14%) was purchased from Alfa-Aesar. All the solvents including methanol, toluene, hexane, and acetone were of analytical grade and purchased from Beijing Chemical Reagents Cooperation. Chemicals were used as received without further purification. Cadmium myristate (Cd(myr)2) was self-made according to literature methods.33 2.2. Stock Solutions. Sulfur solution. S powder (0.4 mmol) was added into a flask with 10 mL of ODE. After degassing for 10 min at room temperature, the solution was heated to 130 °C under Ar flow. The temperature was maintained for 5 min, and then the sulfur solution was cooled to room temperature for use. Zinc stearate solution. Zn(St)2 (0.4 mmol) was added to a flask containing 10 mL of ODE. After degassing for 10 min at room temperature, the mixture was heated to 200 °C to dissolve the zinc stearate. The solution was cooled to room temperature, and a slurry formed. The slurry was directly used for ZnS shell growth. 2.3. Noninjection Synthesis of CdS NCs. Zinc-blende (ZB) CdS NCs were prepared using a noninjection synthesis.34,35 In a typical experiment, 1 mmol Cd(myr)2, 0.5 mmol S powder, and 50 g of ODE were loaded into a three-neck round-bottom flask at room temperature. The resulting mixture was degassed at room temperature and then was heated to 240 °C under Ar flow. Serial aliquots were withdrawn to monitor the growth by UV−vis spectroscopy. When the NCs reached the desired size, the reaction was stopped by rapidly cooling to room temperature. Then, the resulting CdS NCs were purified via precipitation by adding acetone. The suspension was centrifuged at 6000 rpm for 10 min, and the supernatant was discarded. The precipitates were redispersed in hexane and deposited

ΔE = Eadditive + Efacet − Eadditive/facet

(1)

where Eadditive, Efacet, and Eadditive/facet are the energies of the isolated additive molecule, the relaxed bare facet, and the adsorption system, respectively. Based on the above definition, a positive ΔE corresponds to the stable adsorption of an additive on the bare facet. B

DOI: 10.1021/cg501747e Cryst. Growth Des. XXXX, XXX, XXX−XXX

Article

Crystal Growth & Design

3. RESULTS AND DISCUSSION 3.1. Reaction Conditions for Shell Growth of Differently Shaped CdS/ZnS NCs by SILAR. The main focus of this work was to study the shape evolution of CdS/ZnS core/ shell NCs. Reaction temperature plays a critical role in the growth of CdS/ZnS NCs with different morphologies. ZnS growth temperatures ranging from 220 to 280 °C were applied to study the effect of the ZnS growth temperature on the shape evolution of CdS/ZnS NCs. The growth schematic is shown in Scheme 1. The CdS core was coated by six ML ZnS. The

pyramidal tetrahedron structure can be obtained. When the temperature is increased to 280 °C, the shape of the CdS/ZnS NCs changes back to dots (Figure 1f). The sizes of the tetrapod-, tetrahedron-, and dot-shaped core/shell NCs are uniform. Moreover, reaction time affects the formation of CdS/ZnS core/shell NCs with different morphologies. The reaction time was extended to 15 min. As shown in Supporting Information Figure S1, only at 220 °C are the tetrapod-shaped NCs formed. When the ZnS growth temperature is raised from 240 to 280 °C, the annealing process causes the shape of the core/shell NCs to change from triangular to spherical. 3.2. Structural Characterization. To identify the core/ shell structure, we investigated the crystallographic properties by XRD. Figure 2 demonstrates the XRD patterns of CdS core

Scheme 1. Schematic Illustration of Differently Shaped CdS/ ZnS NCs with Different ZnS Shell Growth Temperatures

temperature for the first two MLs of the ZnS shell was 220 °C to avoid the heterogeneous nucleation of the ZnS NCs. The temperature for the final four MLs of the ZnS shell was raised to 240, 260, 270, and 280 °C, respectively. Figure 1 shows TEM images of the CdS core and CdS/ZnS core/shell NCs. The plain CdS cores are mainly monodisperse spherical particles (dots), as shown in Figure 1a. When the ZnS growth temperature was kept at 220 °C, the CdS/ZnS NCs appears to be nearly triangular in shape (Figure 1b). The low temperature caused insufficient growth of the shell. By increasing the ZnS growth temperature to 240 °C, the tribranched morphology which is characteristic of a tetrapod is obtained.42 When the growth temperature reaches 260 °C, the length of the branches become shorter (Figure 1d), and the shape tends to be between that of a tetrapod and a tetrahedron. In Figure 1e, the triangular shape of the CdS/ZnS NCs under TEM indicates that a

Figure 2. XRD patterns of CdS core and differently shaped CdS/ZnS core/shell NCs.

and CdS/ZnS core/shell NCs. All XRD patterns show obvious size-broadening effects, indicating the finite size of ZB phase

Figure 1. TEM images of (a) the CdS core and the corresponding differently shaped CdS/ZnS NCs with different ZnS shell growth temperatures: (b) 220 °C, (c) 240 °C, (d) 260 °C, (e) 270 °C, and (f) 280 °C. C

DOI: 10.1021/cg501747e Cryst. Growth Des. XXXX, XXX, XXX−XXX

Article

Crystal Growth & Design

Figure 3. HRTEM images and SAED patterns of differently shaped CdS/ZnS core/shell NCs with different shell growth temperatures: (a), (f) 220 °C; (b), (g) 240 °C; (c), (h) 260 °C; (d), (i) 270 °C; and (e), (j) 280 °C. Scale bar: 5 nm.

Figure 4. Evolution of UV−vis and PL spectra of the core/shell NCs with different growth temperatures of the ZnS shell: (a) 240 °C; (b) 280 °C. (c) UV−vis and PL spectra of the CdS/ZnS NCs with 6 MLs of ZnS shell at different growth temperatures.

Figure 5. Physical adsorption structures of CH3CH2NH2 on CdS (a) (110) and (b) (111) facets; chemical adsorption structures of CH3CH2NH2 on CdS (c) (110) and (d) (111) facets.

of the wurzite (WZ) bulk ZnS rather than the CdS-core material. They are also verified by HRTEM and SAED measurements (Figure 3a−j). The HRTEM images reveal the CdS/ZnS core/shell NCs with excellent crystallinities. As

structure with strongly characteristic (111), (220), and (311) peaks. When growing the ZnS shell onto the CdS core, three diffraction peaks occurred between 25 and 30°, and all the diffraction peaks are very consistent with the diffraction peaks D

DOI: 10.1021/cg501747e Cryst. Growth Des. XXXX, XXX, XXX−XXX

Article

Crystal Growth & Design

Table 1. Bonding Energies (in a.u.) and Fermi Energy Levels (in eV.) of Each Bare Facet; Adsorption Energies and Relevant Geometrical Parameters for CH3CH2NH2 Absorbed on CdS (110), (111), and (001) Facets (110) −9.8297 (−9.8670) −4.5 (−4.4) 7.6 (5.9) 30.5 (26.1) 2.736, 2.841 (2.747, 2.854) 2.488, 2.650 (2.545, 2.635) 2.398 (2.376)

a

c

BE FEb ΔEPA,d ΔECA,e (rNH•••S)PA,f (rNH•••S)CA,f (rN−Cd)CA,f

(111)

(001)

−9.7983 (−9.8353) −4.5 (−4.4) 9.4 (6.6) 23.0 (21.7) 2.872, 2.802 (2.850, 2.772)

−9.1955 (−9.3286) −4.8 (−4.6) 277.2 (211.3)

2.452 (2.409)

a

Binding energies for the bare slabs, units in hartree. bFermi energetic levels, units in eV. cValues in parentheses obtained in bulk solvent with 78.2 dieletric constant. dAdsorption energies by physical adsorption, units in kcal/mol. eAdsorption energies by chemical adsorption, units in kcal/mol. f Significant distances between the atoms, units in Å.

Table 2. Mülliken Charges (Units of e) of Hydrogen (Attached to Nitrogen), Nitrogen Atoms, and CH3CH2NH2 as a Separate Molecule or a Fragment in the Studied Complex CH3CH2NH2 (110)

PA CA

(111)

PA CA

(001) a

PA

qH1

qH2

qN

q(CH3CH2NH2)

q(Cd48S48)

0.166 (0.193)a 0.193 (0.207) 0.109 (0.109) 0.209 (0.213) 0.225 (0.250) 0.201 (0.213)

0.168 (0.195) 0.195 (0.210) 0.255 (0.268) 0.205 (0.213) 0.228 (0.252) 0.192 (0.207)

−0.443 (−0.513) −0.484 (−0.541) −0.571 (−0.576) −0.467 (−0.515) −0.511 (−0.523) −0.481 (−0.542)

0.000 (0.000) 0.000 (−0.013) 0.153 (0.177) 0.093 (0.041) 0.152 (0.191) −0.003 (−0.024)

− − 0.000 0.013) −0.153 (−0.177) −0.093 (−0.041) −0.152 (−0.191) 0.003 (0.024)

Values in parentheses obtained in bulk solvent with 78.2 dieletric constant.

labeled in the image, the measured lattice d-spacings of 3.3 and 3.1 Å spacing correspond to the (100) and (002) planes of WZ structure of ZnS, respectively. These are consistent with the results of XRD. The SAED patterns consist of broad diffuse rings, also confirming the good crystallinities of the CdS/ZnS NCs. 3.3. Optical Characterization. The shape-controlled synthesis of core/shell NCs and the investigation of their optical properties are of importance in furthering the progress of nanoscience. The growth of core/shell NCs was monitored by UV−vis and PL spectra (Figure 4). For the growth of the first two ML ZnS shells at 220 °C, the first exciton absorption peak and the band-edge PL peak shift to the largest wavelength. For the PL spectra of the CdS core, the deep-trap emission is distinct before the ZnS shell growth and eliminated completely with ZnS coating due to the removal of structural defects. When the ZnS growth temperature is 240 °C, a slight blue shift of the two peaks can be seen when the outer four MLs of ZnS are coated onto the CdS/ZnS NCs (Figure 4a). As shown in Figure 4b, when the ZnS growth temperature is 280 °C, a continuous blue shift of the two peaks is observed. Figure 4c shows the UV−vis and PL spectra of the CdS/ZnS NCs with six MLs of the ZnS shell at different growth temperatures. When the ZnS growth temperature is 220 or 240 °C, the alloying between CdS core and ZnS shell is not obvious, but the dramatic alloying happened when the growth temperature kept increasing. At 280 °C, the first exciton absorption peak shifts back to 399 nm, similar to peak position of the CdS core. The blue shift of the absorption peak indicates that the substantial alloying of CdS and ZnS should shift the band gap

of CdS toward that of ZnS. At high temperature, the Zn atoms may diffuse into the Cd-rich region at the interface between CdS and ZnS, thereby causing a blue shift.26 3.4. Calculations. In order to study how the solventmediated CdS nanocrystalline facet rising occurs, and to explain the formation of differently shaped CdS/ZnS NCs, a periodic DFT method was used to calculate the geometries and energies of (110), (111), and (001) bare facets and their complexes with the additive. The facet−additive interaction via physical adsorption can occur through interaction of the additive with each facet. The potential hydrogen bonding between an NH bond and the most accessible/outermost S atoms of the facets should be considered. Afterward, further interaction between N atom with lone-pair electron and the neighboring Cd atom of the facet occurs via chemical adsorption, thus forming a stronger Cd−N covalent bond. Figure 5 and Supporting Information Figures S3−S6 show the optimized structures of both bare and additive−ZB CdS (110), (111), and (001) facets in vacuum and aqueous solutions, and the corresponding key values are summarized in Table 1. From the present calculations, the absolute values of both the total energies and the binding energies of the three bare facets follow the trend (110) > (111) > (001). These show that both (110) and (111) bare facets are more stable than the (001) facet. For the additive adsorbed physically to the facet, the potential intermolecular distances are presented in Table 1. For the additive−CdS (110) facet complex, the NH···S hydrogen bonding distances are 2.736 and 2.841 Å in vacuum, which are extended by ca. 0.01 Å in the bulk solvent. For the additive−CdS (111) facet complex, the NH···S distances are E

DOI: 10.1021/cg501747e Cryst. Growth Des. XXXX, XXX, XXX−XXX

Article

Crystal Growth & Design

only along the (111) facet. Thus, CdS/ZnS NCs changed from tetrapod-shape to tetrahedron- and dot-shapes. A 280 °C temperature is appropriate for preparing dot-shaped CdS/ZnS NCs. To the best of our knowledge, the tetrapod- and tetrahedron-shaped CdS/ZnS NCs are reported here for the first time. More importantly, compared with the previous works43−47 which focused on the control of the sizes and morphologies by adjusting the various reaction conditions, the as-prepared CdS/ZnS NCs with well-defined morphologies could be obtained by only tuning the ZnS growth temperature, and series of periodic density functional theory calculations were carried out to successfully explain the growth mechanism.

obviously longer than those of the additive−CdS (110) facet. The physical adsorption (PA) energy is 7.6 kcal/mol in vacuum for the additive−CdS (110) facet complex, which is slightly lower by 1.8 kcal/mol than the PA for the additive−CdS (111) facet complex. The difference of these two PA energies decreases when the bulk solvent is added, as seen in Table 1. A quite high PA energy is expected for the additive−CdS (001) facet complex. One of the possible reasons is that sulfide ions array on the whole (001) facet so that the interaction energies (including hydrogen bonding energy) are quite large. The stronger chemical adsorption (CA) energies can be observed through the covalent cross-link between facet outermost Cd and N atoms of the additive. First, the bond length of Cd−N is 2.398 Å with a (110) facet and 2.376 Å with a (111) facet in vacuum. The bulk solvent can decrease the distances up to 2.376 and 2.409 Å, respectively. It is noteworthy that changes from physical to chemical adsorption of the additive are exothermic, corresponding to formation of more stable complexes. The CA energies are 30.5 in vacuum, 26.1 kcal/mol in bulk solvent for the additive−CdS (110) facet complex and 23.0 in vacuum, 21.7 kcal/mol in bulk solvent for the additive−Cd (111) facet complex. Thus, with CA, the interaction between the additive and the CdS (110) facet is more stable. Besides the hydrogen bonding effect, the charge transfer effect always contributes to complex stability. Thus, Mülliken charge analysis was also performed to explore the stable modes between the additive and different facets of CdS core, as seen in Table 2. For the additive bound on the facets, N−H bonds are strongly polarized with more positive charges on H of the amino group and more negative charges on the N atoms when compared with the cases of the free molecule. For the PA complexes, the total charge of the additive fragment is 0.000 in vacuum and 0.013 e (0.001 e corresponding to 0.627 kcal/mol) in the bulk solvent, which corresponds to nearly no electron transfer to the Cd48S48 bulk with (110) facet in vacuum and small partial electron transfer to the bulk. A similar phenomenon is also observed for the Cd48S48 bulk with the (111) facet. However, for the Cd48S48 bulk with a (001) facetcontaining complex, there is small partial electron transferred to the additive, which is attributed to large number of S atoms located on the facet. Note that the additive chemical adsorption on the (110) and (111) facets make the additive bear more positive charges, which mean that more electron transfer to Cd48S48 bulk occurs. Note that the bulk solvent could influence the electron movement between the additive and Cd48S48 bulk. For the complexes involving chemical adsorption, the amount of electron transfer is ca. 0.02−0.04 e, as shown in Table 2. Therefore, the growth mechanism of differently shaped CdS/ ZnS NCs can be explained based on our DFT calculations. Due to the different interaction between the organic ligand and each facet of CdS core, with increasing the ZnS growth temperature, the interaction between the organic ligand and a certain facet such as the (111) facet became weaker so that the organic ligand could no longer cap this facet, resulting in ZnS shell growing only along this facet. When the ZnS growth temperature was 240 °C, the ZnS shell preferred to grow along the (111) facet of the CdS core, because the (111) facet could not be capped by the organic ligand. Thus, the tetrapodshaped CdS/ZnS were formed. With increasing of the ZnS growth temperature, the interaction between the organic ligand and the (110) facet also became weaker and the organic ligand could no longer cap the (110) facet, making it harder to grow

4. CONCLUSIONS In this study, we applied the SILAR technique to obtain CdS/ ZnS core/shell heterostructured NCs. SILAR provides a facile method to grow differently shaped core/shell NCs. Shell growth temperature was identified as the key parameter for successful growth of uniform tetrapod-, tetrahedron-, and dotshaped CdS/ZnS core/shell heterostructured NCs. Periodic DFT was carried out to explore the binding energies between the organic ligand and different facets of the CdS core considering the physical adsorption, chemical adsorption, hydrogen binding, and charge transfer effects. These insights allow for further study of colloidal synthesis of core/shell NCs with uniform morphologies. Based on the superior experimental and theoretical features, we believe that the approach described in this study can be extended to the preparation of more sophisticated heterostructured NCs.

■

ASSOCIATED CONTENT

S Supporting Information *

TEM images, HRTEM images, SAED patterns, wild-type, and optimized models of bare facets. This material is available free of charge via the Internet at http://pubs.acs.org/.

■

AUTHOR INFORMATION

Corresponding Authors

*Tel., Fax: +86 1088 875298. E-mail address: zhangrubo@bit. edu.cn. *Tel., Fax: +86 1088 875298. E-mail address: [email protected]. cn. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Nature Science Foundation of China (No. 21275018 and 21203008), Research Fund for the Doctoral Program of Higher Education of China (RFDP) (No. 20121101110049), and the 111 Project (B07012) for financial support. The authors also thank Derek LaMontagne for his help with the experiments and discussion.

■

REFERENCES

(1) Demortière, A.; Schaller, R. D.; Li, T.; Chattopadhyay, S.; Krylova, G.; Shibata, T.; dos Santos Claro, P. C.; Rowland, C. E.; Miller, J. T.; Cook, R.; Lee, B.; Shevchenko, E. V. J. Am. Chem. Soc. 2014, 136, 2342−2350. (2) Diroll, B. T.; Murray, C. B. ACS Nano 2014, 8, 6466−6674. (3) Soni, U.; Pal, A.; Singh, S.; Mittal, M.; Yadav, S.; Elangovan, R.; Sapra, S. ACS Nano 2014, 8, 113−123. F

DOI: 10.1021/cg501747e Cryst. Growth Des. XXXX, XXX, XXX−XXX

Article

Crystal Growth & Design (4) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Rev. Mater. Sci. 2000, 30, 545−610. (5) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019−7029. (6) Fang, X. S.; Bando, Y.; Gautam, U. K.; Zhai, T. Y.; Gradečak, S.; Golberg, D. J. Mater. Chem. 2009, 19, 5683−5689. (7) Kang, S. J.; Lee, G. H.; Yu, Y. J.; Zhao, Y.; Kim, B.; Watanabe, K.; Taniguchi, T.; Hone, J.; Kim, P.; Nuckolls, C. Adv. Funct. Mater. 2014, 24, 5157−5163. (8) Hu, L. F.; Brewster, M. M.; Xu, X. J.; Tang, C. C.; Gradečak, S.; Fang, X. S. Nano Lett. 2013, 13, 1941−1947. (9) Han, S. C.; Hu, L. F.; Liang, Z. Q.; Wageh, S.; Al-Ghamdi, A. A.; Chen, Y. S.; Fang, X. S. Adv. Funct. Mater. 2014, 24, 5719−5727. (10) Rieger, T.; SchäPers, T.; Grützmacher, D.; Lepsa, M. I. Cryst. Growth Des. 2014, 14, 1167−1174. (11) Chen, Z. L.; Zhang, H.; Du, X. H.; Cheng, X.; Chen, X. G.; Jiang, Y.; Yang, B. Energy Environ. Sci. 2013, 6, 1597−1603. (12) Du, X. H.; Chen, Z. L.; Li, Z. B.; Hao, H. X.; Zeng, Q. S.; Dong, C. W.; Yang, B. Adv. Energy Mater. 2014, 4, 1400135. (13) Chen, Z. L.; Zhang, H.; Zeng, Q. S.; Wang, Y.; Xu, D. D.; Wang, L.; Wang, H. Y.; Yang, B. Adv. Energy Mater. 2014, 4, 1400235. (14) Wang, J. J.; Liu, P.; Seaton, C. C.; Ryan, K. M. J. Am. Chem. Soc. 2014, 136, 7954−7960. (15) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Nat. Mater. 2005, 4, 435−446. (16) Wu, K. F.; Chen, Z. Y.; Lv, H. J.; Zhu, H. M.; Hill, C. L.; Lian, T. Q. J. Am. Chem. Soc. 2014, 136, 7708−7716. (17) Zhou, H. L.; Qu, Y. Q.; Zeid, T.; Duan, X. F. Energy Environ. Sci. 2012, 5, 6732−6743. (18) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Chem. Rev. 2010, 110, 389−458. (19) Talapin, D. V.; Koeppe, R.; Götzinger, S.; Kornowski, A.; Lupton, J. M.; Rogach, A. L.; Benson, O.; Feldmann, J.; Weller, H. Nano Lett. 2003, 3, 1677−1681. (20) Zhong, H. Z.; Scholes, G. D. J. Am. Chem. Soc. 2009, 131, 9170−9171. (21) Nair, P. S.; Fritz, K. P.; Scholes, G. D. Small 2007, 3, 481−487. (22) Li, J. J.; Wang, Y. A.; Guo, W. Z.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. G. J. Am. Chem. Soc. 2003, 125, 12567− 12575. (23) Mahler, B.; Lequeux, N.; Dubertret, B. J. Am. Chem. Soc. 2010, 132, 953−959. (24) Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, Y.; Wong, C.; Harris, D. K.; Wei, H.; Han, H. S.; Fukumura, D.; Jain, R. K.; Bawendi, M. G. Nat. Mater. 2013, 12, 445−451. (25) Xia, X.; Liu, Z. L.; Du, G. H.; Li, Y. B.; Ma, M. J. Phys. Chem. C 2010, 114, 13414−13420. (26) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463−9475. (27) Xie, R. G.; Kolb, U.; Li, J. X.; Basché, T.; Mews, A. J. Am. Chem. Soc. 2005, 127, 7480−7488. (28) Meyns, M.; Iacono, F.; Palencia, C.; Geweke, J.; Coderch, M. D.; Fittschen, U. E. A.; Gallego, J. M.; Otero, R.; Juárez, B. H.; Klinke, C. Chem. Mater. 2014, 26, 1813−1821. (29) Puzder, A.; Williamson, A. J.; Zaitseva, N.; Galli, G.; Manna, L.; Alivisatos, A. P. Nano Lett. 2004, 4, 2361−2365. (30) Chou, H. L.; Tseng, C. H.; Pillai, K. C.; Hwang, B. J.; Chen, L. Y. Nanoscale 2010, 2, 2678−2684. (31) Azpiroz, J. M.; Matxain, J. M.; Infante, I.; Lopez, X.; Ugalde, J. M. A. Phys. Chem. Chem. Phys. 2013, 15, 10996−11005. (32) Shanavas, K. V.; Sharma, S. M. J. Phys. Chem. C 2012, 116, 6507−6511. (33) Yang, Y. A.; Chen, O.; Angerhofer, A.; Cao, Y. C. J. Am. Chem. Soc. 2006, 128, 12428−12429. (34) Yang, Y. A.; Chen, O.; Angerhofer, A.; Cao, Y. C. J. Am. Chem. Soc. 2008, 130, 15649−15661. (35) Cao, Y. C.; Yang, J. H. J. Am. Chem. Soc. 2004, 126, 14336− 14337.

(36) Delley, B. J. Chem. Phys. 2000, 113, 7756−7764. (37) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (38) Inada, Y.; Orita, H. J. Comput. Chem. 2008, 29, 225−232. (39) Delley, B. Mol. Simul. 2006, 32, 117−123. (40) Nunzi, F.; Angelis, F. D. J. Phys. Chem. C 2011, 115, 2179− 2186. (41) Kusama, H.; Orita, H.; Sugihara, H. Langmuir 2008, 24, 4411− 4419. (42) Fiore, A.; Mastria, R.; Lupo, M. G.; Lanzani, G.; Giannini, C.; Carlino, E.; Morello, G.; De Giorgi, M.; Li, Y.; Cingolani, R.; Manna, L. J. Am. Chem. Soc. 2009, 131, 2274−2282. (43) Xu, X. J.; Hu, L. F.; Liu, S. X.; Wageh, S.; Al-Ghamdi, A. A.; Alshahrie, A.; Fang, X. S. Adv. Funct. Mater. 2015, 25, 445−454. (44) Han, S. C.; Hu, L. F.; Gao, N.; Al-Ghamdi, A. A.; Fang, X. S. Adv. Funct. Mater. 2014, 24, 3725−3733. (45) Wageh, S.; Maize, M.; Han, S. C.; Al-Ghamdi, A. A.; Fang, X. S. RSC Adv. 2014, 4, 24110−24118. (46) Amiri, O.; Hosseinpour-Mashkani, S. M.; Mohammadi Rad, M.; Abdvali, F. Superlattices Microstruct. 2014, 66, 67−75. (47) Amiri, O.; Emadi, H.; Hosseinpour-Mashkani, S. M.; Sabet, M.; Mohammadi Rad, M. RSC Adv. 2014, 4, 10990−10996.

G

DOI: 10.1021/cg501747e Cryst. Growth Des. XXXX, XXX, XXX−XXX