Article pubs.acs.org/Langmuir
In Situ Aggregation of ZnSe Nanoparticles into Supraparticles: Shape Control and Doping Effects Gaoling Yang,† Haizheng Zhong,*,† Ruibin Liu,† Yongfang Li,‡ and Bingsuo Zou† †
Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology, 5 Zhongguancun South Street, Haidian District, Beijing, 100081, Peoples’ Republic of China ‡ CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, Peoples’ Republic of China S Supporting Information *
ABSTRACT: The ability to tune the size, shape, and properties of supraparticles is of great importance for fundamental study as well as their promising applications. We previously developed a method to synthesize monodisperse ZnSe supraparticles via “in situ aggregation” of ZnSe nanoparticles through a simple hot-injection method. In the present work, we show that the “in situ aggregation” strategy can be extended to tune the shapes of ZnSe supraparticles, and introduce novel functional magnetic and luminescence properties. Shape control is manipulated with oleic acid as ligands, which balances the attractive interparticles van der Waals forces and steric repulsive forces from the ligands. With the increase of oleic acid concentration, a morphology change from microspheres to asymmetrical multimer and three-dimensional nanoflowers was observed. “Doping” preformed Fe3O4 nanoparticles into ZnSe supraparticles endow them with magnetic properties. The magnetism of these Fe3O4@ZnSe supraparticles depends on the dosage of dopant. Doping of preformed CdS nanocrystals was also studied, resulting in emissive hybrid CdS@ZnSe supraparticles with diameters of 50−100 nm. It is noted that the doping of Fe3O4 and CdS nanoparticles show differing morphologies. The differences can be explained by variance in the lattice mismatches which leads to differing potentials for crystal growth. properties have been of great interest.13−17 Fe3O4 NPs have been known for their size-dependent superparamagnetic properties.18 The preparation and photonic applications of Fe3O4 SPs that are composed by smaller NPs has been recently demonstrated by Yin et al.19−21 However, the average size of these Fe3O4 SPs can be only adjusted within a narrow range from 30 to 180 nm, which does not match some of the special requirements in photonic applications. Here we apply the doping strategy to prepare size-tunable magnetic SPs. Recently, considerable progress has been made in the synthesis of type II nanocrystal heterostructures (NHs) that spatially separate photoexcited electrons and holes in different parts of hybrid NHs.22−24 These NHs are made of two semiconductor materials with a particular alignment of conduction and valence band edges at the interface and the resulting charge separation leads to a strong dipole moment, indirect bandgap radiative emission, and a large offset between absorption and emission spectral profiles.25 The CdS-ZnSe NHs are particularly attractive due to a highly efficient charge
1. INTRODUCTION Controlled assembly of nanoparticles (NPs) into larger supraparticles (SPs) has attracted considerable attention because of the size- and geometry-dependent properties as well as potential photonic applications.1−3 The “in situ aggregation” in solution has been an effective strategy to assemble NPs into monodisperse spherical SPs in one pot synthesis.4−6 In many cases, the sizes of as-fabricated SPs can be tuned by controlling the colloidal chemistry.4 To explore novel functionalization, it has been of great interest to achieve shape control of SPs and modulate their magnetic and optical properties.7,8 The incorporation of impurities into semiconductor provides an additional means to control their optical, electrical, and magnetic properties.9 This has stimulated similar efforts to colloidal semiconductor nanocrystals, which may enhance or expand their applications.10−12 “Doping” of preformed NPs into SPs through an “in situ aggregation” strategy may be an interesting route to functionalize SPs. To our knowledge, there is no report concerning the “doping” effects in the “in situ aggregation” process. Considering the applications of SPs for photonic and biotechnology applications, SPs with magnetic, luminescence properties and/or bifunctional magnetic-optical © 2013 American Chemical Society
Received: November 8, 2012 Revised: January 1, 2013 Published: January 18, 2013 1970
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Scheme 1. Diagram of the Formation Processes of (a) Spherical ZnSe SPs, (b) Nonspherical ZnSe SPs, and (c) Preformed NP Doped ZnSe SPs
separation across the interface.26,27 While, previous works focused on the core/shell and linear or branched nanorods/ wires. We here explored the formation of novel type II NHs by doping of CdS NPs in the in situ aggregation of ZnSe NPs. In our recent work, we have demonstrated that monodisperse spherical ZnSe SPs can be prepared via the hotinjection method through an in situ aggregation. The size can be tuned from 100 to 800 nm by varying the concentration of precursors.28 In this work, we aimed to study their shape-tuning and explore the possibility to functionalize them with novel magnetic and luminescence properties. We synthesized magnetic Fe3O4@ZnSe SPs with tunable sizes through doping of preformed Fe3O4 in the in situ aggregation of ZnSe NPs. To modify their surface functionalization, a common hydrophilic SiO2 shell was applied. By applying the “in situ aggregation” doping strategy, we also synthesized type-II hybrid CdS@ZnSe SPs, which exhibit luminescent emissions from type II chargetransfer state.
NPs. The as-synthesized CdS NPs were precipitated with acetone followed by centrifugation. 2.4. Synthesis of ZnSe Colloidal Supraparticles and Their Functionality. To prepare colloidal ZnSe SPs, a fixed amount of zinc oleate was added into ODE in a 25-mL three-necked flask. The mixture was degassed at 120 °C for 30 min and then heated to 300 °C under Ar flow in less than 10 min. Selenium solution (selenium powder dissolved in 1 mL of TBP) was quickly injected. After the injection, the reaction was kept at 300 °C for 5 min to get a light yellow colloidal solution. As the solution was cooled to room temperature, the ZnSe SPs were precipitated by adding 10 mL of ethanol into the colloidal solution, centrifuged, washed repeatedly, and redispersed in chloroform. The preparation of nonspherical ZnSe SPs followed a similar procedure, except that a certain amount of oleic acid was added together with ODE in a 25-mL three-necked flask. The feeding amount of oleic acid changed from 0.5 to 5 mL, aimed to tune the shape of ZnSe SPs. Preparation of magnetic ZnSe SPs followed a similar procedure, except that the selenium solution was made by a fixed amount of selenium and Fe3O4 NPs powder mixture dissolved in 1 mL of TBP. The SiO2 surface layer coating was achieved as follows: A specified amount of magnetic ZnSe SPs (20 mg) was added to a mixture of 80 mL isopropyl alcohol plus 7.5 mL deionized water. After ultrasonication for several minutes, NH3·H2O was added (up to pH 8−9) with suitable amount (0.2 mL) of TEOS dropped into the solution. After precursor addition, the reaction system was kept stirring at room temperature for 2 h to obtain the gels. The formed flocculent precipitate was centrifuged, and the upper layer liquid was decanted; then the isolated solid was dispersed in deionized water. The above centrifugation and isolation procedure was repeated several times for material purification. Preparation of hybrid CdS@ZnSe SPs followed a similar procedure, except that the selenium solution was made by a fixed amount of selenium and CdS NPs mixture dissolved in 1 mL of TBP. 2.5. Characterization. The as-synthesized ZnSe and doped ZnSe SPs were characterized by a Hitachi F-4800 scanning electron microscope (SEM) with an X-ray energy dispersed spectrometry (EDS) for compositional analysis. Transmission electron microscopy (TEM) observations were performed with a JEOL-1011 transmission electron microscope, accompanied by selected area electron diffraction (SAED). X-ray diffraction (XRD) patterns were recorded with a Rigaku D/max 2500Pc X-ray powder diffractometer with monochromatized Cu Kα radiation (λ = 1.5406 Å). The percentage of dopant was determined by inductively coupled plasma-atomic emission spectrometer (ICP-AES) using Perkin−Elmer Optima 2100 DV instrument. The doped SPs were repeatedly purified to remove excess precursors. The purified SPs were dissolved in toluene and it was then evaporated and the dried SPs were digested in concentrated HNO3 for ICP-AES measurement. UV−vis absorption spectra were recorded on a Hitachi U-3010 spectrophotometer. Photoluminescence (PL) spectra were measured using a laser with a wavelength of 405 nm as the excitation source at room temperature. Magnetic characterization was carried out using a Vibrating sample magnetometer (VSM)
2. EXPERIMENTAL SECTION 2.1. Materials. Zinc oleate, ethanol, hexane, isopropyl alcohol, and toluene (Beijing Chemical Reagent Ltd. Co., P. R. China) were analytical grade reagents. Selenium powder (99.9%) was a high-purity regent (Beijing Chemical Regent Ltd. Co., P. R. China). 1-Octadecene (ODE, tech.90%, Alfa), tributylphosphine (TBP, tech.90%, Aladdin), and oleic acid (OA, tech.90%, Alfa), iron(III) chloride hexahydrate (West gansu chemical ltd. Co., P. R. China, 99%), cadmium oxide (CdO, Alfa, 99%), iron powder reduced (Tianjin JinKe Fine Chemical Industry Research Institute, P. R. China, 98%), sulfur (Sigma, 99.98%), laurylamine (Sinopharm Chemical Reagent Co., Ltd.), octadecylamine (ODA, tech.97%, Aldrich), tetraethoxysilane (TEOS, tech.98%, Alfa) were used as received without further purification. 2.2. Synthesis of Fe3O4 Nanoparticles. A facile chemical approach developed by Li et al. was used to prepare monodisperse Fe3O4 NPs in the presence of oleic acid and laurylamine mixture.29 In a typical reaction for Fe3O4 NPs, 1.15 g (4 mmol) of FeCl3·6H2O and 0.12 g (2 mmol) of iron powder were added to the hexane solution containing 8 mL of laurylamine (40 mmol) and 3.5 mL of oleic acid (12 mmol) at room temperature. The resulting mixture was sealed in a Teflon-lined stainless steel autoclave with a capacity of 50 mL and maintained at 180−190 °C for over 3 h, and then the autoclave was allowed to cool to room temperature naturally. The product was precipitated with ethanol followed by centrifugation, and Fe3O4 NPs were obtained. 2.3. Synthesis of CdS Nanoparticles. The synthetic method is similar to Peng’s previous report.30 A mixture (4 g in total) of CdO (0.0128 g, 0.10 mmol), oleic acid (0.5 mmol), and technological-grade ODE was heated to 300 °C. A solution of sulfur (0.0016 g, 0.05 mmol) in ODE was swiftly injected into this hot solution, and the reaction mixture was allowed to cool to 250 °C for the growth of CdS 1971
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with fields up to 20 000 Oe at a temperature of 300 K. All solid samples for the measurements were prepared by drying the purified product under vacuum.
3. RESULTS AND DISCUSSION 3.1. Synthetic Strategy. Monodisperse spherical SPs with various compositions have been obtained through “in situ aggregation” strategy by controlling the nucleation and growth steps.31−36 Recently, we synthesized monodisperse ZnSe spherical SPs by using a hot injection method.28 The materials characterizations indicated that final ZnSe SPs are made of small NPs. The synthetic chemical investigations demonstrated that colloidal ZnSe SPs were formed by aggregation of the NPs which were generated by nuclei growth. On the basis of “two stage” theoretical models,32 the formation of polycrystalline colloidal ZnSe SPs may incorporates two dynamical processes. As shown in Scheme 1a, the first process is nucleation of primary particles in supersaturated solution, which was accomplished by injecting Se precursor into a hot solution of Zn precursor and further growth by diffusive capture of solute species. The second process is the aggregation of these precursors to form the secondary particles of colloidal microspheres. According to the understanding of NP assemblies, the aggregation process is related to the interparticle interactions including van der Waals forces between NPs and the interaction of their surface ligands.37 Uncoated NPs have strong van der Waals forces and tend to aggregate when in inert nonpolar solvents because of the reduction of total surface free energy. Aggregation can be prevented by protecting nanoparticles via “steric” or “electrostatic” repulsion. It has been learned that surfactant ligands have a strong influence on the driving force of aggregation.38−40 For our “in situ aggregation” of ZnSe NPs system, the fresh ZnSe NPs with high free surface energy become unstable after they are formed, and the van der Waals forces provide the driving force for the aggregation of these primary NPs to form ZnSe SPs. The presence of long chain surfactant on the NP surface could introduce steric repulsion force, which is opposite to the driving force for NP aggregation. Therefore, it is reasonable that the in situ aggregation of ZnSe NPs can be controlled by adjusting the amount of oleic acid (see Scheme 1b). Recent work by Tang et al. demonstrated that the “in situ aggregation” is self-limiting process and can tolerate large size and shape distribution of NPs.1 This provides a chance to dope preformed NPs into SPs by introducing the preformed NPs (see Scheme 1c). Therefore, we conducted experiments to study the influence of oleic acid on the “in situ aggregation” of ZnSe NPs and apply “doping” strategy to synthesize functional ZnSe SPs. 3.2. Synthesis of Nonspherical ZnSe Supraparticles. On the basis of the above discussions, ODE was chosen as solvent and oleic acid was introduced as ligands to investigate the influence of ligands on the ZnSe SPs. The resulting products were characterized by XRD and EDS measurements (see Figure S1 in the Supporting Information), which confirmed the formation of ZnSe SPs. Figure 1a−f shows the SEM or TEM images of typical samples of the ZnSe SPs, which were prepared with different amount of oleic acid in the synthesis system. Without oleic acid, spherical ZnSe SPs was produced (see Figure 1a). When oleic acid was presented in the reaction system, nonspherical ZnSe SPs with complex structures including dimer, trimmers, tetramers, and nanoflowers were formed. The number of spheres that composed
Figure 1. SEM images of the ZnSe SPs prepared at 300 °C for 20 min with different dosage of oleic acid as surfactants: (a) injection of 0.08 g of Se dissolved in 1 mL of TBP into 0.31 g of zinc oleate in 5 mL of ODE; (b) injection of 0.08 g of Se dissolved in 1 mL of TBP into a mixture of 0.31 g of zinc oleate in 5 mL of ODE and 0.5 mL of OA; (c), (d) injection of 0.08 g of Se dissolved in 1 mL of TBP into a mixture of 0.31 g of zinc oleate in 5 mL of ODE and 1 mL of OA; (e) injection of 0.08 g of Se dissolved in 1 mL of TBP into a mixture of 0.31 g of zinc oleate in 5 mL of ODE and 2 mL of OA; (f) injection of 0.08 g of Se dissolved in 1 mL of TBP into a mixture of 0.31 g of zinc oleate in 5 mL of ODE and 5 mL of OA. The inset shows an enlarged picture of a typical nanoflower.
multimers increased from 2−4 to 6−8 with the increase of oleic acid from 0.5 to 1 mL in the system (see Figure 1b,c). High magnification SEM imaging reveals that the ZnSe SPs with multimer structures are aggregated by the formed microspheres (see Figure 1d). It is also noted that the average diameter of colloidal spheres formed in the same reaction time increased from 100 to 200 nm to 1 μm. With an increase of the oleic acid to 2 mL, some ZnSe nanoflowers appeared as byproducts (see Figure 1e). When oleic acid further increases to 5 mL, only ZnSe nanoflowers were produced (see Figure 1f). This is similar to the observation that ZnSe NPs partially fused into nanoflowers in the NPs synthesis at a limited ligands protection domain.40 These results demonstrated that the “in situ aggregation” of ZnSe nanoparticles can be controlled by adjusting the amount of oleic acid via balancing the attractive interparticles van der Waals forces and steric repulsive forces from the ligands. When oleic acid was used, the interparticle van der Waals forces of freshly formed ZnSe NPs induced high surface free energy, which provides the driving force for the formation of the ZnSe microspheres. When an insufficient amount of ligands was below the critical ligand concentration for stabilization of individual NPs was used, nonspherical SPs were formed because of the aggregation of unfinished spheres. Dimer ZnSe SPs were observed when a relatively low ligand concentration was used, tetramers or multimer nonspherical ZnSe SPs with larger diameters were often yielded when more 1972
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Figure 2. (a) SEM image and (b) EDS spectrum of Fe3O4@ZnSe SPs with Fe3O4/(ZnSe + Fe3O4) molar ratio of 13%; (c) XRD patterns of Fe3O4 NPs and Fe3O4@ZnSe SPs with Fe3O4/(ZnSe + Fe3O4) molar ratio of 27%; and (d) magnetization curves of Fe3O4 NPs and magnetic ZnSe SPs with different dosage of dopants. Inset is a photograph of a magnet attracting magnetic colloidal Fe3O4 NP doped ZnSe SPs in toluene.
samples was 13% and 27%, respectively, as determined by ICPAES measurements (see Table S1 in the Supporting Information). A comparison of the XRD patterns (see Figure 2c) of Fe3O4 NPs and doped ZnSe SPs demonstrated the coexistence of Fe3O4 NPs with ZnSe SPs. The average size of Fe3O4 NPs is calculated to be ∼6.0 nm from the half-width of the (311) reflection peaks of the XRD pattern using the Scherrer formula where the k value was taken as 0.89. The influence of Fe3O4 NPs doping on the final ZnSe SPs was also studied. Figure S2 in the Supporting Information shows the SEM images of ZnSe SPs with and without Fe3O4 NP doping. It is clear that the size and shape remain almost the same with Fe3O4 NP doping. The magnetic property of the Fe3O4 NP doped ZnSe SPs was investigated using a VSM at 300 K in the applied magnetic field sweeping from −20 to 20 kOe. Figure 2d shows the magnetization curves of the Fe3O4 NPs and Fe3O4 NPs doped ZnSe SPs. For all of the samples, no magnetic hysteresis loops and no remanence were observed from the field-dependent magnetization plots, which illustrated that they are superparamagnetic. The saturation magnetization (Ms) value for Fe3O4 NPs is 87.4 emu/g, the Ms values for Fe3O4 NPs doped ZnSe SPs were 1.6 emu/g for the samples with Fe3O4/(ZnSe + Fe3O4) molar ratio of 13% and 31.6 emu/g the samples with Fe3O4/(ZnSe + Fe3O4) molar ratio of 27%. These results indicate that the magnetization of Fe3O4 NPs doped ZnSe SPs was lower than that of Fe3O4 NPs, additionally, the magnetism of Fe3O4 NP doped ZnSe SPs enhanced with the increase in weight ratio of Fe3O4 NPs. Such parameters mean that asprepared magnetic doped ZnSe SPs have strong magnetic responsivity and can be separated easily from the solution with
ligand was used. A model developed by Privman, Goia, and Matijevi31 has revealed that the growth of the final supraparticles by the aggregation of small particles must be coupled with the rate of formation of the precursors. In our reaction systems, the ZnSe precursors were supplied by the reaction between zinc oleate with TBPSe in ODE solvent. Because of the high reactivity and large nucleation, the precursors were consumed quickly after hot-injection. Then, the insufficient supply of ZnSe precursor inhibited the aggregation process, producing ZnSe microspheres. The reaction is slightly slowed in the presence of oleic acid, which induces delayed precursor supply and subsequently results in continuous aggregation of the microspheres into multimers. 3.3. Preparation of Magnetic Colloidal ZnSe Supraparticles. Magnetic SPs have attracted significant interest because of their unique superparamagnetic properties, which have found applications in increasingly diverse areas including catalytic carrier, purification of proteins, fluorescence imaging, drug delivery, and biosensors.41−44 There are various techniques for the preparation of magnetic microspheres; however, the multistep reactions involved make the preparation tedious and time-consuming.45−47 As shown in Scheme 1c, the formation of Fe3O4 NPs doped ZnSe SPs was accomplished by injecting Se precursor into a hot solution of Zn precursor in the presence of preformed Fe3O4 NPs. A typical SEM image of the resulting Fe3O4 NPs doped ZnSe SPs, shown in Figure 2a, indicate that Fe3O4 NPs doped ZnSe SPs are quite uniform and monodisperse. A typical EDS spectrum of Fe3O4 NPs doped ZnSe SPs, shown in Figure 2b, revealed that Fe element does exist in the SPs. It should be noted that the Fe3O4/(ZnSe + Fe3O4) molar ratio of these two 1973
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Figure 3c) of the SiO2@ZnSe microspheres, it is clearly seen that the SiO2 layer is about ∼20 nm. In addition, the diffraction rings of SiO2@ZnSe SPs diffuse and weaken in compassion to those of uncoated ones (see inset of Figure 3a,b). This is the distinguished feature being coated by SiO2 layers. 3.5. Preparation of Hybrid CdS@ZnSe Supraparticles. Hybrid CdS@ZnSe supraparticles were prepared by doping formed CdS nanocrystals in the synthetic process of ZnSe SPs. Figure 4a shows a typical SEM image of the type-II hybrid CdS@ZnSe SPs with a TEM image as inset. The as-synthesized SPs are random shaped and have diameter of 50−100 nm. Figure 4b shows a typical EDS spectrum obtained at 20 KV, along with peak assignments for Zn, Se, Cd, and S elements. To verify the phase transformation after doped, powder XRD analysis was also performed (see Figure S4 in the Supporting Information). The XRD patterns have three diffraction peaks from (111), (220), and (311) plane, which are in agreement with the standard spectra for both of CdS and ZnSe with cubic phase. The UV−vis absorption and emission spectra of CdS nanocrystals and corresponding CdS@ZnSe SPs in toluene are shown in Figure 4c. The CdS nanocrystals exhibits typical absorption and emission features with a distinctive first (1s) exciton peak at ∼425 nm. In CdS@ZnSe type II NHs the 1s exciton absorption peak becomes much less pronounced and a new band at ∼535 nm and a broad absorption tail extending into the red and near-IR are formed. The red tail in the absorption spectra of CdS@ZnSe type II NHs suggest the existence of spatially indirect transitions, lower in energy than the bulk band gap of either material, due to the type II band
the help of external magnetic force. When the magnetic Fe3O4@ZnSe SPs were dispersed in toluene giving a brown suspension, upon applying an external magnetic field, the particles were readily harvested within 10 s and the solution became transparent. 3.4. Surface Coating of SiO2 Shells. Because the synthesis of ZnSe SPs was performed in nonpolar solvent, colloidal ZnSe SPs have poor dispersion performance in water, which inhibits future applications. As one common shell material, SiO2 was widely used in surface coating at present because of its mature ability in surface functionalization. We prepared SiO2 coated colloidal ZnSe SPs using the sol−gel method. After being coated with SiO2, SiO2@ZnSe SPs can be well dispersed in water (see Figure S3 in the Supporting Information). The asprepared SiO2@ZnSe SPs were characterized by TEM with SAED (see Figure 3a,b). From the enlarged TEM image (see
Figure 3. TEM images of colloidal (a) ZnSe SPs corresponding SAED patterns (inset); (b) SiO2@ZnSe SPs, and corresponding SAED pattern (inset); and (c) enlarged TEM image of SiO2@ZnSe SPs.
Figure 4. (a) SEM image of CdS@ZnSe type II NHs and their corresponding TEM image as an inset; (b) EDS spectrum of CdS@ZnSe type II NHs; (c) Absorption and emission spectra of CdS nanocrystals (blue) and CdS@ZnSe type II NHs (red); (d) Schematic drawing of a type II alignment for CdS and ZnSe NCs, facilitating charge separation of electrons and holes in different materials. CBM stands for conduction band minima, VBM for valence band maxima. 1974
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magnetization reached 31.6 emu/g. After being coated with a 20-nm thick SiO2 shell, colloidal ZnSe SPs displayed excellent aqueous dispersity. In addition, hybrid CdS@ZnSe SPs were also synthesized and the emission peak of hybrid CdS@ZnSe SPs significantly red-shifted from ∼490 nm (CdS nanocrystalsonly) to ∼590 nm, implying the formation of type-II band alignments. The observed morphology difference of Fe3O4 and CdS NPs into ZnSe SPs was proposed and discussed. The nonspherical shape as well as the incorporation of magnetic and fluorescent properties endows ZnSe SPs with great potential applications in photonic devices, photocatalysis, drug targeting, bioseparation, and diagnostic analysis.
alignment. This weak indirect transition is very different from a strong spatially direct transition in CdS nanocrystal alone, which results in the development of a sharp band-edge absorption feature as observed experimentally. Furthermore, its emission peak position (∼590 nm) is significantly redshifted 100 nm from that of the CdS NPs (∼490 nm). The bandwidth is mostly governed by the inhomogeneous broadening from the size distribution of the sample. In the case of type-II NHs, the size inhomogeneity can be considered as a convolution of the size distributions of both components of NHs, so the PL full width at half-maximum of type-II NHs is broad.25 These absorption and emission spectral features suggest the existence of a type II band alignment in the CdS@ZnSe NHs. 3.6. Comparison of Supraparticles Doping. It is noted that the doping Fe3O4 NPs and CdS NPs via “in situ aggregation” strategy produced ZnSe SPs different morphologies. To understand the morphological difference, a possible explanation was proposed and discussed. As shown in Figure 5,
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ASSOCIATED CONTENT
S Supporting Information *
The results of ICP-AES measurements of Fe3O4@ZnSe SPs, EDS spectrum and XRD pattern of ZnSe SPs; SEM images of magnetic Fe3O4@ZnSe SPs; photograph of colloidal ZnSe SPs and SiO2@ ZnSe SPs; and XRD patterns of CdS NPs and CdS@ZnSe SPs. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +86-10-68918188; E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 5. Sketch highlighting the influence of lattice mismatchs on the “doping” preformed Fe3O4 and CdS NPs in the “in situ aggregation” of ZnSe NPs.
ACKNOWLEDGMENTS This work is financially supported under NSFC Research Grant Nos. 51003005, 512002011, and 91023039 and BIT funds.
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the lattice mismatch between Fe3O4 (8.396 Å) and ZnSe (5.6676 Å) is very large (−48.1%). According to the crystal growth theory, the epitaxial growth model was determined by the surface energy as well as lattice mismatches between two different materials.48 Even for the soft NP cores, epitaxial growth can tolerate the lattice mismatches of ∼12%.49 In our system, ZnSe and Fe3O4 have similar cubic phase with large lattice mismatches up to 48%. Therefore, it is not possible for epitaxial growth of ZnSe onto Fe3O4 NPs and the “in situ aggregation” of ZnSe NPs was not influenced with the presence of Fe3O4 NPs. In the case of CdS NPs, CdS and ZnSe are cubic phase with a lattice mismatch of −2.6%. Epitaxial growth between ZnSe and CdS is energy favorable and has been observed in the recent reports.26 In addition, epitaxial growth of ZnSe onto preformed CdS NPs is also preferred from XRD pattern of CdS@ZnSe NHs. This assumption is also in agreement with the observed type II emission from hybrid CdS@ZnSe NHs.
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4. CONCLUSIONS Nonspherical ZnSe colloidal SPs with multimers as well as nanoflower structures were synthesized through a hot-injection method with the assistance of oleic acid as the ligand and ODE as the solvent. The units of multimers increased from 2−4 to 6−8 with the increasing oleic acid. This work also provides a new strategy to synthesize multiple functional SPs. Magnetic ZnSe colloidal SPs were prepared by doping Fe3O4 NPs in the initial stage of the synthesis system. Their magnetic properties study indicated that the magnetism of the Fe3O4 NPs doped ZnSe SPs depend on the dosage of dopants and the saturation 1975
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