Spontaneous Transformation of CdSe Nanoparticles into

Dec 29, 2014 - Here, CdSe NPs are used as the starting point of synthesis; the presence of EDTA triggers the decomposition of primary CdSe NPs, and re...
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Spontaneous Transformation of CdSe Nanoparticles into Nonspherical Se Crystals: Role of the Precursor Ligand Dawei Deng*,†,‡ and Junsheng Yu*,‡ †

School of Life Science and Technology, China Pharmaceutical University, Nanjing 210009, P. R. China School of Chemistry and Chemical Engineering, Key Laboratory of Analytical Chemistry for Life Science, Nanjing University, Nanjing 210093, P. R. China

‡

S Supporting Information *

ABSTRACT: In this report, nonspherical Se crystals were synthesized from the transformation of CdSe nanoparticles (NPs) under ambient conditions, including nanowires (), microscale or nanoscale rods (−), crossheads, crosses (+), and other unusual highly anisotropic structures. Here, CdSe NPs are used as the starting point of synthesis; the presence of EDTA triggers the decomposition of primary CdSe NPs, and resultant gradual release of Se2− anions into solution; finally, highly pure well-crystallized nonspherical hexagonal Se (h-Se) crystals are formed via the oxidation of the released Se2− anions in air (this redox reaction will stimulate further the release of Se2− anions) and the subsequent spontaneous crystallization of Se monomers. Some key variables such as the concentrations of EDTA and CdSe NPs, and the ligand of primary NPs, were explored systematically. The experimental results show that the ligand nature of the NP precursor influences the transformation rate of CdSe NPs to Se crystals and dominates the shape and aspect ratio of Se product, while the concentrations of EDTA and CdSe NPs only influence the size of the product. Meanwhile, we also investigated intensively the transformation process of CdSe NP precursors to multiarmed Se crystals, aside from the detailed characterizations on their sizes, shapes, and crystal structures.

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such as CdE (E = Se or Te) NPs → E nanowires,9,10 CdE NPs → CdE nanowires and even nanosheets,11−13 CdTe QDs → CdS QDs, etc.14 Particularly, in the study of CdE NPs → E nanowires,9,10,15 the resulting nanowires exhibited favorable size uniformity, in which the main advantage of NPs as a starting reagent is the relatively slow release of molecular componentsthe building blocks for subsequent materials, compared with molecular precursors. Inspired by these interesting achievements, in this report, using Se as an example, we explore further how to achieve the shape control of Se crystals. As we know, selenium is an important elemental semiconductor and exhibits many useful and interesting properties, including high photoconductivity (∼8 × 104 S cm−1), nonlinear optical response, high thermoelectric or piezoelectric responses, and catalytic activity.16 Besides these intriguing properties, Se nanomaterials are also excellent chemical and physical templates for the preparation of other more complicated nanostructures, such as nanocables, nanotubes, and so on.17−19 Hence, the related studies of selenium are attracting increasing attention. Currently, many excellent solution-based synthesis strategies have been developed,20−30 such as the refluxing

INTRODUCTION Over the past two decades, considerable efforts have been devoted to fabricate inorganic nanocrystals with nonspherical, branched, or non-centrosymmetric shapes,1−3 because they not only provide opportunities to explore possible new phenomena arising from unusual structures but also are useful for practical applications in fabricating nanoscale electronic and optoelectronic devices.4 Up to now, various nonspherical shapes, e.g., rods, tubes, disks, prisms, polyhedra, branched multipods, etc., have been obtained.1−7 Owing to a higher level of complexity, branched rodlike nanocrystals can be used to construct new complex superstructures with more advanced functionality.5,6 Hence, the synthesis of new branched nanocrystals is a significant and challenging scientific task. As a result of continuous efforts, branched nanocrystals have been prepared from various types of inorganic materials−metal oxides, metal chalcogenides, as well as noble metals.2−6 However, there are only a few reports on branched nanocrystals of elemental semiconductors, for example, Se, Te. Applying nanoparticles as the starting reagent for the synthesis of nonspherical crystals is a new research area, which opens up novel opportunities for the control of the shape and composition of materials.8 In this research field, the group of Kotov has realized a series of transformations from primary NPs to secondary nanostructures via the ligand-depleted routes, © XXXX American Chemical Society

Received: August 12, 2014 Revised: December 3, 2014

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were recorded and shown in Figure S1, Supporting Information. Their size was determined by transmission electron microscopy (TEM) measurements and/or referring to the literature data (Figures S2 and S3, Supporting Information).31−33 (II). Transformation of CdSe NPs into Se Crystals. First, ethanol was added into a certain amount of crude ligand-capped CdSe NP solution (the volume ratio of ethanol/CdSe NP solution is 2:1). As a result, the solution became turbid immediately. After centrifugation at 5000 rpm for 3 min, the precipitates of CdSe NPs were obtained and then redispersed into 5 mL of water solution containing EDTA (pH 9) (after being purified with ethanol, there is still a certain amount of ligand on the NP surface or free in solution that will influence the decomposition of primary CdSe NPs and the subsequent spontaneous growth of Se crystals). The concentrations of EDTA and CdSe NPs in solution were controlled at 5 mM and 2 mM, respectively, except for additional illustration. Subsequently, the dispersions of CdSe NPs were stored in the dark under ambient conditions (∼25 °C). During this storage, the spontaneous transformation of CdSe NPs to Se crystals occurred under the simultaneous assistance of added EDTA and dissolved oxygen (initially, the dispersions turned turbid; intermediately, the upper solution became gradually colorless and clear; finally, dark (or brick red) precipitates appeared on the bottom of the bottle after about 48 h of storage). The color of Se products with a low aspect ratio is dark, while the color of Se nanowires with a high aspect ratio is brick red. Characterization. TEM, energy-dispersive X-ray (EDX) analysis, and selected-area electron diffraction (SAED) were performed on a Philips FEI Tecnai G2 20 S-TWIN or a JEOL JEM-200CX. Scanning electron microscope (SEM) images were obtained using a LEO 1530 VP field-emission microscope or a Philips FEI Quanta 200. Powder Xray diffraction (XRD) measurement was carried out using a Philips X′Pert PRO X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) was investigated by using a VG ESCALAB MK II spectrometer with a Mg Kα excitation (Binding energy calibration was based on C 1s at 284.6 eV). Raman spectrum was obtained at room temperature on a Renishaw inVia Raman spectrometer (England). Fourier transform infraed (FT-IR) spectra and 1H NMR spectra were recorded with a TENSOR 27 FT-IR spectrophotometer (Bruker) and a 300 MHz spectrometer (Bruker AV-300) at ambient temperature in D2O, respectively. Absorption and emission spectra were measured using a Shimadzu 3100 UV−vis-near-IR spectrophotometer and a Shimadzu RF-5301 fluorescence spectrometer, respectively. All optical measurements were performed at room temperature.

process, the solvo(hydro)thermal methods, sonochemical approaches, microwave-assisted methods, photothermally assisted solution method, biomolecule-assisted methods, and other methods. As a result, high quality Se nanorods,20 nanowires,21,22 nanotubes,23−25 and nanobelts26−29 have been prepared. However, few strategies were developed for the synthesis of Se crystals with other unusual shapes.20,30 And most of the existing strategies were based on the assistance of templates or externally added force (such as ultrasonic, microwave, or heat).20−30 In this work, by using aqueous CdSe NPs as the starting reagent, we extensively investigated the influence of various experimental variables on the spontaneous transformation of primary CdSe NPs into secondary Se crystals, including the concentrations of EDTA and NP precursor, and the ligand of NPs (here, EDTA was used as the decomposition reagent of primary CdSe NPs9,10). It was found that the shapes, sizes, and aspect ratios of the produced Se crystals could be controlled well by selecting the ligand of the NP precursor and varying the concentrations of EDTA and primary NPs. Noteworthy is the fact that to our knowledge, Se crystals with some unusual shapes (e.g., crossheads, crosses, pentapods, hexapods, and other more complex structures) have not been reported.9,15,20−30 In addition, we investigated further the crystal structure of the Se product and the transformation process of primary CdSe NPs → Se crystals. These experimental results may provide a better fundamental understanding of the transformation mechanism of CdE (E: Se, Te) NPs into E crystals.

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EXPERIMENTAL SECTION

Chemicals. CdCl2·2.5H2O (99+%), selenium powder, sodium borohydride (99%), absolute ethanol, L-cysteine hydrochloride monohydrate (L-Cys, 99+%), mercaptamine hydrochloride (MA, 98%), 1-thiolglycerol (TG, 90%), thiolglycolic acid (TGA, 90+%), 3mercaptopropionic acid (MPA, 98%) trisodium citrate dihydrate (TSC, 99+%), ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA, 99+%), sodium hydroxide (96+%), and sulfuric acid (95− 98%). All chemicals are commercially available products and used as received. Redistilled water was used in all preparations. Synthesis of Variously Shaped Se Crystals. The typical synthetic procedure can be divided into two main parts. (I). The Synthesis of CdSe NPs (i.e., the NP Precursor). These water-soluble NP precursors were synthesized by following a previously reported procedure for citrate-stabilized CdSe NPs.31 In brief, H2Se gas generated by the reaction of ethanol solution of NaHSe with added dilute H2SO4 passed to nitrogen-saturated CdCl2 aqueous solution in the presence of various ligands as stabilizing agents with a N2 flow, in which the ethanol solution of NaHSe was prepared by the reaction of Se powder with excessive NaBH4 in ethanol at 40 °C. The total volume of NPs solution is 100 mL. Here, six ligands were selected, including MA, L-Cys, TG, TGA (and MPA), and TSC. The precursor molar ratios of Cd/Se/ligand and the starting pH values were used for the preparation of water-soluble CdSe NPs as follows:

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RESULTS AND DISCUSSION

Synthesis of Multiarmed Se Crystals. In our recent studies, L-Cys has been observed to play a special role in stabilizing water-soluble NP colloids because of its unique molecular structure, containing a mercapto terminus, an amino terminus, and a carboxylic acid terminus.34,35 So in this study, LCys was chosen first as the stabilizer to prepare CdSe NP precursors, i.e., the starting reagent for the growth of Se crystals (the detailed characterizations of these primary NPs were shown in Figure S2, Supporting Information). As expected,9,10 the presence of EDTA initiates the transformation of CdSe NPs into Se crystals (the corresponding transformation process will be presented in Figures 4−6). After incubation at room temperature for ∼35 h, a dark precipitate of Se appeared on the bottom of the bottle, and correspondingly, the upper solution became colorless and clear. The shape and composition of the dark precipitate were characterized fully by means of SEM and TEM techniques. As shown in Figure 1, the spontaneous transformation product of destabilized L-Cys-capped CdSe NPs consists of a large quantity of exclusively anisotropic crystals (Figure 1A,E), whose shapes include typically ∼50% rods (−), ∼15% crossheads, and ∼30% crosses (+). Occasionally, Se

Cd/Se/L‐Cys (or TGA) = 1:0.5:2 ([Cd2 +] = 4 mM, pH = 12)

Cd/Se/TG = 1:0.75:2 ([Cd2 +] = 4 mM, pH = 12) Cd/Se/MA = 1:0.5:2 ([Cd2 +] = 4 mM, pH = 5)

Cd/Se/TSC = 1:0.25:2 ([Cd2 +] = 2 mM, pH = 11) After 30 min of stirring at room temperature, water-soluble CdSe NPs stabilized by various single ligands were obtained (the colors of thiolcapped CdSe NP solutions and TSC-capped CdSe NP solution are yellow and brown, respectively). The absorption spectra of CdSe NPs B

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crystals in the presence of 5, 7.5 10 mM of EDTA are ∼800, ∼600, and ∼250 nm, respectively). In addition, we also observed that the times needed for fulfilling the transition of CdSe NPs → nonspherical Se crystals in the presence of 5, 7.5, 10 mM of EDTA were different, and ∼35 h, ∼30 h, ∼25 h, respectively. That is, the higher the concentration of EDTA, the shorter the time needed to complete this transformation process. On the basis of these experimental observations, we propose that the increase of EDTA concentration might induce the faster decomposition of primary NPs and the resultant faster generation of Se monomers. As a result, a higher degree of supersaturation of Se and a greater number of Se condensation centers (seeds) are achieved, which will quickly deplete Se monomers.9,10 Thus, the as-prepared nonspherical Se crystals have a smaller size in diameter and length in the presence of higher EDTA concentration. Precursor Concentration. TEM images in Figure S5, Supporting Information indicate clearly that all as-produced Se products contain multiarmed structures, although the concentration of primary NPs (the value refers to the Se content of the NPs in solution) was changed; with the increase of precursor concentration, the average size of Se crystals increases ([CdSe] = 0.5 mM, the average diameter of arm is 200 nm; 1 mM, 500 nm; 2 mM, 800 nm). In addition, as expected, increasing the NP precursor concentration will prolong the time needed to complete the transformation process (the corresponding transformation times are ∼20, 25, and 35 h respectively for the precursor concentrations of 0.5, 1, and 2 mM). Here, from the experimental observations in Figures S4 and S5, it can be concluded that the concentrations of NP precursor and EDTA only influence the size of the asprepared Se crystals and almost do not change their morphology and aspect ratio. Ligand Nature of the NP Precursor. In all above experiments, the NP precursors used are the same, namely, LCys-stabilized CdSe NPs. To better understand the growth of multiarmed Se crystals, we changed the ligand of starting CdSe NPs. Here, 2-mercaptoethylamine (MA), 1-thioglycerol (TG), thiolglycolic acid (TGA), and trisodium citrate (TSC) were selected (Figure S1). The absorption spectra (Figure S1a) and TEM images (Figures S2 and S3) of the CdSe NPs stabilized by various ligands all show that they have a similar ultrasmall size (∼2 nm in diameter) except for TSC-stabilized ones (∼2.8 nm).31−33 When these CdSe NPs were used as the starting reagents, the dark (or brick red) precipitates of Se also were obtained via this room temperature incubation route (see the Experimental Section). As presented in Figure 2, an obvious dependence of the morphology of Se product on the precursor ligand nature was observed. When MA was used as the capping ligand of starting NPs, the resulting product is inhomogeneous dumpy Se rods with low aspect ratio or their aggregates (Figure 2A). For comparison, Figure 2B shows again the SEM image of Se crystals obtained from L-Cys-stabilized CdSe NPs. If TG was used, the as-prepared product also consists of highly anisotropic Se crystals (Figure 2C), in which radialized bunches, rods, crossheads, and crosses of Se can be observed. SEM image in Figure 2D indicates that the as-prepared product from TGAstabilized CdSe NPs is inhomogeneous Se nanowires (similarly, if using MPA-stabilized CdSe NPs as the precursor, the asprepared product also is inhomogeneous Se nanowires (data not shown)). However, when CdSe NPs stabilized by TSC were used as the starting reagent, the product is uniform Se nanowires (Figure 2E).

Figure 1. (A−D) Typical SEM images of nonspherical Se crystals prepared from L-Cys-stabilized CdSe NPs. (E) Low-magnification TEM image. (F and G) High-magnification TEM images of single crosshead and cross. (H) The corresponding EDX spectrum (the signals of Cu should be attributed to TEM grids).

crystals with other more complicated shapes (∼5%) such as pentapods, hexapods, and so on have been observed (Figure 1D and the inset). The typical size of the resulting nonspherical Se crystals is ∼800 nm in diameter and ∼4 μm in length. The typical arm−arm angles for crossheads, crosses, pentapods, and hexapods are identical (90°), which may be an indication of the synthetic relationship among them. Elemental analysis from energy-dispersive X-ray (EDX) spectrum confirms that all the products are made from pure Se rather than CdSe (Figure 1H). As we know, the crystal structure of hexagonal selenium is composed of hexagonally packed, one-dimensional (1D) spiral chains of Se atoms, which easily results in a 1D anisotropic growth of Se.16,20−29 Hence, the growth of these unusual Se structures is quite unexpected and difficult to achieve, even in the similar studies.9,15 In addition, primary CdSe NPs (∼2.0 nm in diameter) are not detected in the SEM and TEM images in Figure 1A−G, indicating an app. 100% conversion efficiency of CdSe NPs to nonspherical Se crystals. Influence Factors. To the best of our knowledge, the nonspherical Se crystals as presented in Figure 1 have not been obtained in previous reports.9,15,20−29 How to controllably generate these nonspherical Se crystals? In the present study, we noted that the synthesis begins with partially destabilized LCys-capped CdSe NPs, whereas highly pure Se crystals are obtained finally at room temperature in the presence of EDTA. Hence, three key experimental variables, such as the EDTA concentration, the precursor (CdSe NPs) concentration, and the precursor ligand nature, were selected to explore their effects on the spontaneous growth of Se crystals. EDTA Concentration. The introduction of EDTA induces the transformation of CdSe NPs into Se crystals.9,10 Therefore, its role was investigated first. As shown in Figure S4, Supporting Information, although the concentration of EDTA used is different, all products contain multiarmed Se crystals; the increase of EDTA concentration results in the decrease in the average size of the prepared nonspherical Se crystals (for instance, the average diameters of the arms of the nonspherical C

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influence of the solution pH on the crystallization of Se monomer: the experimental results in Figure S6 indicate that when the solution pH is equal or higher than 8.5, the Se monomer formed from the oxidation of Se2− anions by dissolved oxygen (the standard redox potentials for Se2−/Se and O2/OH− pairs are −0.924 and 0.401 V, respectively9,36) can crystallize into hexagonal Se particles spontaneously (hence, in this study, pH 9 was selected). (ii) the unique role of the EDTA: the data in Figure S7 demonstrate that on the one hand, in N2 gas, EDTA can act as the stabilizer to prepare CdSe NPs (∼3 nm in diameter), whereas on the other hand, under ambient conditions, the as-prepared EDTA-capped NPs are not stable, and will gradually decompose and transform into 1D Se nanowires after 120 h of storage, different from thiol- or TSC-stabilized CdSe ones (in this study, we have observed that different from EDTA, thiols, or TSC added cannot induce the transformation of CdSe NPs into Se crystals (not shown)). These experimental observations could be attributed to three reasons, namely, the low solubility product constant Ksp (10−33) of CdSe (25 °C), the high formation constant between EDTA and Cd2+ ions (Kform, equal to 1016.4),9,36 and the easy oxidation of released Se2− anions by dissolved oxygen. Another aspect of these results is to indicate the key role of the dissolved oxygen for the EDTA-induced transformation of CdSe NPs to nonspherical Se crystals. Here, according to the results in Figure 2, Figure S7, Table 1, and some experimental observations, we analyze further the roles of the precursor ligands during the transition from CdSe NPs to Se crystals, by combining our previous report (the −SH terminus in organic small molecule might play a more important role in inducing the spontaneous assembly of Se crystals than other groups (e.g., −NH2, −OH, or −COO−),37 in which Se crystals were formed from the oxidation of Se2− anions by dissolved oxygen). Specifically, (i) trisodium citrate (and EDTA) without −SH terminus as the precursor ligand: the low transformation rate (or the low release rate of Se components) makes the growth of Se crystals more dependent on its intrinsic propertythe unique 1D infinite helical chains of covalently bound Se atoms.16 Thus, 1D nanowires with a high aspect ratio will be formed (Figures 2E and S7) (in these two cases, TSC and EDTA might have a weak influence on the shape control of Se particles). (ii) L-Cys (or TG) with −SH and other groups: the suitable (or high) release rate of Se components favors the formation of Se nanorods with lower

Figure 2. Typical low- and high-magnification (inset) SEM images of the as-prepared Se crystals from CdSe NPs stabilized by different single ligand: (A) MA, (B) L-Cys, (C) TG, (D) TGA, and (E) TSC.

To understand deeply the fundamental roles of the ligands on the growth of Se crystals, we gave further the detailed descriptions on the starting CdSe NPs, the transformation time of CdSe → Se, and the shape of the resulting Se product in Table 1. These data show that aside from the perceptible morphology distinctions, in the present experiment, the time needed for the transformation of CdSe NPs → Se crystals is also quite different (here, the time refers to the period when the solution color changes from initial yellow or brown to ultimate colorless and clear), even if the NP precursors stabilized by various ligands have the similar diameter (∼2 nm). The order of the transformation rate for CdSe → Se under same conditions is MA > L-Cys > TG > TGA > TSC as listed in Table 1. Thus, a conclusion can be made safely that in this study, as compared to the concentrations of EDTA and NP precursor, the ligand nature of precursor plays a more important role in influencing the transformation rate of CdSe NPs → Se crystals and dominating the shape of Se crystals. Meanwhile, these results also confirm that using NPs as the starting reagent may afford us a facile way to control the release rates of the molecular components (the building blocks for subsequent materials), in comparison with molecular precursors.9,10 Next, to reveal further the key roles of the precursor ligands on the shape control of Se crystals during the spontaneous transformation, we explored some other related factors. (i) the

Table 1. Detailed Descriptions on the Starting CdSe NPs, the Transformation Time of CdSe → Se and the Shape of Se Product, and the Molecular Formula of Ligand

D

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Figure 3. TEM image (A) and corresponding SAED (B) of Se rods. TEM image (C) and corresponding SAED (D) of single Se cross. (E) Typical XRD pattern of the as-prepared Se crystals from the transformation of L-Cys-stabilized CdSe NPs.

lattice mismatch. XRD analysis in Figure 3E confirms that these nonspherical crystals are pure single crystal elemental Se with the hexagonal structure (JCPDF No. 06-0362), by further considering the Raman spectrum in Figure S9, Supporting Information. All the reflection peaks could be indexed to the hexagonal Se, and no impurity was detected. In addition, the XRD patterns (Figure S10, Supporting Information) and SAED patterns (Figure S1, Supporting Information) taken from the as-prepared Se crystals with other shapes also indicate that they are single crystalline, have the hexagonal structure, and preferentially grow along the [001] direction. Transformation Process of CdSe NPs into Multiarmed Se Crystals. To explore further the shape evolution and the accompanied chemical changes in the transition of L-Cysstabilized CdSe NPs → multiarmed Se crystals started by the addition of EDTA, the intermediate samples were investigated systematically by using a combination of TEM, SAED, EDX, UV−vis absorption, XRD FT-IR, and NMR. Figure 4 gives the TEM images and EDX spectra of five samples taken from the different intermediate stages of the reaction. TEM image in Figure 4A shows that the starting material for the growth of multiarmed Se crystals is uniform NPs with an extremely small diameter (∼2 nm). The elemental analysis from EDX spectrum and the SAED pattern (inset of Figure 4A) indicate that these NPs are cubic CdSe, having a certain amount of L-Cys E

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are amorphous Se (a-Se).16,38 With an extension of the reaction time to 4 h, a few small multiarmed submicrometer crystals could be detected (Figure 4C). EDX spectrum and SAED patterns indicate that they are hexagonal Se (h-Se) crystals. As shown in Figure 4C,D, at this stage, the dispersion is a complicated mixture of nanoscale particles, sub-micrometer spherical colloids, and sub-micrometer Se crystals. Here, it should be mentioned that we observed that the aggregates of CdSe NPs (∼2 nm) were mixed with many nanocrystallites with a larger diameter (>20 nm). The SAED patterns (insets of Figure 4D) taken from these nanocrystallites could be assigned to the hexagonal phase of selenium. These small h-Se nanocrystallites might serve as seeds for the growth of multiarmed Se crystals in the following stage through a solid−solution−solid transformation process (that is, in the present synthetic system, a-Se colloids would dissolve into solution phase gradually because a-Se has a higher free energy (5−10 kJ/mol) relative to h-Se;16 subsequently, Se monomer released from a-Se could be deposited selectively on the surfaces of h-Se nanocrystallites and grew into nonspherical Se crystals). Hence, the formation of these nanocrystallites may be the key point in the transformation from CdSe NPs to multiarmed Se crystals. As presented in the TEM images in Figure 4E,F, when the reaction time was lengthened further to more than 6.5 h, sub-micrometer multiarmed Se crystals were formed continuously and grew gradually into microscale Se crystals at the expense of the dissolution of amorphous Se and the decomposition of remnant CdSe NPs. After about 35 h of storage, the transformation process would be completed: the starting CdSe NPs were depleted, and unusual multiarmed Se crystals were obtained, as shown in Figure 1. In addition, as schemed in Figure 5, some TEM images have suggested that

Figure 4. Typical TEM images of the samples taken from reaction dispersion at different aging times: (A) 0, (B) 2, (C and D) 4, (E) 6.5, and (F) 10 h. Inset of panel A: the SAED pattern of primary L-Cyscapped CdSe NPs. Inset of panel B: the SAED pattern of one amorphous Se particle. Inset of panel D: the SAED patterns of h-Se nanocrystallite (I) and a random aggregate of h-Se nanocrystallites (II). (G) The EDX spectra of the intermediate samples taken at different aging times, confirming the gradual transformation of L-Cyscapped CdSe NPs into pure Se crystals (here, the signals of elemental S could be attributed to the L-Cys molecules on the NP surface).

Figure 5. Schematic illustrations of the stepwise transformation from rod to crosshead, and finally to cross or the direct transformation from rod to cross.

the arms of Se cross may not be formed at the same time and possibly result from the further shape transformation of small Se rods formed at the early stage. In this study, UV−vis spectra (Figure 6A) and XRD patterns (Figure 6B) also were used to follow the accompanying color change and crystallization process in the CdSe NP → Se crystal transition. The absorption spectra in Figure 6A indicate that (i) initial L-Cys-stabilized CdSe NPs show a sharp excitonic peak at 420 nm along with two smaller peaks at 390 and 360 nm, similar to those for the ultrastable (CdSe)33 and (CdSe)34 magic clusters grown selectively in the organic phase.32,33 This suggests that primary CdSe NPs have a narrow size distribution

molecules on their surface (the molar ratio of Se to Cd is close to 1:1; in this case, the total concentration of the remnant L-Cys ligand in reaction solution is about 4 mM). As the reaction proceeds for 2 h, some larger spherical nanoparticles (∼200 nm in diameter) were observed in Figure 4B. EDX spectrum and SAED pattern (inset of Figure 4B) indicate that these large NPs F

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Figure 6. Temporal evolutions of UV−visible absorption spectra (A) and XRD patterns (B) recorded for samples taken at different aging time.

decomposition of CdSe NPs and the subsequent spontaneous growth of Se crystals. Furthermore, the data from the FTIR and NMR measurements (Figures S12 and S13, Supporting Information) also support that primary CdSe NPs in the reaction solution are still capped by the ligand molecules; the ligand nature will influence the stability of primary NPs, and thus their decomposition rates; the precursor ligand may also affect the spontaneous crystallization of the Se monomer formed from the oxidation of the released component (after purified with ethanol, there is still a certain amount of ligand molecules on the NP surface or free in solution, inducing the ligand-surface interactions); meanwhile, EDTA seems to just act as the decomposition reagent of primary CdSe NPs. At last, we further illustrate the chemical transformation mechanism for the CdSe NP → nonspherical Se crystal transition, based on the detailed investigations described above. On the one hand, although the solubility product constant Ksp of CdSe (25 °C) is as low as 10−33,9,36 EDTA is a strong coordination agent that can coordinate with Cd2+ ions to form a stable complex because of the high formation constant (Kform, equal to 1016.4).36 Thus, the added EDTA may exchange partly surface ligand molecules of CdSe NPs, and bind strongly with the surface Cd2+ to form water-soluble Cd(EDTA)2− complex; as a result, Se2− anions are released gradually into solution (eq 1).9,10 On the other hand, importantly, Se2− anions released from CdSe NPs can be easily oxidized into Se by dissolved oxygen (eq 2) due to the strongly reducing character. The standard redox potentials for Se2−/Se and O2/OH− pairs are −0.924 and 0.401 V, respectively.36 This redox reaction will stimulate greatly Se2− anions release into solution.9,10 In addition, in the control experiments, we also observe that (i) in the absence of EDTA, the colloidal solutions of CdSe NPs are stable for months and even years under air in the dark at room temperature; (ii) in N2 gas, CdSe NPs remain unaltered even in the presence of 5 mM of EDTA (data no shown). Therefore, the simultaneous presence of EDTA and dissolved oxygen offers the possibility for the spontaneous transformation of primary CdSe NPs into secondary Se crystals.

and a small average size (∼2 nm in diameter), and even have an unique stable atomic structure with well-defined numbers of constituent atoms; (ii) upon prolonging the aging time, the absorbance of the reaction system at 420 nm decreases, implying that the concentration (or the number, rather than the size in this case) of CdSe NP precursor decreases gradually due to the transformation of CdSe NPs → Se crystals. Analogous phenomena have been observed for other untrasmall NP precursors (not shown). These observations suggest that a part of the NP precursorsthe CdSe NPs close to the water/air interface might decompose preferentially, since the concentration of dissolved oxygen at there should be higher than that in the interior of the reaction solution (it has been proven that the EDTA-induced transformation of CdSe into Se highly depends on the presence of the dissolved oxygen); (iii) when the reaction time was close to 4 h, the dispersion exhibits an enhanced absorption in the range from 450 to 700 nm, compared other intermediate samples, which corresponds to the stage−the sufficient formation of amorphous Se colloids from the decomposition of CdSe NPs. These findings demonstrate that the physical structure and chemical nature of the ligand play critical roles in the selective growth, stability, and decomposition rate of the CdSe NPs.33 Figure 6B shows the XRD patterns obtained from the corresponding intermediate samples. XRD pattern of primary NPs (black solid line) matches approximately cubic CdSe.31−33 The full width at half-maximum of the (111) peak is broad, the diffraction peaks of the (220) and (311) planes between 40° and 50° merge into a broad peak, and the intensities of the diffraction peaks are low, which all suggest that primary L-Cyscapped CdSe NPs have an ultrasmall size.31−33 XRD pattern (blue solid line) indicates that at the early stage (∼2 h) of the synthetic process, CdSe NPs decomposed due to the presence of EDTA and transformed into amorphous Se in air.9,10,16,38 Subsequently, a-Se began to crystallize spontaneously in our synthetic systems. As a result, the corresponding diffraction peaks of h-Se crystals appear (green solid line).16 With further prolonging of the aging time, the intensities of the diffraction peaks of h-Se increase gradually (dark yellow and red solid lines). Finally (∼35 h, purple solid line), highly pure Se crystals with single crystal hexagonal structure were obtained. The combined characterizations described above have confirmed the transformation process of CdSe NPs → Se crystals−primary CdSe NPs → metastable a-Se spheres → stable nonspherical h-Se crystals. In other words, the transition of primary CdSe NPs into nonspherical Se crystals could be divided into two sequential steps: the EDTA-induced

CdSe(NP) (ligand) + EDTA4 − → Cd(EDTA)2 − + Se 2 − + ligand

2Se 2 − + O2 + 2H 2O → 2Se( ↓ ) + 4OH− G

(1) (2)

DOI: 10.1021/cg5012007 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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CONCLUSION In summary, nonspherical Se crystals have been synthesized by using CdSe NPs as the precursor, including nanowires, micro(nano)scale rods, crossheads, crosses, and other unusual highly anisotropic structures. The ligand nature of the NP precursor was observed to dominate the release rate of molecular components and the shape and aspect ratio of the as-prepared Se crystals since it influences the properties of primary NPs and the crystallization of Se monomer, while the concentrations of EDTA and NP precursor only influence the size of Se crystals. Meanwhile, we revealed further the crystal structure of nonspherical Se crystals, as well as the unique transformation process from primary CdSe NPs to amorphous Se, and finally to unusual nonspherical Se crystals. Here, the growth of nonspherical Se crystals spontaneously proceeds under ambient conditions, depending on its intrinsic anisotropic nature and the synthetic variables without using any externally added force. Hence, the present route for the synthesis of these nonspherical Se crystals is facile. Furthermore, these unusual Se structures might initiate novel unique application for hexagonal Se, especially as micro(nano)electronic interconnecting devices. And the findings might be helpful for understanding how to fabricate nonspherical, branched, or noncentrosymmetric crystals from nanoscale particles.

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ASSOCIATED CONTENT

S Supporting Information *

Additional TEM and SEM images and other spectra of primary CdSe NPs and produced secondary Se crystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(J.Y.) E-mail: [email protected]. *(D.D.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (Nos. 20275016 and 81371627) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars.

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DOI: 10.1021/cg5012007 Cryst. Growth Des. XXXX, XXX, XXX−XXX