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Jul 8, 2015 - (NRF) grants funded by the Korean government (Grant Nos. .... (25) Owen, J. S.; Park, J.; Trudeau, P.-E.; Alivisatos, A. P. Reaction. Ch...
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Direct Cd-to-Pb Exchange of CdSe Nanorods into PbSe/CdSe Axial Heterojunction Nanorods Dongkyu Lee,† Whi Dong Kim,† Seokwon Lee,† Wan Ki Bae,*,‡ Sangheon Lee,*,§ and Doh C. Lee*,† †

Department of Chemical and Biomolecular Engineering (BK21+ Program), KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-338, Korea ‡ Photo-Electronic Hybrids Research Center and §Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea S Supporting Information *

ABSTRACT: We report synthesis of PbSe nanorods (NRs) and PbSe/ CdSe axial heterojunction NRs via direct Cd-to-Pb cation exchange in CdSe NRs. Use of suited ligand−cation combinations enables the cation exchange while keeping the nanomaterial morphology intact. For example, solvation of Cd2+ using oleylamine (OLA) allows for the cation exchange process, which would not be possible by using oleic acid instead of OLA. A mild cation exchange process, such as mixing Pb-oleate and OLA with CdSe NRs at 130 or 150 °C, results in anisotropic replacement of CdSe into PbSe along the ⟨0001⟩ direction of wurtzite CdSe, and a partial conversion leads to the formation of heterostructure NRs containing axial CdSe/PbSe heterojunctions. While the cation exchange proceeds at both tips of CdSe NRs, exchange appears to be faster on (0001̅) planes. Binding energy calculation based on density functional theory reveals that OLA binds strongly to the (0001̅) facet of CdSe NRs, leading to asymmetric cation exchange. This protocol to convert CdSe nanocrystals directly into PbSe broadens the design range of CdSe/PbSe heterojunction nanomaterials potentially with various morphologies because template CdSe nanocrystals can be prepared in different shapes via colloidal synthesis.

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INTRODUCTION Colloidal growth of inorganic nanocrystals (NCs) via arrested precipitation has proven to be versatile and powerful, yielding size- and shape-controlled NCs with atomic precision.1−4 More recently, progress in the synthesis of colloidal NCs has expanded from the growth of single-component materials to the design of heterostructure NCs (HNCs), whose properties can be exquisitely tuned with combinations and dimensions of the heterostructures. The staple of the colloidal synthesis of HNCs is to find the chemistry best suited for the growth of desired crystal components. For example, to prepare a CdSe/ CdS dot-in-rod nanostructure, CdS shell needs to grow on a CdSe core in a way that the shell grows into a nanorod shape via a suitable growth chemistry.5 Over the years, new precursors have been developed for the synthesis of HNCs composed of different material combinations. Cation exchange, a phenomenon where cations in an inorganic solid are replaced with another type of cation, is an example of a powerful tool to design HNCs. This solid-state reaction draws much attention particularly because design of complex HNCs is possible when anisotropic seed nanoparticles are available, and a relatively mild exchange process keeps the shape anisotropy intact.6 The versatility of cation exchange has motivated the intensive study of its molecular-level mechanism for the process.7 High surface area of NCs reduces subreaction © 2015 American Chemical Society

activation barriers; therefore, the synthesis of unique metastable products becomes possible.8 The low activation barrier also enables completely reversible cation exchange,9−11 which suggests that intermediate species during cation exchange reactions are important. Base, or lack thereof, has been reported to play a critical role in cation exchange. For instance, conversion from a divalent cation such as Cd2+ to monovalent Cu+ or Ag+ proceeds rapidly in the presence of hard base such as methanol, which helps solvate hard divalent cations better.11,12 Conversely, the reverse reaction is stimulated by soft basic surfactants, such as tributylphosphine, which successfully solvate the soft monovalent cations. From the atomic viewpoints, the vacancies or halides on the NC surface act as active sites where the conversion process is considerably faster.13 Understanding cation exchange on a molecular level should allow the protocol to be applied in various heterojunction NCs, with one notable example being CdSe/PbSe, where the cation exchange can result in type-II heterostructures with energy band levels of staggering offset. Open type-II heterostructure nanorods (NRs) based on CdSe and PbSe with both Received: April 26, 2015 Revised: July 8, 2015 Published: July 8, 2015 5295

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Figure 1. Representative TEM images of CdSe NRs and converted NCs of (a−c) NR2 or (d−f) NR9. Starting from (c,d) CdSe NRs seeds, NCs were reacted with either (a,b) PbCl2−OLA complex at 100 °C or (e,f) Pb-oleate-OLA at 150 °C. In all cases, reaction lasted for 1 h.

the NRs are cylindrical, volume of the NRs reduces from ∼731 nm3 to 230 nm3 for NR2 and ∼1936 nm3 to 195 nm3 for NR9. This implies that NRs undergo significant shape transition via etching or ripening. However, the increased diameter of PbSe NCs obtained from the NR9 sample indicates that there is more than a simple etching/ripening process that is responsible for the transformation. Chlorine could be held accountable for the transformation, as the halogen molecules can replace surface alkylphosphonic acids on the surface of CdSe NCs and leave chlorine on the NC surface.25 In a recent study, it was reported that metal chloride can induce surface etching of CdSe NRs to detach phosphonates on the nanocrystal surface.26 Therefore, we speculate that the cation exchange proceeds toward both unpassivated end and side facets, which results in unstable wurtzite(w)-CdSe/rock-salt(rs)-PbSe heterojunctions in sideplanes with severe lattice mismatch.27 The lattice mismatch causes instability of the interfaces leading to morphology change for the nanostructures to lower overall interface and surface energy. On the other hand, it is also possible that the difference in diffusivity of Cd2+ and Pb2+ may lead to the formation of voids within NRs during cation exchange, causing NRs to break up into smaller nanoparticles, a process similar to a previously reported Ti-to-Cu cation exchange.28 Large surface area of the exposed side planes in NRs could facilitate the void formation during the reaction. Since chlorine is considered to help break the anisotropy of CdSe NRs during the cation exchange, we reasoned that chlorine-free Pb precursor should be examined. When Pboleate, a widely used precursor in colloidal Pb chalcogenide NC synthesis, is used alone,29 no sign of cation exchange is observed: the 1S peak from the absorption spectra of CdSe NCs does not shift (Figure S2). The inactivity of Pb-oleate on this particular cation exchange can be understood in the context of the hard−soft acid−base (HSAB) theory.30 The HSAB theory tabulates the interactions of molecules or ions within solvents of varying polarizability and chemical hardness. Validity of the theory has been examined for numerous cation exchange reactions.7,31

components exposed to the external environment enable facile carrier extraction, rendering the nanostructures suitable for the study of exciton dynamics−photocatalysis relationship.14−17 While many groups have reported direct pathways for complete or partial Pb-to-Cd cation exchange,18−21 Cd-to-Pb conversion has been possible mostly by a multistep process via monovalent cation intermediates (e.g., Cu+).11 So far, direct Cd-to-Pb cation exchange has been achieved by using Pb(CH3COO)2 aqueous solution to synthesize CdS/PbS core/shell nanoparticles22 or by using PbCl2-oleylamine (OLA) in an effort to prepare isotropic PbSe NCs with superior stability. 23 A noncolloidal approach based on evaporation-induced cation exchange was reported to produce PbSe-CdSe nanodumbbells.24 However, to the best of our knowledge, no colloidal approach has been available for partial cation exchange of CdSe into PbSe in anisotropic nanomaterials. In this article, we examine ligand−cation interactions and their consequences in the resulting product from cation exchange reactions in CdSe NRs. Our primary focus lies on the morphology changes during the cation exchange process because heterostructure NRs with controlled dimension are possible only when the morphology can be controlled during the cation exchange reaction. Furthermore, density functional theory (DFT) calculations are carried out to elucidate how the ligand−cation interactions affect anisotropy and asymmetry of cation exchange.

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RESULTS AND DISCUSSION Figure 1 shows transmission electron microscopy (TEM) images of products after mixing CdSe NRs with PbCl2−OLA or Pb-oleate-OLA at elevated temperatures. When reacted with PbCl2−OLA, CdSe NRs appear to turn into spherical PbSe NCs for both NR samples with aspect ratios of ∼2 (denoted as NR2) and ∼9 (denoted as NR9). As shown in histograms of Figure S1 of the Supporting Information, the average diameter and length of the initial CdSe NRs are 7.7 ± 0.7 nm and 15.7 ± 2.1 nm for NR2, and 6.6 ± 0.6 nm and 56.6 ± 5.6 nm for NR9, while the diameters of PbSe NCs converted from NR2 and NR9 are 7.6 ± 0.9 nm and 7.2 ± 0.9 nm, respectively. Assuming 5296

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Scheme 1. Cd-to-Pb Cation Exchange When (a) PbCl2−OLA, (b) Pb-Oleate, or (c) Pb-Oleate-OLA Is Used as Pb Source

Figure 2. (a) XRD patterns of nanoparticles produced at 100 °C using PbCl2−OLA. (b,c) XRD patterns of (b) CdSe NR9 or (c) NR2 and their cation exchange products at various temperatures using Pb-oleate and OLA as precursors. Reaction duration was 1 h for all cases. Using Pb-oleate and OLA, partially exchanged NCs are formed at lower temperatures (100 or 130 °C), while higher temperature (150 °C) leads to full conversion of w-CdSe into r-PbSe. PbCl2−OLA complex evoked complete exchange even at low temperature. Solid vertical lines located above and below the XRD patterns of nanoparticles represent the standard patterns from the Joint Committee on Powder Diffraction Standards (JCPDS) for rs-PbSe (JCPDS file No. 06−0354) and w-CdSe (JCPDS file No. 08−0459), respectively.

°C,29,32 while the complete formation of Cd-oleate (hard−soft) requires much higher temperature of ∼300 °C.33 Therefore, Pb-oleate by itself cannot drive enough Cd-to-Pb cation exchange because oleic acid is less capable of solvating Cd2+, which diffuses out of NRs during the conversion. Because OLA can form strong hard−hard interaction with Cd2+, additional OLA in the mixture of CdSe NCs and Pb-oleate can provide a

Scheme 1 summarizes how the HSAB theory explains Cd-toPb conversion in our systems. Since Cd2+ and OLA are harder acid−base combination than Pb2+ and oleic acid, Cd-OLA or Pb-oleate is more stable than Pb-OLA or Cd-oleate complexes. Typical metal−ligand complex formation procedures manifest the comparative magnitude of the interactions; for example, transparent Pb-oleate (soft−soft) complex is formed at ∼150 5297

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6.2 nm in size, which are in close proximity to average diameters of NRs measured from TEM analysis. The HSAB theory predicts that unstable Pb-OLA bonding in PbCl2−OLA is readily cleaved, while stable Cd-OLA bonding forms during the cation exchange reaction in the presence of PbCl2−OLA. The large difference in solvation energy drives the complete conversion even at a lower temperature.23 However, the cation exchange using Pb-oleate is unfavorable since the bonding of Pb-oleate is more stable than that of Cd-oleate. Under this circumstance, OLA gives auxiliary thermodynamic driving force by forming stable bonding with Cd2+. Since electrostatic hard−hard interactions (e.g., Cd-OLA) are more energetically favorable than the π-bonding soft−soft combinations (e.g., Pb-oleate),30 OLA can act as a “chemical switch” in this cation exchange process. The role of OLA is similar to TBP: it promotes the cation exchange in Cu2S NCs by solvating soft Cu+ ions.11 The small difference in bonding energies between soft−soft Pb-oleate interaction and hard− hard Cd-OLA interaction makes the cation exchange slower than the case of conversion using PbCl2−OLA, enabling facile synthesis of HNCs. The moderate driving force may hamper most of unfavorable cation exchange pathways, resulting in exclusive formation of the most stable interface between wCdSe and rs-PbSe. The w-CdSe/rs-PbSe interface with a very small lattice mismatch also helps keeping the NR shape relatively intact. The two cation exchange reaction pathways with different Pb precursors are summarized in Scheme 2.

driving force for the cation exchange. In our cation exchange reaction, the injection of OLA seems to catalyze cation exchange, which is noticed by color change from red−brown to dark-brown and evidenced in increased absorption in the infrared range (Figure S2a). X-ray diffraction (XRD) patterns (Figure S2b) also show that in the presence of OLA, mixture of Pb-oleate and zinc blende(zb)-CdSe NCs results in complete transformation into rs-PbSe NCs. Morphology of the products from cation exchange using Pboleate-OLA complex provides a hint for the impact of precursor−cation interactions on the shape of NRs. With Pboleate-OLA present in the reaction at 150 °C, the resulting PbSe NCs appear to retain the original anisotropy of the respective CdSe NR seeds, as shown in Figure 1, panels e and f, whereas the NR seeds turn into isotropic PbSe particles when PbCl2 is used as a Pb source even at lower temperature, for example, 100 °C. While NRs converted at 150 °C using Pboleate-OLA partially retain their original anisotropy, reactions at a lower temperature result in partial conversion with NR shape remarkably intact (Figure S3). Histrograms in Figure S4 show quantitative change of the morphology during the cation exchange. Upon reaction at 150 °C, the aspect ratio of NR2 decreases from 2 to 1.6, whereas the NR9 sample shows more drastic change in both diameter and length: ∼20% decrease in each dimension from the original CdSe NR seeds. In contrast, NRs reacted at 100 °C maintained their original morphology. More drastic degradation of NR9 compared to NR2 products may be due to stacking faults more frequently observed in CdSe NRs with higher aspect ratio.34 These stacking faults provide defect sites such as corners, edges, or kinks on side-walls of CdSe NRs, making the side-walls of CdSe NRs more reactive because of their low coordination number. As was the case for chlorine-containing Pb precursors, unstable w-CdSe/rs-PbSe interfaces form and ultimately cause the breakdown of original rod-shape. Figure 2 shows XRD patterns of CdSe NR samples before and after reacting CdSe NR solution with PbCl2−OLA or Pboleate-OLA at various temperatures. When reacted in solution containing Pb-oleate and OLA at 150 °C, w-CdSe NRs turn into rs-PbSe nearly completely in 1 h. Partial exchange can be observed at a lower reaction temperature, as evidenced in XRD peaks indexed to be from both CdSe and PbSe crystal structures. XRD analysis reveals that CdSe NRs reacted with Pb-oleate-OLA convert to PbSe gradually more with increasing temperature, but not completely at 100 or 130 °C. On the other hand, when PbCl2−OLA is used, the reaction appears to yield complete conversion into PbSe even at a temperature as low as 100 °C. The relative Pb compositions were identified by inductively coupled plasma-mass spectrometer (ICP-MS) measurements, which show the similar trend with XRD patterns of increasing Pb signal as reaction temperature increases (Figure S5). TEM images (Figure 1), XRD patterns (Figure 2), and absorption spectra (Figure S6) of NR2, NR9, and their cation exchange derivatives altogether point to the fact that, with increasing reaction temperature, rs-PbSe portion grows: the first excitonic peak of CdSe NRs becomes smaller, while absorption in the infrared (IR) range increases. The 1S peak of CdSe NRs is positioned at ∼640 and 630 nm for NR2 and NR9, respectively. According to empirical fitting functions that correlate 1S peak position and size of quantum confined nanostructures,35 the 1S peak positions correspond to 7.3 and

Scheme 2. Schematic Illustration of Cation Exchange Reaction Pathways When Two Different Pb Precursors Are Used. Red and Black Segments Represent CdSe and PbSe, Respectively

Figure 3 shows high-angle annular dark field-scanning TEM (HAADF-STEM) images of CdSe NRs partially cationexchanged into PbSe by mixing with Pb-oleate-OLA. Elemental mapping using a STEM mode indicates that the interfaces are formed parallel to the (0001) plane of w-CdSe, and cation exchange initiates at tips of the NRs. As the {111} facets of rsPbSe form nearly strain-free interface with {0001} facet of wCdSe (the {0001} plane of hcp corresponds to {111} plane of fcc36), we speculate that the cation exchange proceeds anisotropically in ⟨111⟩PbSe or ⟨0001⟩CdSe direction forming {111}PbSe/{0001}CdSe interfaces. This presumption is in agreement with a previous report that rs-PbSe/zb-CdSe HNCs exhibit sharp interfaces of constituted {111}PbSe/{111}CdSe,27 5298

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Figure 3. Representative (a,d) HAADF STEM and (b,c; e,f) elemental mapping images of (a−c) partially converted NR2 and (d−f) NR9. Green, orange, and sky blue dots represent energy dispersive spectroscopy (EDS) signals from cadmium, lead, and selenium, respectively. In both NR2 and NR9 cases, cation exchange reaction was performed using Pb-oleate-OLA at 130 °C for 1 h.

Figure 4. HAADF images of partially converted NR2 using Pb-oleate-OLA at 130 °C for 1 h. Heterostructure NC shows either only one tip converted into PbSe or both tips converted with different extent of conversion.

and rs-PbSe/w-CdSe heterogeneous nanodumbbells feature {111}PbSe/{0001}CdSe heterojunction.24 The lattice strain of the {111}PbSe/{0001}CdSe interface can be predicted based on a lattice mismatch at bulk (m), which is defined as absolute difference between two lattice spacing values (d1 and d2) relative to the average of the lattice spacings:37 m=

with a low-angle X-ray incident beam. Therefore, a stronger (111)PbSe peak of cation exchanged nanoparticles indicates that the cation exchange proceeds along the ⟨111⟩PbSe or ⟨0001⟩CdSe direction, which was visually evidenced in STEM images in Figure 3. The tendency of enhanced XRD peak intensity along growth direction coincides with a previous report, in which ⟨200⟩-grown PbSe NRs resulted in stronger (200)PbSe peak intensity than typical PbSe NCs.39 Figure 4 shows additional high-resolution HAADF images with either only one or both ends of CdSe NRs exchanged to PbSe. Two end facets in a NR show significant difference in the extent of conversion, indicating that this Cd-to-Pb cation exchange proceeds more favorably at one end than the other of a NR. The asymmetry in cation exchange has previously been observed in Cd-to-Cu conversion for CdS NRs,40 resulting from lack of inversion symmetry about the c axis in CdS wurtzite lattice; namely, the (0001) and (0001̅) end facets of the NRs have different atomic population, hence different surface energies.41 Likewise, in our case, the facet-selectivity of Cd-to Pb cation exchange would be a consequence of crystallographic nonequivalence of (0001) and (0001̅) polar end facets of w-CdSe NRs.42 The (0001̅) facet in CdSe NRs is known to be more reactive than the (0001) plane, since Cd termination leads to three dangling bonds per Cd atom.41 Therefore, the formation of {111}PbSe/{0001}CdSe interface at (0001̅) end facet of the nanorods is thermodynamically more favorable, as it displaces a high-energy Cd-terminated surface into a low-energy CdSe/PbSe interface.

|d1 − d 2| 0.5 × (d1 + d 2)

On the basis of the lattice spacing values from JCPDS files, the lattice mismatch at the {111}PbSe/{0001}CdSe interface is evaluated to be only 0.74%, hinting the stability of this particular epitaxial interface. Therefore, it is reasonable to assume that the direct Cd-to-Pb cation exchange occurs preferentially along the ⟨111⟩PbSe direction. The fact that cation exchange proceeds along the ⟨111⟩PbSe direction is also corroborated by XRD analysis: the (111)PbSe/ (200)PbSe ratio in the XRD patterns of cation exchanged NRs is relatively higher than that of PbSe NCs and NRs prepared directly via arrested precipitation method. The ratio of (111)PbSe/(200)PbSe was ∼0.35 for both NR2 and NR9 cases, which show a higher relative (111)PbSe peak intensity than that of ∼0.2 or lower from typical PbSe NCs38 or PbSe NRs.39 Since NRs usually align parallel to the substrate, as shown in TEM images (Figure 1), more lattice planes normal to cation exchange direction will fulfill Bragg condition for diffraction 5299

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Figure 5. Calculated binding energies (Ebinding) of MPA, AA, and MA on the Cd-terminated (0001) and Se-terminated (0001̅) polar end facets for (a) w-CdSe and (b) rs-PbSe/w-CdSe, where the topmost Cd layer is replaced by the Pb layer. The inset shows the lowest binding energy geometries of MA on the Se-terminated (0001̅) polar end facet. Purple, gray, green, brown, blue, and white spheres represent Cd, Pb, Se, C, N, and H atoms, respectively.

likely to stay passivated with alkylphosphonic acid. On the other hand, in the presence of OA, both polar surfaces are likely to be blocked by alkylphosphonic acid due to the low binding strength of OA relative to MPA. The rs-PbSe/w-CdSe case exhibits a similar trend in the order of binding affinities to the w-CdSe case. In particular, the MA binding strength to the (0001̅) surface is higher than the MPA and AA binding strength by 0.58 and 0.73 eV, respectively, whereas the MA binding strength to the (0001) surface is 0.49 eV lower than the MPA binding strength. These results suggest that for the (0001̅) facet, the initial cation exchange becomes thermodynamically more favorable in the presence of OLA rather than in the presence of alkylphosphonic acid and OA, whereas for the (0001) facet, alkylphosphonic acid is more likely to initiate cation exchange than OA and OLA. This investigation of ligand−surface interactions gives immediate insight into the role of OLA on the facet-selective cation exchange, which was manifested in experimental observations. In the presence of OLA, the (0001)̅ polar facet of a w-CdSe NR is primarily covered by OLA, as shown in the inset in Figure 5, panel a, while the (0001) polar facet of a wCdSe NR is primarily covered by alkylphosphonic acid. Since the binding strength of MPA on the (0001) facet is comparable to that of MA on the (0001̅) facet for the rs-PbSe/w-CdSe case, one may suggest that the cation exchange should occur on both (0001) and (0001̅) facets in the presence of OLA (driven by the HSAB theory), which contradicts our experimental observation of the anisotropic cation exchange rates of the two polar facets. The steric hindrance effect can account for this possible contradiction. As proven in recent experimental and

Both STEM images and elemental mapping analysis in Figure 3 reveal that Pb exists at the end and sidewalls of NR9. In the case of NR2, ∼70% have only one side of NRs converted with Pb (Figure S7). The difference in asymmetry between NR2 and NR9 can be explained in a similar manner with Cd-toCu conversion.40 With high fraction of curved interfaces, NR9 exposes reactive {101̅1}-type facets than NR2, instead of {0001} surfaces. Since {101̅1}-type facets lacks asymmetry in reactivity, NR9 shows less asymmetry in cation exchange reaction than NR2. Since OLA plays an important role in Cd-to-Pb cation exchange, the difference in accessibility of OLA to either tip of CdSe NRs is also likely responsible for the difference in reaction rates in either side. Figure 5 exhibits calculated binding energies of methyl phosphonic acid (MPA), acetic acid (AA), and methyl amine (MA) on (0001) and (0001̅) polar end-facets of (a) w-CdSe NRs and (b) rs-PbSe/w-CdSe. Here, 75% of Cd atoms and 25% of Se atoms are passivated by the ligands for the (0001) and (0001̅) surfaces, respectively, so that overall electron counting rules are satisfied.43,44 The binding energy (Ebinding) of a species M on a surface is determined via n × Ebinding = En×M* − (Esurf + n × EM), where En×M* and Esurf are the total energies of the surface with and without n adsorbed M molecules, respectively, and EM is the total energy of the species M in the vacuum. In the case of a w-CdSe NR, the order of binding affinities is AA < MA < MPA on the (0001) surface and is AA < MPA < MA on the (0001̅) surface. These results indicate that OLA molecules are favored to adsorb on the (0001̅) surface by replacing alkylphosphonic acid molecules, whereas the (0001) surface is 5300

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Chemistry of Materials theoretical studies,34,36,42 the packing density of alkylphosphonic acid on the (0001) facet is as high as 75% of surface Cd atoms. Considering that the alkyl chains are long and the repulsive interaction between the alkyl chains is thus significant, we anticipate that the cation exchange on the (0001) facet can be severely hindered by alkylphosphonic acid, although they can be thermodynamically allowed. In contrast, less severe steric hindrance is expected on the (0001̅) facet, where the packing density of alkylphosphonic acid is as low as 25% of surface Se atoms. Our experimental and theoretical analysis suggests that the cation exchange indeed proceeds anisotropically, more readily on the (0001̅) facet, and both thermodynamic effects from the ligand−surface interaction and kinetic effects from the steric hindrance play a key role in the facetselective cation exchange of w-CdSe NR to rs-PbSe/w-CdSe. The strong affinity of OLA to the (0001̅) polar facets of the w-CdSe and rs-PbSe/w-CdSe NRs also accounts for the observed facet-selectivity of Cd-to-Pb cation exchange between the (0001) and (0001̅) end facets. A recent experimental study reported that Lewis bases, such as alcohols and amines, displace Cd-oleate and Pb-oleate from the surface of CdSe and PbSe NCs.45 Likewise, OLA molecules can displace Cd-oleate on the (0001) and (0001̅) facets of CdSe NRs to create surface cation vacancies and activate the surfaces toward the cation exchange moves. Figure S8 shows Fourier transform (FT)-IR spectra of CdSe NR2 before and after the cation exchange with and without additional OLA. While phosphonate peaks remain intact, carboxylates signals are weaker when OLA is added than the case without OLA. The diminished carboxylate peaks suggest the displacement of surface Cd-oleate by OLA. On the basis of the theoretical calculations in which the binding strength of acetic acid is much weaker than MPA or MA, it is likely that oleate binds to unpassivated sites rather than displacing bound phosphonate. The absence of N−H signals implies that adsorbed OLA would be detached in the form of Cd-OLA and solvate Cd2+ during cation exchange reaction. A previous computational study reveals that the polar (0001) end facet of w-CdSe shows higher alkyl phosphonate coverage than (0001̅), causing more oleic acids to bind to w-CdSe NRs preferentially on an unpassivated (0001̅) facet.41 Therefore, we expect that the (0001)̅ facet is more prone to the surface activation process than the (0001) facet because more Cdoleates are exposed to the OLA molecules. In addition, removal of each Cd-oleate from the (0001) facet requires cleaving three Cd−Se bondings as opposed to only one Cd−Se bonding cleaved in the (0001̅) case. Given these theoretical backgrounds, the asymmetry observed in TEM images in Figure 4 derives from the selective affinity of OLA to the (0001̅) polar facet, on which the Cd-to-Pb cation exchange occurs more preferentially than on the (0001) facet.

dimensions. Analysis on structures and morphologies of resulting nanostructures reveals that under a modest reaction condition, for example, in the presence of OLA and absence of chlorine-containing precursors, the axial diffusion of Pb2+ within CdSe NRs occurs along the direction of ⟨0001⟩ of wCdSe. It turns out that the exchange reaction rate at two end facets of w-CdSe NRs (i.e., (0001) and (0001̅) facets) is notably different. DFT calculations corroborate the asymmetry of reactivity in CdSe NRs: the ligand binding energies are significantly different between the (0001) and (0001̅) facets. Both experimental and theoretical results indicate that alkylphosphonic acids on NR surfaces act as a protective layer against the adsorption of OLA on the (0001) facet and diffusion of Pb2+ source. The HSAB theory is an underlying signature that has the potential to provide recipe for numerous anisotropic HNCs. The successful exploitation of the theory will allow for a fine control in an atomic precision in cation exchange reactions and help capitalize on the versatility of colloidal synthesis of HNCs. The theory can easily expand the library of available combinations of HNC components, and also the shape control is possible when the information on ligands and precursors is available. Anisotropic nanostructures with reduced overlap integral of electron and hole wave functions are essentially “open” HNCs with photogenerated electrons and holes readily accessible on the surface. The ultimate test for the benefits of open HNCs will be in the performance of photocatalytic reactions: for more efficient photochemical conversion, electron−hole recombination needs to be minimized while the charge carriers are separated on different parts of a photocatalyst HNC.14

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EXPERIMENTAL SECTION

Chemicals. Cadmium oxide (CdO, ≥99.99%), lead(II) oxide (PbO, 99.999%), lead chloride (PbCl2, 98.0%), selenium powder (Se, 99.99%), 1-octadecene (ODE, 90%), oleylamine (OLA, 70%), oleic acid (OA, 90%), trioctylphosphine (TOP, 97%), trioctylphosphine oxide (TOPO, tech. grade, 90%), and anhydrous toluene (99.8%) were purchased from Sigma-Aldrich. 1-Tetradecylphosphonic acid (TDPA, 98%), n-hexylphosphonic acid (HPA), and n-octadecylphosphonic acid, (ODPA, 97%) were purchased from Alfa Aesar. Tributylphosphine (TBP, ≥93%) was purchased from Wako. Tetrachloroethylene (TCE, 98.0%), n-hexane (95.0%), acetone (99.5%), ethanol (99.5%), methanol (99.5%), chloroform (99.5%), and toluene (99.5%) were purchased from Samchun Chemicals. Characterization. TEM images were acquired using a Tecnai TF30 ST operated at 300 kV. High-resolution TEM (HR-TEM) and HAADF-STEM images were collected using a Titan G2 60−300 operated at 200 kV with two Cs-correctors, in combination with a Super-X EDS detector. TEM samples were prepared by dropcasting hexane solution of NCs onto square mesh copper grids. The grids were dried under ambient condition, followed by being dipped into methanol, which helps cleanse residual organic materials before TEM imaging. UV−vis spectra were collected by a Shimadzu UV 3600 spectrometer. XRD analyses were performed using a RIGAKU Ultima IV at 40 kV and 40 mA with Cu Kα radiation (λ = 1.5406 Å). XRD samples were prepared by dropcasting NC solution in hexane onto glass or Si substrates. ICPMS analysis was performed using an Agilent ICP-MS 7700S. Synthesis of CdSe NCs. Colloidal CdSe NCs were synthesized using a recipe modified from a previously published method.46 CdO (0.51 g, 4.0 mmol), OA (6 mL), and ODE (70 mL) were degassed in vacuum at 150 °C for 90 min, and then it was heated to 300 °C under Ar flow until the mixture became transparent. Se injection solution was prepared by mixing Se (0.25 g), TOP (1.5 mL), and ODE (12.5 mL). The Se precursor was added via dropwise injection with rate of 0.6

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CONCLUSION Direct exchange of Cd2+ of CdSe NRs into Pb2+ is possible when a proper precursor−ligand combination is used for the cation exchange reaction. For example, Pb-oleate by itself does not react with CdSe to yield PbSe within 1 h of reaction at 150 °C, while in the presence of OLA, Pb-oleate reacts to turn CdSe into PbSe at the identical reaction conditions. OLA favorably solvates byproduct Cd2+, which provides thermodynamic driving force to break the weaker Pb-oleate bond and evokes the cation exchange reaction with moderate kinetics. The extent of the cation exchange can be controlled, thus yielding PbSe/CdSe axial heterojunction NRs with variable 5301

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Chemistry of Materials mL/min for 20 min at 240 °C (total 12 mL). Ten min after the injection, the reaction was quenched with ice bath, followed by ethanol-induced precipitation. Collected NCs were washed three more times with toluene and ethanol. The CdSe NCs were redispersed in ODE and stored in an Ar-filled glovebox. Synthesis of CdSe NRs. Colloidal CdSe NRs were prepared via a recipe modified from a method by Wang et al.47 CdO (0.2054 g), ODPA (1.1034 g), and TOPO (2.8966 g) for NR2 or HPA (0.1399 g), TDPA (0.6817 g), and TOPO (3.1784 g) for NR9, were loaded into the reaction flask (50 mL). The flask was degassed in vacuum at 120 °C for 1 h, and then it was heated to 320 °C under Ar blanket until the solution becomes transparent. The transparent solution was cooled to room temperature, and then aging process was performed for at least 24 h under Ar condition. Afterward, the reaction vessel was degassed once more in vacuum at 120 °C for 1 h prior to injection of Se precursor. In an Ar-filled glovebox, Se injection solution was prepared by mixing Se-TBP (0.8 mmol of Se with 0.19 g of TBP), TOP (1.447 g), and anhydrous toluene (0.3 g). The solution was injected into reaction flask at 320 °C. Then, the particles were grown at 300 °C for 8 min followed by cooling to room temperature in the ambient environment. Chloroform (5 mL) was injected into the vessel at ∼60 °C. NCs were collected by methanol-induced precipitation with centrifugation and decantation. The particles were dispersed in toluene and centrifuged to remove residual TOPO. The NR solution was further purified with methanol. The NRs were stored in ODE for cation exchange reaction. Cation Exchange with PbCl2−OLA Complex. We performed cation exchange using PbCl2−OLA complex as Pb source based on a previously reported recipe.23 Briefly, a mixture of PbCl2 (0.0695 g, 0.25 mmol), OLA (1 mL), and ODE (2.5 mL) in a reaction vessel (50 mL) was degassed under vacuum at 100 °C for 1 h, followed by additional heating for 10 min under Ar at 100 °C, which resulted in white slurry solution. CdSe NR (5 mg) solution in 2.5 mL of degassed ODE was prepared in an Ar-filled glovebox. The solution was injected into the vessel, and the reaction was quenched after 1 h by cooling, followed by injection of 5 mL of hexane and 5 mL of OA at 70 and 40 °C, respectively. Resultant NCs were purified with ethanol and redispersed in hexane, followed by additional centrifugation to remove unreacted PbCl2. The NCs were washed twice more using hexane and ethanol, followed by redispersion in hexane or TCE for further characterization. Cation Exchange Using Pb-Oleate and OLA (Pb-OleateOLA). PbO (0.056 g, 0.25 mmol), OA (0.2 mL), and ODE (2.5 mL) in 50 mL flask were degassed under vacuum at 120 °C for 1 h, which resulted in clear and colorless solution. Under Ar, the CdSe QDs or NRs (5 mg) solution in 2.5 mL of degassed ODE and 1 mL of degassed OLA were injected at 100−150 °C. Then, the reaction was quenched after 1 h or later, depending on targeted extent of cation exchange. The crude solution was washed with acetone. Afterward, collected nanoparticles were purified 1−2 times more using acetone and ethanol, and then redispersed in hexane or TCE for characterization. Calculations. We investigated the role of OLA on the facetselective cation exchange via an Ab initio calculation. Figure S9 shows the facet-selective cation exchange viewed as a layer-by-layer exchange process of a series of c-plane CdSe monolayers from one end-facet to the other end-facet, reflecting our experimental observation of {111}PbSe/{0001}CdSe interfaces within CdSe-PbSe binary NRs. Ligands can play two decisive roles for the cation exchange process. First, ligands compete to block surfaces of w-CdSe NR via adsorption before the cation exchange propagates. To evaluate the tendency of site-blocking by a specific ligand, we computed the ligand binding energies on (0001) and (0001̅) polar end-facets of w-CdSe NR. Second, the adsorbed ligands can modulate the thermodynamic driving force of the initial cation exchange process. To get an insight into this thermodynamic effect, we computed the ligand binding energies on the two polar end-facets of rs-PbSe/w-CdSe, where the topmost Cd layer is replaced by the Pb layer. Here, we constrain the initial cation exchange process to the complete cation exchange of the topmost Cd layer to the Pb layer, considering that variations of the

energy cost for cation exchange moves in a typical epitaxial process nearly converge at the second monolayer.48 The stronger ligand− surface binding for the rs-PbSe/w-CdSe case indicates that the initial cation exchange process becomes thermodynamically more facile in the presence of the ligand. The calculations reported herein were performed on the basis of DFT within the Perdew−Burke−Ernzerhof (PBE) functional,49 as implemented in the Vienna Ab-initio Simulation Package (VASP).50 The projector augmented wave (PAW) method with a planewave basis set was employed to describe the interaction between ion cores and valence electrons.51 Surfaces of w-CdSe and rs-PbSe/w-CdSe were modeled as slabs containing six layers of 2 × 2 hexagonal supercell, each of which contains four Cd/Pb atoms and four Se atoms. Surfaces terminated by Cd/Pb and Se atoms, which minimize the surface dangling bonds, were taken as reference surfaces for (0001) and (0001̅) polar facets, respectively. The other side of the slab was passivated by fractional pseudo hydrogen, that is, qH = 1.5 e for each surface Cd dangling bond, and qH = 0.5 e for each surface Se dangling bond. A (4 × 4) Monkhorst−Pack mesh was used to sample the Brillouin zone52 together with a plane-wave energy cutoff of 450 eV. The slab is separated from its periodic images in the vertical direction by a vacuum space of approximately 12 Å. While the bottom two layers of the five-layered slab are fixed at corresponding bulk positions with the predicted lattice constants of a = 4.389 Å and c = 7.152 Å, the upper three layers are fully relaxed using the conjugate gradient method until residual forces on all the constituent atoms become smaller than 5 × 10−2 eV/Å. A cubic supercell of size 12 Å was employed to calculate the energy of the ligand in the gas phase. For computational conciseness, we employed MPA, AA, and MA as ligand models for alkylphosphonic acid, OA, and OLA, respectively. Note that truncation of long alkyl chains down to a methyl group does not alter key interpretations regarding the ligand−surface interactions.41 It is also worth noting that acidic ligands, such as alkylphosphonic acid and OA, are often partially deprotonated and tend to bind in their anionic form (i.e., as phophonates and carboxylates) to excess Cd2+ or Pb2+.20,25,53,54 Partially deprotonated alkylphosphonic acid and OA molecules are charged, and they become tricky to simulate. Taking this into account, we simulated only the simplest case involving passivation by neutral MPA and AA molecules. Nevertheless, we believe that the protonated cases can be a reasonable approximation considering that the calculation results based on the neutral MPA are able to describe anisotropic growth of CdSe NRs.41 In this work, the strong affinity between Cd2+/Pb2+ and alkylphosphonic acid (protonated or deprotonated) and that between Se2− and OLA are identified as a key to interpreting the role of OLA in the facet-selective Cd-to-Pb cation exchange, and these trends are not likely to be switched as a result of the deprotonation of the acidic ligands. In a real growth environment, the charged, deprotonated ligands might also bind to other molecules in the liquid solution, which might mitigate its charging effects and make its passivating ability similar to that of the neutral, protonated ligand.41

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ASSOCIATED CONTENT

S Supporting Information *

Histograms of CdSe NRs size, absorption spectra of CdSe QDs and NRs with their cation exchange derivatives, additional XRD patterns of CdSe QDs and their cation exchange products, TEM images of cation exchange products synthesized at various temperatures, FT-IR spectra and ICP-MS data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01548.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. 5302

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Research Foundation (NRF) grants funded by the Korean government (Grant Nos. NRF-2011-0030256 and NRF-2014R1A2A2A01006739), the Korea CCS R&D Center (KCRC) grant funded by the Korean Ministry of Science, ICT, and Future Planning (No. NRF2014M1A8A1049303), and the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted from the Ministry of Trade, Industry, and Energy, Republic of Korea. (No. 20133030011330). S.L. acknowledges the financial support from the Basic Science Research Program through the NRF of Korea funded by the Ministry of Education (No. NRF2013R1A6A3A04059268). This work was financially supported by the internal funding (2E25393) of Korea Institute of Science and Technology.

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