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Janus Nanoparticle Structural Motif Control via Asymmetric Cation Exchange in Edge-Protected Cu S@IrS Hexagonal Nanoplates 1.81

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Jongsik Park, Jisol Park, Jaeyoung Lee, Aram Oh, Hionsuck Baik, and Kwangyeol Lee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02752 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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Janus Nanoparticle Structural Motif Control via Asymmetric Cation Exchange in Edge-Protected Cu1.81S@IrxSy Hexagonal Nanoplates Jongsik Park,†,‡ Jisol Park,†,‡ Jaeyoung Lee,† Aram Oh,†,# Hionsuck Baik,# and Kwangyeol Lee*,† †

Department of Chemistry, Korea University, Seoul 02841, Korea

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Korea Basic Science Institute (KBSI), Seoul 02841, Korea

‡

These authors contributed equally.

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ABSTRACT: Post-synthetic transformation of nanoparticles has received great attention, because this approach can provide an unusual route to elaborately composition-controlled nanostructures while maintaining the overall structure of the template. In principle, anisotropic hetero-nanoparticles of semiconductor materials can be synthesized via localized, i.e., single site, cation exchange in symmetric nanoparticles. However, the differentiation of multiple identical cation exchange sites in symmetric nanoparticles can be difficult to achieve, especially for semiconductor systems with very fast cation exchange kinetics. We posited that single-site cation exchange in semiconductor nanoparticles might be realized by imposing a significant kinetic hurdle to the cation exchange reaction. The different atomic arrangements of the core and crown in core-crown structures might further differentiate the surface energies of originally identical cation exchange sites, leading to different reactivities of these sites. The first cation exchange site would be highly reactive due to the presence of a formed interface, thereby continuing to act as a site for cation exchange propagation. Herein, we present the proof-of-concept synthesis of Janus nanoparticles by using edge-protected Cu1.81S@IrxSy hexagonal nanoplates. The Janus nanoparticles comprising {Au2S-Cu1.81S}@IrxSy or {PdS-Cu1.81S}@IrxSy exhibited dissimilar structural motifs due to the disparate cation exchange directions. This synthetic methodology exploiting cation exchange of surface-passivated semiconductor nanoparticles could fabricate the numerous symmetry-controlled Janus heterostructures.

KEYWORDS: Janus nanoparticle · binary metal sulfide · cation exchange reaction · asymmetric · copper sulfide


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Chemical transformation of nanocrystals via post-synthetic modification has been intensively investigated for the design and production of complex nanomaterials that cannot be obtained via conventional synthetic methodologies.1-4 Cation exchange, in particular, has been a preferred post-synthetic method for altering the composition of semiconductor nanoparticles while maintaining the overall structural parameters.5-9 In general, the feasibility of cation exchange is largely determined by the thermodynamic parameters, such as the lattice energy, of the semiconductors,10, 11 where the similarity of the anion sub-lattices of the original semiconductor and the final structure play a pivotal role in determining the efficiency of cation exchange.12, 13 In principle, locally confined cation exchange on a structurally controlled semiconductor nanocrystal can lead to a variety of complex semiconductor systems with sophisticated compositional hierarchy or anisotropy. Although there are several interesting examples of locally confined cation exchange reactions that produce semiconductor systems with unusual compositional hierarchy or anisotropy,14,

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it is quite difficult to induce confined cation

exchange in a symmetrical nanostructure with multiple reaction sites of identical reactivity. This situation is further exacerbated in semiconductor systems such as Cu2-xS nanocrystals that easily undergo cation exchange with a variety of metal cations of noble metals (Au+, Pd2+, Ag+, Rh3+, Ru3+)16-24 and non-noble metals (Zn2+, Hg2+, Cd2+, In3+, Sn4+).25-31 For example, all six corners of the hexagonal Cu2-xS nanoplate are equally preferred sites for cation exchange, and in the case of Au cation exchange, all six corners react simultaneously with Au cations to generate a hexagonal ring structure after Cu removal via acid etching.32 We posited, however, that locally confined cation exchange might be accomplished under kinetically challenged conditions. The additionally formed grain boundary at the very first cation exchange site of a semiconductor nanoparticle would be the sole preferred site for further cation exchange due to the strain-


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induced high energy, which would in turn produce Janus semiconductor nanoparticles. Because cation exchange cannot occur at very low temperatures, the reaction kinetics must be slowed by imposing physical barriers on the cation exchange sites. Herein, we demonstrate that a thin layer of a different material on a symmetrical semiconductor template successfully acts as a strong deterrent to cation exchange, leading to local confinement of the cation exchange reaction. In order to demonstrate the concept of kinetically-controlled cation exchange, we selected the symmetric hexagonal Cu2-xS nanoplate system as a template for cation exchange with Au or Pd cations.33-37 Physical protection of the six corners of a Cu2-xS nanoparticle was provided by a thin iridium sulfide (IrxSy) crown formed on Cu2-xS to form Janus nanoparticles {i.e., MyS-Cu2-xS}@IrxSy (M = Au, Pd). It is further demonstrated that the different atomic arrangements of IrxSy and Cu1.81S at the corners offer the possibility for anisotropic diffusion in the cation exchange reaction. Moreover, due to the differences in the crystal systems of gold sulfide and palladium sulfide, the direction of metal atom diffusion differs for the two cations; thus, the two Janus nanoparticles from the respective metal atoms adopt disparate structural motifs. Overall, this study provides insight for breaking the symmetry of symmetrical semiconductor nanocrystals by imposing structural deterrents to the cation exchange diffusion pathway, resulting in structural motif-varied Janus-type heteronanostructures.

Results and Discussion A representative transmission electron microscopy (TEM) image of the Cu1.81S@IrxSy core@crown nanostructures (CSIS) is shown in Figure 1a. The size of the CSIS particles was 73 ± 2 nm, which is smaller than that of the Cu1.81S nanoplates that were synthesized under the same


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conditions without addition of the Ir precursor (Figure S1). To demonstrate the existence of the IrxSy crown, line profile (Figure 1b) and corresponding elemental mapping analyses were performed for the CSIS (Figure 1c). Based on these analyses, while the Ir atoms were distributed over the entire nanocrystals, the highest concentration of Ir atoms was found at the edges of Cu1.81S, indicating a core-crown structural motif (i.e., CSIS). Enlarged elemental mapping analysis of CSIS is also shown in Figure S2 to demonstrate the edge-specific Ir-coating. The scanning TEM (STEM) image of the CSIS (Figure 1d) was also acquired, where the brightness of the STEM image depends on the square of the atomic number (Z), thus making the Ir atoms in the crown layer easily observable. In order to analyze the crystal phases in more detail, enlarged high-resolution STEM (HRSTEM) images of the hexagonal nanoplates of CSIS were obtained, as shown in Figures 1e and f. All the measured lattice distances for the embedded copper sulfide phase were 0.19 nm, which could be matched with the {008} and {064} facets of the Cu1.81S phase. The fast-Fourier transform (FFT) pattern was in good agreement with the simulated Cu1.81S FFT pattern. On the other hand, the measured lattice distances of the Ir-rich crown were 0.31 and 0.28 nm, which are similar to those of pure IrS2 (d{011} = 0.30 nm, d{002} = 0.28 nm). The zone axis of IrS2 was found to correspond to the [100] direction. The HRSTEM images of various crown/shell locations of CSIS show that the crown of CSIS is composed of crystalline IrS2 phase (Figure S3). To elucidate the exact phase of Ir-rich crown, we obtained the high power powder X-ray diffraction (PXRD) analysis of CSIS (Figure S4). Due to the relatively small amount of IrS2 crown as compared to the Cu1.81S phase, the majority of peaks of PXRD pattern come from the Cu1.81S phase. In addition to PXRD pattern of Cu1.81S phase, peaks could be observed near the 27o and 30o, which match well with the major peaks of IrS2 phase. Therefore, we concluded that the composition of crown is mainly comprised of IrS2 phase.


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In order to further understand the composition and the phase of the Ir-containing crown, the embedded Cu1.81S phase was removed by chemical etching in 3 M HCl solution (Figure S5). The TEM images of the CSIS after various etching times indicate that the inner Cu1.81S core phase was dissolved under the acidic conditions, but the outer IrxSy crown exhibited high resistance to the harsh conditions. Because of the Scherrer’s equation, the PXRD peaks originated from the flimsy IrxSy crowns should become broader rather than that of Cu1.81S. A rather broad PXRD peak was observed at 30° for the sample subjected to 30 min etching, and the peaks originating from Cu1.81S shifted to higher angles, indicating incomplete removal of the Cu1.81S phase and lattice compression for the remaining Cu1.81S phase. When the etching process was further extended to 3 h, the Cu1.81S phase was completely removed to yield hollow cages. The thoroughly etched sample also showed broad XRD peaks near 20°, 30°, and 52°, which is similar to the PXRD pattern of the pure IrS2 phase. We obtained the temporal TEM images and performed X-ray photoelectron spectroscopy (XPS) analysis of CSIS intermediates to elucidate the formation mechanism of IrxSy crown structure (Figure S6). The average size distributions of overall CSIS are quite similar between intermediates at 2 min and 30 min. In addition, the XPS analysis of intermediates shows that the intensities of Ir peaks are gradually increased with increased reaction time. These results reveal that the cation exchange reaction of Ir to form the IrxSy crown structure proceeds slowly at the reactive sites of Cu1.81S template and partially exchanged domains occur only at the surface. We examined whether the formation of IrxSy crown structure is possible through the two-step synthesis method (Figure S7). However, Ir cations could not exchange with the Cu cations in the Cu2-xS, forming Ir-based nanoparticles separately. In general, the incorporation of foreign cations into Cu2-xS involves nucleation of template nanoparticles, followed by incorporation of incoming


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cations.38 We concluded that the formation of IrxSy crown structure follows a vacancy-assisted mechanism, thereby requiring the incorporation of CuSCN and Ir(acac)3 precursor at the same time. The amount of Ir precursor introduced into the system affects the diagonal length and thickness of the CSIS, as shown in Figure S8. The decrease in the size of the nanoplates driven by the addition of the Ir precursor might be due to facilitate the reaction kinetics of CuSCN. The initially added precursor could affect the decomposition kinetics of Cu precursor, resulting the different formation of Cu1.81S nanoplates with various aspect ratio.39,

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Furthermore, as the

amount of Ir precursor was increased, the thicker IrxSy crown structure was obtained. The elemental mapping analysis and corresponding line profile analysis of thicker CSIS in Figure S9 clearly demonstrate the edge-specific coating of IrxSy on the hexagonal Cu1.81S nanoplate. The XRD data and selected area electron diffraction (SAED) pattern of thicker CSIS also reveal that highly crystalline IrS2 phase is found over the entire crown part of CSIS (Figure S10). In order to explain the different atomic arrangements of the Cu1.81S and IrS2 phases, the detailed atomic orientations at the corners and edges are illustrated in Figure 2a. The crystallographic planes along the interface were identified as (040) (corresponding to [100]) for Cu1.81S and (002) (corresponding to [100]) for IrS2, based on the HRSTEM and FFT patterns in Figure 1e-f. These epitaxial interfaces could be formed due to the similar sulfur sublattices shared by Cu1.81S (triclinic, space group P-1) and IrS2 (orthrombic, space group Pnma), resulting in the formation of the IrS2 crown on the Cu1.81S edges (Detailed information of crystal structure is described in Figure S11). Moreover, the Ir atoms were located at the octahedral hole sites, whereas the Cu atoms were situated in trigonal or tetragonal holes. Recently, it was reported that the coordination number of entering ion affects the diffusion rate of cations, resulting in formation of


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core-shell nanostructures.30 In addition, the ionic radius of Ir(IV) (63 pm) is similar to that of Cu(I) (77 pm), and thus the anionic framework is preserved even if the Ir atoms are located at the octahedral sites. Interestingly, the unit cell of IrS2 (zone axis: [100]) has an anisotropic atomic arrangement, in contrast to that of Cu1.81S (zone axis: [100]), which has anisotropic atomic arrangement, even though both species share the same anion sublattice (Figure S12). Due to this difference in the orientations of the crown and core, the activities of the corners of Cu1.81S become differentiated. The detailed atomic orientations of each corner are illustrated in Figure 2b. At the (A) corners, only half of the octahedral sites are occupied by Ir atoms, where the Ir atoms are bound with S atoms. The vacant sites can thus serve as the diffusion route for cation exchange because the absence of Ir atoms induces low-energy barriers. At the (B) corners, however, the fully occupied Ir atomic arrangements make it difficult for cations to diffuse into the Cu1.81S phase from outside. As a result, the difference in the crystal packing system of IrS2 and Cu1.81S at the corners offers the possibility of anisotropic diffusion for the cation exchange reaction. Previously, we reported the isotropic cation exchange of hexagonal Cu1.94S nanoplates with Au cations to yield Au-based hexagonal rings (Figure S13).32 The facile transformation between the Cu1.94S and Cu1.81S phases has been previously demonstrated, and the choice of the phase was dependent on the relative thermodynamic stabilities of the phases under the specific reaction conditions.41-43 On the other hand, there is no significant difference in the atomic arrangement of the Cu1.94S and Cu1.81S phases (Figure S14). Therefore, diffusion of the Au cations into the hexagonal Cu1.81S is also feasible. However, the presence of the thin IrxSy crown and the differentiation of the corners in terms of the availability for cation exchange suggests that


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anisotropic cation exchange of the hexagonal Cu1.81S@IrxSy nanoplates with Au cations is feasible. To confirm the postulated feasibility of the anisotropic cation exchange reaction, the CSIS templates were reacted with Au cations (Figure 3). As expected, the pathway for cation exchange of Au with CSIS was quite different from that of bare copper sulfide without the IrxSy crown. The products acquired from cation exchange of CSIS with Au had the Janus {Au2SCu1.81S}@IrxSy nanostructure (JAC), as shown in Figure 3a. The different levels of brightness in the scanning TEM (STEM) image in Figure 3b indicate Au2S, IrxSy, and Cu1.81S, respectively. Complete separation of the Au2S and Cu1.81S phases was detected in the corresponding elemental mapping analysis (Figure 3c). The line profile analysis of JAC presented in Figure S15a indicates a completely phase-segregated structural motif, in which the Au segment and unreacted Cu-rich segment are clearly differentiated. The energy-dispersive X-ray spectroscopy (EDS) data for JAC (Figure S15b) indicates the following atomic composition: 16% Au, 46% Cu, 4% Ir, and 34% S. To elucidate the diffusion pathway in JAC, the HRSTEM image and corresponding FFT pattern for Figure 3b were acquired (see Figure 3d, e). As shown in Figure 3b, the Au cation exchange started from the active corner of the CSIS, and diffusion of the Au cations proceeded to the other side of the active corner. Analysis of the HRSTEM image of the bright Au-rich region (Figure 3d) indicated lattice distances of 0.28, 0.24, and 0.17 nm, which can be indexed to the {002}, {111}, and {220} facets of face-centered-cubic (fcc) Au2S (Cubic, space group Pn3m), respectively. The magnified HRSTEM image in the inset of Figure 3d matched well with the unit cell model of Au2S. The FFT pattern in Figure 3e reveals the coexistence of the Au2S and Cu1.81S phases, where cation exchange diffusion in Au2S and Cu1.81S occurred in the [002] and [080] directions, respectively. The high-resolution TEM (HRTEM) image of JAC in Figure


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S16 also confirms the crystallinity of IrS2 phase. The crystalline domains and corresponding FFT patterns confirm the coexistence of crystalline IrS2 phase and Au2S phase. Therefore, we conclude that the crown of CSIS structure is mainly composed of crystalline IrS2 phase. In order to achieve successful cation exchange, the anion sublattice at the interface should be similar.44 As shown in Figure 3f, the similar atomic orientations of Cu1.81S and Au2S allow cation exchange diffusion to occur at the interface. Furthermore, the lattice parameters of anion framework of Au2S (a = 3.8 Å and b = 3.8 Å) are larger than those of the anion framework of Cu1.81S (a = 3.4 Å and b = 3.4 Å). The discrepancy in the lattice distance of anion framework along the c-axis (Cu1.81S: 3.8 Å and Au2S: 5.3 Å) generates a large strain between Cu1.81S and Au2S. Therefore, all the measured lattice distances of the Au2S phase adjoined to Cu1.81S the phase were slightly shorter than those of the pure Au2S phase (d{111} = 0.29 nm, d{002} = 0.25 nm, and d{220} = 0.18 nm), resulting in compressive strain at the Au2S phase and tensile strain at the Cu1.81S phase. Modulation of the thickness of the IrxSy crown is important for controlling the energy barrier when the cations enter the CSIS (Figure S17). When thicker CSIS templates were used by employing a higher Ir precursor concentration, diffusion of the Au cations was severely inhibited. To expand the synthesis concept to a different Janus particle, cation exchange of CSIS with Pd cations was attempted. Because several palladium sulfide phases (PdxSy) have crystal packing systems similar to that of Au2S, cation exchange with Pd was expected to be feasible.45-47 The reaction of Pd cations with CSIS also generated a Janus nanostructure as shown in Figure 4. The HAADF-STEM image of Janus {PdS-Cu1.81S}@IrxSy (JPC) in Figure 4a indicates that the diffusion of Pd cations into CSIS also started at the active corner of CSIS. The corresponding elemental mapping data (Figure 4b) and line profile analysis (Figure S18a) of JPC also clearly


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demonstrated the characteristics of the Janus heterostructure. We also tested the cation exchange reaction of Pd precursor with the bare Cu1.81S nanoplates in order to demonstrate the indispensable role of IrxSy shell in differentiating the cation exchange pathway. As shown in Figure S19, we found that the Pd cations are exchanged with Cu cations at the identical active sites of bare Cu1.81S nanoplates. Due to the low barrier exchange energy with Cu, the insertion of Pd cations was facilitated at the active corner sites, forming several domains of PdxSy. Therefore, the existence of IrxSy not only differentiates the activities of corner sites of Cu1.81S, but also provides activation barriers to slow down the cation exchange reaction. The EDS data for JPC (Figure S18b) indicated an atomic composition of 9% Pd, 51% Cu, 3% Ir, and 37% S. The content of Cu in the PdS region was less than that in the Au2S region in JAC; in contrast with the Au2S region contiguous to Cu1.81S, most of the Pd cations could be readily exchanged with Cu cations due to the smaller lattice mismatch with the few unexchanged Cu species. Throughout the cation exchange process of Au and Pd cations with CSIS template, the morphology of CSIS was maintained, indicating that the amount of sulfur is nearly preserved. Since 2 equivalent of Au cations and 1 equivalent of Pd are required to form Au2S phase and PdS, respectively, the measured elemental count of Au was twice of Pd. However, the Janus structure obtained by Pd exchange exhibited a totally different diffusion pathway from that of the Au case. The diffusion pathway was tilted by 60° relative to that of JAC, as shown by the STEM image in Figure 4c. The HRTEM image and FFT pattern of JPC in Figure 4c are shown in Figures 4d and f. The measured lattice distances in the PdS region were 0.34 and 0.25 nm, which are comparable to those of the pure PdS phase (d{002} = 0.35 nm, d{022 } = 0.24 nm). The FFT pattern in Figure 4e reveals the coexistence of PdS and Cu1.81S phases. In order to elucidate the formation of JPC, we obtained temporal TEM images (Figure 4f). In the


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initial stage, the insertion of Pd cations into CSIS formed a small PdS domain at one corner of the hexagonal nanoplate. This crystal domain further grew along the edge of the CSIS until the PdS phase completely filled the edge. After this point, another small PdS domain appeared on top of the PdS trapezoidal domain and grew further until it touched the other edge of the hexagonal nanoplate. This different diffusion pathway of the Pd cations is driven by the unit cell arrangement of PdS. The unit cell models of PdS and Cu1.81S are shown in Figure S20. The measured lattice parameters of anion framework of PdS (a = 3.6 Å and b = 3.3 Å) is shorter than that of anion framework of Cu1.81S (a = 4.4 Å and b = 3.8 Å), but the a/b ratio of PdS (a/b = 1.1) is similar to that of Cu1.81S (a/b = 1.2), resulting in the different diffusion direction, tilted by 60o, from the case of Au. We examined the temporal structural evolution of the reaction intermediates of JAC and JPC to visualize the diffusion process, as shown in Figure 5. At the reaction time of 2 min, a small amount of Au cations diffused into Cu1.81S to form a highly crystalline triangular Au2S domain (Figure 5a). However, the incorporation of Au cations into the CSIS resulted in large lattice strain due to the discrepancy in the lattice distance of anion framework along the c-axis (Cu1.81S: 3.8 Å and Au2S: 5.3 Å). Therefore, the major PXRD peaks of the Au2S phase were shifted to higher angles than those of pure Au2S (Figure 5d (ii)). At the reaction time of 5 min (Figure 5b), a well-defined Au2S-Cu1.81S Janus structural morphology was observed. The decrease in the amount of Cu1.81S gradually reduced the lattice strain, as confirmed by the shift of the major PXRD peaks in Figure 5d (iii) to lower angles. Interestingly, anisotropically elongated hexagonal nanoplates were formed at the reaction time of 10 min (Figure 5c). A more detailed analysis of the elongated Au2S hexagonal nanoplates is presented in Figure S21. The disparity between the lattice distance of anion framework of Au2S and Cu1.81S induces elongation of the hexagonal


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nanoplates. Furthermore, the PXRD peaks observed in Figure 5d (iii) indicate the presence of the pure Au2S phase, where the Au cations fully replace the Cu cations. The elongation of nanoplates could be explained through the disparate ionic radius between Au and Cu. In general, the radius of inserted cation affects the local lattice strain during the cation exchange reaction.48 In our systems, the radii of Au(I) (137 pm) is much larger than that of Cu(I) (77 pm), inducing the local lattice strain when the Au cations are inserted into the CSIS. The anisotropically elongated Au2S@IrxSy nanoplates could be observed until the entire Cu1.81S template is exchanged. In the case of JPC, the mechanism of diffusion and insertion of the Pd cations was slightly different from that of Au the cations, as briefly discussed in relation to Figure 4 (see Figures 5e−g). In the initial stage (at a reaction time of 5 min), some Cu cations were replaced by a small amount of Pd cations to form the PdS phase (Figures 5e and h (ii)). After full occupation of the edge sites of CSIS with formation of the PdS phase at 10 min, the PdS phase domain expanded along the edge direction. In contrast with the Au2S case, the relatively similar ionic radius of Pd(II) (87 pm) led to the facile formation of the Janus structures without the strain effect. The PXRD data for PdS acquired during the reaction also support the notion that cation exchange between Pd and Cu occurs without producing any lattice mismatch (Figure 5h). Analyses of the diagonal lengths of JAC and JPC during the reaction suggest that only insertion of the Au cations induces elongation of the CSIS template (Figure S22). Although the formation of Au2S and PdS phase via cation exchange of Cu1.81S phase is thermodynamically favorable,49 the anion sublattice packing system is different between Cu1.81S (hcp) and Au2S, PdS (cubic), which predicts an unfavourable condition in terms of kinetics. Due to this discrepancy, the formation of Au2S phase occurs with generating the twinning


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boundaries32 or transforming to heteronanostructures with a Janus motif in order to minimize the overall surface energy.17 In our case, the IrxSy crown motif not only differentiates the activities of identical corner sites of Cu1.81S, but also maintains the overall anion framework by strongly binding the sulfur anions with Ir atoms at the surface. Surprisingly, it was difficult to find fully cation-exchanged PdS hexagonal nanoplates even with a prolonged reaction time (Figure S23a) and increased amount of Pd precursor (Figure S23b); only Pd-rich nanoparticles or nanoplates with unevenly distributed PdxSy grains on the CSIS were detected. Due to the increased reaction kinetics, which is dependent on the concentration of precursor, the kinetics of cation exchange reaction was also facilitated. Therefore, the PdCu nanoparticles with different alloy compositions were formed between the remnant Pd precursors and exchanged Cu species, which could be demonstrated by the broad peaks near the 40o of PXRD analysis (Figure S23c). In addition, the distorted anion framework of PdS with the [001] zone axis limits diffusion of the Pd cations and facilitates growth along only one side of the nanoplate (Figure S24). From these experiments, we show that the intrinsic properties of the PdS unit cell structure might prohibit the formation of fully exchanged PdS phase. On the other hand, a higher reaction temperature would provide enough thermal energy to overcome the energy barrier for inserting the Au and Pd cations into the other corners of the CSIS structure (Figure S25). Both Au and Pd cations could diffuse into the corners on both sides and form symmetrically cation-exchanged structures. Thus, the degree of diffusion is affected by the reaction temperature and the intrinsic unit cell structure of the material. The intrinsic unit cell parameters of Au2S and PdS affected the respective diffusion pathways when the cations entered the CSIS template. When we consider only the atomic orientations of each phase according to their zone axes, it is, however, difficult to fully explain the well-


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controlled cation exchange pathway because several other diffusion pathways are also feasible (Figure S26). Therefore, it is postulated that an additional critical factor determining the diffusion pathway might be also operative. To fully describe the different diffusion pathways of the Au and Pd cations in Cu1.81S in terms of thermodynamic stability, we examined the interanion distance of the sulfur sub-lattices at the epitaxial interfaces of Au2S/Cu1.81S and PdS/Cu1.81S (Figure 6). Formation of an alloy phase is possible when the mismatch between the sulfur sublattices is small, which provides a favorable chemical environment for cation exchange and formation of the interfaces.44 The crystallographic planes along the interface were identified on the basis of the HAADF-STEM and HRTEM images in Figure 1d and 2c. From the view at the corner of the CSIS structure, the lattice distances of anion framework along the [002] direction of Au2S are a = 3.8 Å and b = 3.8 Å (a/b = 1) and those along the [010] direction of Cu1.81S are a = 3.4 Å and b = 3.4 Å (a/b = 1). Therefore, both anion frameworks possess tolerable lattice mismatches (11.2%), which should facilitate the formation of the alloy phase. The lattice distances of anion framework along the [-110] Au2S, where the Au2S atomic arrangement is tilted by 60° from the [002] Au2S direction, are a = 3.6 Å and b = 2.5 Å (a/b = 1.42), which generate a large lattice mismatch with copper sulfide phase. From the view at the edge site of the CSIS structure, however, the lattice distances of anion framework along the [100] direction of PdS are a = 3.3 Å and b = 3.3 Å (a/b = 1.03) and those tilted by 60° from the [010] direction of Cu1.81S are a = 3.4 Å and b = 3.4 Å (a/b = 1), leading to a relatively small mismatch. In contrast, the lattice distances of anion framework of PdS tilted by 60° from the [100] direction of PdS (a = 3.3 Å and b = 2.8 Å; a/b = 1.2) show a relatively large lattice mismatch (17.0%) with those of the [010] direction of Cu1.81S. Therefore, the Au cations would diffuse into Cu1.81S along the corner direction and the Pd cations would diffuse into Cu1.81S along the edge direction.


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Conclusions In conclusion, we demonstrated that asymmetric cation exchange in symmetric semiconductor nanoparticles can be accomplished by imposing a kinetic constraint on the cation exchange reaction. Coating the surface of Cu1.81S hexagonal nanoplates with an IrxSy crown provided a kinetic barrier for cation exchange on the Cu1.81S nanoplates with six identical corners, all of which are potential cation exchange sites. Furthermore, the Janus nanoparticles comprising Au2S-Cu1.81S and PdS-Cu1.81S exhibited dissimilar symmetries, because the cation exchange direction depends on the compatibility between the unit cell structures of Cu1.81S and those of Au2S or PdS. We expect that the synthetic methodology exploiting cation exchange reactions of surface-passivated semiconductor nanoparticles could fabricate the numerous symmetrycontrolled Janus heterostructures.

Materials and Methods Reagents. CuSCN (copper(I) thiocyanate, 99%), Ir(acac)3 (iridium(III) acetylacetonate, 99.9%), and oleylamine (98%) were purchased from Sigma-Aldrich. HAuCl4 (gold(III) chloride hydrate, 99%) and K2PdCl4 (potassium tetrachloropalladate(II), 99%) were purchased from Strem. All reagents were used as received without further purification. Material characterization. TEM and HRTEM studies were carried out on a TECNAI G2 F30ST microscope and a Tecnai G2 20 S-twin microscope. Aberration-corrected imaging and high spatial resolution energy-dispersive spectroscopy (EDS) were performed at FEI Nanoport in


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Eindhoven using a Titan Probe Cs TEM 300 kV instrument with Chemi-STEM technology. EDS elemental mapping data were collected using a higher efficiency detection system (Super-X detector with XFEG) that integrates four FEI-designed silicon drift detectors (SDDs) very close to the sample area. Compared to the conventional energy dispersive X-ray (EDX) detector with Schottky FEG systems, the X-ray generation of ChemiSTEM with the X-FEG is up to five times and up to ten times higher than the X-ray collection with the Super-X detector. All scanning transmission electron microscopy (STEM) images and compositional maps were acquired with the use of high angle annular dark field-scanning TEM (HAADF-STEM). Powder X-ray diffraction (PXRD) patterns were collected to probe the crystal structures of the synthesized nanocrystals using a Rigaku Ultima III diffractometer system with graphite monochromatized Cu-Kα radiation at 40 kV and 40 mA. Preparation of Cu1.81S nanoplates. The Cu1.81S hexagonal NPs were prepared by a previously reported method using CuSCN as precursor.33 A slurry of CuSCN (0.25 mmol, Aldrich, 99%) and oleylamine (15.20 mmol, Aldrich, 98%) was placed into a 100 mL Schlenk tube. After placing the solution under vacuum at 60 °C for 10 min, the solution was charged with 1 atm Ar. The Schlenk tube was directly placed in a hot oil bath that was preheated to 240 °C. After heating at 240 °C for 30 min, the reaction mixture was cooled to room temperature with magnetic stirring. Toluene (15 mL) and methanol (25 mL) were added, followed by centrifugation at 4000 rpm for 5 min. The resulting precipitates were further purified twice by washing with ethanol/toluene (v/v = 10 mL/5 mL). Preparation of Cu1.81S@IrxSy core@crown nanoplates (CSIS). For synthesis of the 70 nm hexagonal Cu1.81S@IrxSy core@crown nanoplates (CSIS), a slurry of CuSCN (0.25 mmol, Aldrich, 99%) and Ir(acac)3 (0.025 mmol, Strem, 99%) in oleylamine (15.20 mmol, Aldrich,


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98%) was placed into a 100 mL Schlenk tube. After placing the solution under vacuum at 60 °C for 10 min, the solution was charged with 1 atm Ar. The Schlenk tube was directly placed in a hot oil bath that was preheated to 240 °C. After heating at 240 °C for 30 min, the reaction mixture was cooled to room temperature with magnetic stirring. Toluene (15 mL) and methanol (25 mL) were added, followed by centrifugation at 4000 rpm for 5 min. The resulting precipitates were further purified twice by washing with ethanol/toluene (v/v = 10 mL/5 mL) to furnish the precipitated Cu1.81S@IrxSy. Chemical Etching process to remove Cu1.81S phase. The selective chemical etching of Cu2xS

phase process is modified from our previous reported literatures.20 The samples were

dispersed in a mixture of 2 mL of toluene, 2 ml of ethanol, and 2 ml of 3 M hydrochloric acid solution. The mixture was placed in a preheated oil bath at 60 ºC for 1 h. Finally, the precipitates were centrifuged and washed with 10 ml of ethanol for two times, then dried under vacuum. Synthesis of Janus {Au2S-Cu1.81S}@IrxSy nanostructure (JAC). In a typical synthesis of the Janus {Au2S-Cu1.81S}@IrxSy nanostructure (JAC), the previously synthesized Cu1.81S@IrxSy nanoplates (0.05 mmol) in 1 mL toluene were mixed with HAuCl4 (0.08 mmol, Strem, 99%) in 10 mL oleylamine. The solution was stirred vigorously in a preheated (60 °C) oil bath. The degree of Au diffusion could be modulated by controlling the reaction times. The reaction mixture was cooled to room temperature and 15 mL of toluene and 25 mL of methanol were added, followed by centrifugation at 4000 rpm for 5 min. The NCs were re-dispersed in toluene and ethanol, then collected by centrifugation before further characterization. Synthesis of Janus {PdS-Cu1.81S}@IrxSy nanostructure (JPC). In a typical synthesis of the Janus {PdS-Cu1.81S}@IrxSy nanostructure (JPC), the previously synthesized Cu1.81S@IrxSy (0.05


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mmol) in 1 mL toluene was mixed with K2PdCl4 (0.03 mmol, Strem, 99%) in 10 mL oleylamine. The solution was stirred vigorously in a preheated (180 °C) oil bath. The degree of Pd diffusion could be modulated by controlling the reaction times. The reaction mixture was cooled to room temperature and 15 mL of toluene and 25 mL of methanol were added, followed by centrifugation at 4000 rpm for 5 min. The NCs were re-dispersed in toluene and ethanol, then collected by centrifugation once to adequately remove the ligands before further characterization. Cation exchange of Au and Pd with pure Cu2-xS (Figure S11 and S16). The previously synthesized Cu1.81S nanoplates (0.05 mmol) in 1 mL toluene were mixed with HAuCl4 (0.08 mmol, Strem, 99%) in 10 mL oleylamine. The solution was stirred vigorously in a preheated (60 °C) oil bath. The degree of Au diffusion could be modulated by controlling the reaction times. The reaction mixture was cooled to room temperature and 15 mL of toluene and 25 mL of methanol were added, followed by centrifugation at 4000 rpm for 5 min. The NCs were redispersed in toluene and ethanol, then collected by centrifugation before further characterization. In the case of cation exchange of Pd, we changed the Au precursor to K2PdCl4 (0.03 mmol, Strem, 99%) precursor and the reaction temperature from 60 ºC to 180 ºC.


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Figure 1 Characterization of Cu1.81S@IrxSy core-crown nanostructure (CSIS). a) TEM image, b) line profile analysis, and c) corresponding elemental mapping analysis of CSIS. Colors indicate Ir (cyan), Cu (purple), and S (yellow), respectively. d) STEM and e, f) enlarged STEM images of CSIS and the corresponding FFT, simulated FFT patterns of the regions marked e) and f) in Figure 1d, respectively. The respective regions indicate the Cu1.81S and IrxSy phases.


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Figure 2 Atomic orientations of CSIS a) at the edge and b) at the different corners (A and B) showing different atomic orientations according to each zone axis of Cu1.81S and IrS2, respectively.


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Figure 3 Characterization of Janus {Au2S-Cu1.81S}@IrxSy nanostructure (JAC). a) STEM and b) enlarged STEM image of JAC. The directions of cation diffusion in the Au2S and Cu1.81S phases are indicated by white arrows. c) Elemental mapping analysis of JAC. The colors indicate Au (red), Cu (purple), Ir (cyan), and S (yellow). d) HRSTEM and e) FFT pattern of JAC in Figure 3b. The Inset in Figure 3d indicates the idealistic atomic arrangements of Au2S. f) Atomic


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arrangements of JAC according to the zone axes and corresponding lattice distances of the anion sublattice.

Figure 4 Characterization of Janus {PdS-Cu1.81S}@IrxSy nanostructure (JPC). a) HAADFSTEM image and b) corresponding elemental mapping analysis of JPC. Colors indicate Pd (blue), Cu (purple), Ir (cyan), and S (yellow). c) STEM image of JPC. Diffusion direction of PdS phase is indicated by white arrow. d) HRTEM image and e) FFT pattern of both PdS and Cu1.81S for rectangle in Figure 4c. f) TEM images of JPC with different reaction times, indicating the Pd cation exchange diffusion pathway.


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Figure 5 TEM images of intermediates with varying reaction times and the corresponding PXRD patterns for samples with a) Au cation and b) Pd cation incorporated.


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Figure 6 Atomic models of Au2S, Cu1.81S, and PdS, respectively, with different diffusion pathways. The sulfur atoms are shown in yellow. a) Lattice mismatches at the interface of Au2SCu1.81S in terms of diffusion from a corner and an edge. b) Lattice mismatches at the interface of PdS-Cu1.81S in terms of diffusion from a corner and an edge.


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ASSOCIATED CONTENT SUPPORTING INFORMATION This supporting information is available free of charge on the ACS Publications websites via the Internet at http://pubs.acs.org. Figures S1- S26 give more details on characterization of our synthesized materials. For example, additional TEM, line profile analysis, HRTEM, XRD, elemental mapping analysis, EDS are given. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡ J. Park and J. Park contributed equally to this work.

ACKNOWLEDGMENT


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This work was supported by NRF-2017R1A2B3005682, Korea University Future Research Grant (KU-FRG), Korea University Grant, KBSI project E38300, and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1A6A3A01008861, 2018R1A6A3A01013426). The authors thank Korea Basic Science Institute (KBSI) for the usage of their HRTEM instrument.

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47. Liu, Y.; Sun, C.; Bolin, T.; Wu, T.; Liu, Y.; Sternberg, M.; Sun, S.; Lin, X. -M. Kinetic Pathway of Palladium Nanoparticle Sulfidation Process at High Temperatures. Nano Lett. 2013, 13, 4893-4901. 48. Trizio, L. D.; Li, H.; Casu, A.; Genovese, A.; Sathya, A.; Messina, G. C.; Manna, L. Sn Cation Valency Dependence in Cation Exchange Reactions Involving Cu2-xSe Nanocrystals. J. Am. Chem. Soc. 2014, 136, 16277-16284. 49. Beberwyck, B. J.; Surendranath, Y.; Alivisatos, A. P. Cation Exchange: A Versatile Tool for Nanomaterials Synthesis. J. Phys. Chem. C 2013, 117, 19759-19770.

Table of contents IrxSy-protected Cu1.81S hexagonal nanoplates undergo direction-controlled, asymmetric cation exchange reactions to give Janus nanoparticles with dissimilar symmetries.


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Asymmetric Cation Exchange i - ACS Publications

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