Exploiting Crystallographic Regioselectivity to Engineer Asymmetric

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Exploiting Crystallographic Regioselectivity to Engineer Asymmetric Three-Component Colloidal Nanoparticle Isomers using Partial Cation Exchange Reactions Julie L. Fenton, Benjamin Steimle, and Raymond E. Schaak J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03338 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Journal of the American Chemical Society

Exploiting Crystallographic Regioselectivity to Engineer Asymmetric Three-Component Colloidal Nanoparticle Isomers using Partial Cation Exchange Reactions Julie L. Fenton, Benjamin C. Steimle, and Raymond E. Schaak* Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802 Supporting Information Placeholder 21

Here, cations in the seed nanocrystal exchange with other cations in solution under mild conditions. When cation exchange reactions are arrested prior to reaching completion, the products can have modified compositions in certain regions while maintaining size, shape, and crystal structure features.12,22 Such reactions have produced a variety of asymmetric and multicomponent nanocrystals, including nanorods of one material with a tip or stripes of another material,23,24 core@shell structures,19,25 sandwich-type nanocrystals,15,26 and Janus structures.16,21 However, it remains challenging to achieve controllable regioselectivity, where different regions of a seed particle having the same composition can be selective modified. Here, we show that crystallographic relationships among precursor and product phases during partial cation exchange reactions enable multiple distinct materials to be asymmetrically integrated into uniform colloidal nanoparticles at precise locations to produce complex heterostructured nanoparticles. Spherical 27 ± 1 nm Cu1.8S nanocrystals were synthesized by thermally decomposing CuCl and di-tert-butyl disulfide in oleylamine at 180 ºC;15 complete experimental details and characterization data are included in the Supporting Information. Figure 1a shows both TEM and HRTEM images of the nanoparticles, which are single-domain crystals of roxbyite copper(I) sulfide, Cu1.8S. Cu1.8S contains cation vacancies and highly mobile Cu+ cations that facilitate exchange with other monovalent and divalent cations while maintaining both particle morphology and crystal structure features. Figures 1b and 1c show the products of partial cation exchange with ZnCl2 and Cd(C2H3O2)2, respectively, in oleylamine, trioctylphosphine (TOP), and benzyl ether at 50 ºC. The Zn2+ exchange produces a sandwich-like 26 ± 1 nm ZnS– Cu1.8S–ZnS structure, while the Cd2+ exchange produces hemispherical 27 ± 1 nm CdS–Cu1.8S Janus particles. Both types of particles are known,15,22 and their formation is rationalized by considering optimal lattice matching across the ZnS/Cu1.8S and CdS/Cu1.8S interfaces.22

ABSTRACT: The precise placement of different materials

in specific regions of a nanocrystal is important for many applications, but this remains difficult to achieve synthetically. Here we show that regioselectivity during partial cation exchange reactions of metal chalcogenide nanocrystals emerges from crystallographic relationships between the precursor and product phases. By maximizing the formation of low-strain interfaces, it is possible to rationally integrate three distinct materials within uniform spherical and rod-shaped colloidal nanoparticles to produce complex asymmetric heterostructured isomers. Through sequential partial exchange of Cu+ in Cu1.8S nanocrystals with Zn2+ and Cd2+, five distinct ZnS/CdS/ Cu1.8S nanosphere and nanorod isomers are accessible.

The ability to design and then rationally synthesize populations of uniform complex colloidal nanocrystals with multiple components integrated at precise locations underpins many applications, including photocatalytic chemical transformations,1,2 non-linear optics,3 nanoscale locomotion,4 and nanomedicine.5,6 These and other systems exploit compositional asymmetry, where different regions of a nanoparticle have different materials that can interact synergistically through interfacial electron transfer or exciton confinement, as well as magnetic, optical, and plasmonic coupling.1,7–11 Implicit in the synthesis of such nanocrystals is the ability to achieve asymmetric integration of multiple targeted materials while maintaining other key features, including shape, size, and monodispersity. Regioselective reactions that target precise locations of a nanocrystal are therefore important tools for enabling the rational synthesis of such nanoscale heterostructures. Cation exchange reactions provide a powerful approach for achieving regioselective modification of metal chalcogenide, metal pnictide, lanthanide fluoride, and halide perovskite nanocrystals, which span several classes of useful materials for solar energy harvesting, catalysis, quantum confinement, magnetism, and upconversion.12–

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Figure 1. (A) TEM and HRTEM images of single-crystalline Cu1.8S nanoparticles, along with the crystal structure of roxbyite Cu1.8S and the pseudohexagonal unit cell used for comparison with wurtzite ZnS and CdS. (B) and (C) show TEM images, HRTEM images, and STEM-EDS element maps [Cu (Kα) is red, Zn (Kα) is green, Cd (L) is blue] for the products formed from partial cation exchange of Cu1.8S with Zn2+ (ZnS–Cu1.8S–ZnS sandwich spheres) and Cd2+ (CdS–Cu1.8S Janus spheres), respectively. The products in (B) and (C) subsequently react with Cd2+ and Zn2+, respectively, to form derivative heterostructured spheres: (D) ZnS–(CdS–Cu1.8S)–ZnS and (E) CdS–(ZnS–Cu1.8S–ZnS). The lattice fringes highlighted in the HRTEM images reveal the crystallographic orientations of the Cu1.8S, ZnS, and CdS regions, which are shown (along with the observed lattice planes) in the crystal structures at the top.

In the ZnS–Cu1.8S–ZnS sandwich structure, analysis of the HRTEM image in Figure 1b reveals that the ZnS and Cu1.8S regions stack such that the a-axis directions of the hcp sublattices are aligned. Consistent with this observation, the a-axis lattice parameter of wurtzite ZnS (a = 3.81 Å) matches closely with the a-axis lattice parameter of the pseudohexagonal subunit of roxbyite Cu1.8S (a = 3.87 Å), while the c-axis lattice parameters differ significantly (cCu1.8S = 6.71 Å, cZnS = 6.23 Å).28,29 In contrast, the CdS and Cu1.8S regions of the CdS–Cu1.8S Janus structure stack such that the c-axis directions of the hcp sublattices are aligned (Figure 1c). Here, the c-axis lattice parameter of wurtzite CdS (c = 6.72 Å) matches more closely with the c-axis lattice parameter of the pseudohexagonal

The HRTEM images in Figure 1 confirm that all of the material components have single-crystalline domains. The sulfur sublattice of roxbyite Cu1.8S adopts a distorted hexagonal close packed (hcp) structure,27 and cation exchange is known to generate products that maintain this hcp sulfur sublattice.28 Accordingly, the crystal structures of ZnS and CdS in the partially exchanged nanoparticle products are both wurtzite, which also contains an hcp sulfur sublattice. The roxbyite Cu1.8S unit cell (Figure 1a) is large and complex,27 but a smaller pseudohexagonal subunit with average lattice constants of a = 3.87 Å and c = 6.71 Å can be defined for comparison with the hexagonal wurtzite structures of ZnS (a = 3.81 Å, c = 6.23 Å) and CdS (a = 4.13 Å, c = 6.72 Å).28,29


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partial cation exchange of Cu+ with Cd2+ to form 53 ± 3 × 20 ± 1 nm single-tipped Janus CdS–Cu1.8S nanorods (Figure 2b) while partial exchange with Zn2+ forms 53 ± 2 × 19 ± 1 nm dual-tipped, striped ZnS–Cu1.8S–ZnS– Cu1.8S–ZnS nanorods (Figure 2c).22 The long direction of the Cu1.8S nanorods corresponds to the c-axis direction of the roxbyite pseudohexagonal unit cell that was defined in Figure 1a. Interfaces perpendicular to the length of the rod, therefore, occur along the a-axis directions of that cell. The preferential formation of multiple ZnS stripes within the ZnS–Cu1.8S–ZnS–Cu1.8S–ZnS rod maximizes a crystallographically favorable shared interface along the a-axes between the ZnS and Cu1.8S segments. However, on the CdS–Cu1.8S Janus rods, the shared interface between the CdS and Cu1.8S segments includes both aaxis and c-axis directions. Diagonal interfaces are observed for both ZnS–Cu1.8S and CdS–Cu1.8S, which is attributed to other closely lattice matched planes, as shown in the crystal structure insets at the top of Figure 2. Interestingly, most of the interfacial contact between CdS and Cu1.8S segments in the Janus rods is along the more strained a-axis directions, which is in contrast to the CdS– Cu1.8S interface that forms in the spherical Janus particles. For nanorods, it is known that the tips can be more reactive than the side facets,24 which suggests a competition between Cd2+ exchange along the higher-reactivity tips versus the more crystallographically favored sides. The majority of the Janus CdS–Cu1.8S nanorods consists of Cu1.8S, so partial exchange of the residual Cu+ with Zn2+ should mimic the behavior observed on the pristine rods, with ZnS and Cu1.8S interfacing along their a-axis directions. The TEM and HRTEM images in Figure 2d validate this prediction, showing the formation of complex multi-striped 52 ± 2 × 19 ± 1 nm nanorods consisting of sequential CdS–ZnS–Cu1.8S–ZnS segments. Interestingly, the internal ZnS band forms between the CdS tip and the remaining Cu1.8S domain, creating both ZnS– CdS and ZnS–Cu1.8S interfaces along the a-axis direction. This suggests that the more strained a-axis CdS/Cu1.8S interface in the Janus CdS–Cu1.8S rods may serve as a higher-energy initiation point for cation exchange with Zn2+, though the product structure (containing a new ZnS–CdS interface along the same a-axis directions) would be similarly strained. On the dual-tipped, striped ZnS–Cu1.8S–ZnS–Cu1.8S– ZnS nanorods, the high-energy Cu1.8S facets at the ends of the rod have already been converted to ZnS, and are unavailable for further exchange with Cd2+. Using the principles of crystallographic regioselectivity outlined above, subsequent partial exchange of Cu+ for Cd2+ on these rods should then interface CdS and Cu1.8S along the favorable c-axis direction, parallel to the length of the nanorod. Indeed, the TEM and HRTEM images in Figure 2e confirm the formation of complex multi-component 53 ± 2 × 19 ± 1 nm nanorods of ZnS–(CdS–Cu1.8S)–ZnS– (CdS–Cu1.8S)–ZnS. Similar to the spherical ZnS–(CdS–

subunit of roxbyite Cu1.8S (c = 6.71 Å), while the a-axis lattice parameters differ significantly (aCu1.8S = 3.87 Å, aCdS = 4.13 Å).28,29 For both ZnS–Cu1.8S–ZnS and CdS– Cu1.8S, the observed interfaces minimize lattice distortion. However, for the same roxbyite Cu1.8S seed particles under identical reaction conditions, Zn2+ and Cd2+ exchange with Cu+ in different crystallographic directions. Regioselectivity, or the specific regions of the spherical Cu1.8S nanocrystals that are replaced upon partial cation exchange, therefore emerges from these crystallographic relationships among the various phases. These observations22 suggest that more complex heterostructured nanoparticles could be designed and synthesized by considering the crystallographic preferences of the ZnS/Cu1.8S and CdS/Cu1.8S interfaces. Based on the orientation of the Cu1.8S segment in the ZnS–Cu1.8S–ZnS sandwich particles, the direction that exchange with Cd2+ preferentially targets is aligned along the exposed edges of the particle, and is thus preserved for a subsequent exchange step. Partial exchange of Cu+ with Cd2+ in the ZnS–Cu1.8S–ZnS sandwich particles would therefore be expected to transform the residual Cu1.8S region into hemispherical CdS–Cu1.8S. The TEM and HRTEM images in Figure 1d confirm the formation of this predicted product, which can be described as 26 ± 1 nm particles of ZnS– (CdS–Cu1.8S)–ZnS. The Cu1.8S region remains oriented to share favorable interfaces with the single-crystalline ZnS caps (along the a-axis) and the newly formed CdS domain (along the c-axis). Since the Cd2+ partially replaces the Cu1.8S that is aligned with the ZnS region along the a-axes, the CdS and ZnS regions also share an interface along the a-axes of both wurtzite-type materials. Starting instead with the CdS–Cu1.8S Janus particles, the preferred c-axis Zn2+ exchange direction remains exposed in the single-crystalline Cu1.8S hemisphere. Exchange of Cu+ with Zn2+ in the hemispherical CdS–Cu1.8S Janus particles would therefore be expected to transform the residual Cu1.8S region into a ZnS–Cu1.8S–ZnS sandwich structure. Indeed, the TEM and HRTEM images in Figure 1e confirm the formation of 27 ± 1 nm (ZnS– Cu1.8S–ZnS)–CdS heterostructured nanoparticles. The Cu1.8S that remains is aligned with both the CdS hemisphere (along the c-axes) and the newly formed ZnS domains (along the a-axes), maximizing the most favorable crystallographic interfaces. This sequence of exchanges, however, results in alignment of the new ZnS domains and the CdS hemisphere along their c-axis directions, rather than their a-axis directions. While the (ZnS–Cu1.8S– ZnS)–CdS Janus particle has been observed previously,22 the ZnS–(CdS–Cu1.8S)–ZnS variant has not. Crystallographic regioselectivity is also applicable to related nanorod systems. Roxbyite Cu1.8S nanorods having average dimensions of 53 ± 2 × 19 ± 1 nm were synthesized by reacting Cu(NO3)2·3H2O with a mixture of dodecanethiols in trioctylphosphine oxide and octadecene at 180 ºC (Figure 2a).30 The Cu1.8S nanorods undergo



Figure 2. (A) TEM and HRTEM images of single-crystalline Cu1.8S nanorods. (B) and (C) show TEM images, HRTEM images, and STEM-EDS element maps [Cu (Kα) is red, Zn (Kα) is green, Cd (L) is blue] for the products formed from partial cation exchange of Cu1.8S with Cd2+ (CdS–Cu1.8S Janus rods) and Zn2+ (ZnS–Cu1.8S–ZnS–Cu1.8S–ZnS striped rods), respectively. The products in (B) and (C) subsequently react with Zn2+ and Cd2+, respectively, to form derivative three-component heterostructured nanorods: (D) CdS–ZnS–Cu1.8S–ZnS and (E) ZnS–(CdS–Cu1.8S)–ZnS–(CdS–Cu1.8S)–ZnS. The crystal structure insets at the top show representative lattice planes and also highlight the observed interfaces (black dashed lines) that differ from those shown in Figure 1. All interfaces occur at consistent angles and correspond to closely lattice matched planes.

Cu1.8S)–ZnS particles, the Cu1.8S region aligns favorably with both the ZnS stripes (along the a-axes) and with the newly formed CdS regions (along the c-axis), while the ZnS and CdS regions share an interface along the a-axis. Both CdS–ZnS–Cu1.8S–ZnS and ZnS–(CdS–Cu1.8S)– ZnS–(CdS–Cu1.8S)–ZnS can therefore form as heterostructured nanoparticle isomers. Partial cation exchange of Cu+ with Cd2+ was also carried out on the multisegment CdS–ZnS–Cu1.8S–ZnS nanorod isomer shown in Figure 2d. Among the CdS, ZnS, and Cu1.8S segments that are present, reactivity with Cd2+ is expected to be localized to Cu1.8S because only Cu+ is amenable to exchange with Cd2+ under the mild reaction conditions. Likewise, the Cu1.8S region would be

expected to convert to a Janus CdS–Cu1.8S segment because of the preference for CdS and Cu1.8S to align along the c-axis direction. Indeed, the STEM-EDS element maps in Figure 3a verify the formation of 54 ± 2 × 20 ± 1 nm CdS–ZnS–(CdS–Cu1.8S)–ZnS nanorods. The TEM image in Figure 3b indicates that despite three distinct cation exchange steps and compositional complexity, the morphology, uniformity, and size of the original Cu1.8S nanorods are maintained. In conclusion, we demonstrated the ability to precisely target certain regions of morphologically and compositionally identical nanoparticles to produce complex heterostructured nanoparticle isomers. By exploiting the unique crystallographic relationships among structurally


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HR-TEM imaging, S/TEM imaging, and EDS mapping were performed at the Materials Characterization Lab of the Penn State Materials Research Institute.

similar phases, we achieve regioselectivity through mutiple partial cation exchange reactions, facilitated by the powerful morphologic and crystallographic retention enabled by nanoparticle cation exchange processes.

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Figure 3. Starting with the CdS–ZnS–Cu1.8S–ZnS striped nanorods in Figure 2D, partial cation of the Cu+ in the Cu1.8S region with Cd2+ forms CdS–ZnS–(CdS–Cu1.8S)–ZnS nanorods, shown in the TEM images and STEM-EDS element maps [Cu (Kα) is red, Zn (Kα) is green, Cd (L) is blue, and S (Kα) is yellow].

ASSOCIATED CONTENT Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website. - Complete experimental details and additional characterization data, including powder X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) single-element maps and spectra (Figures S1-S10).

AUTHOR INFORMATION Corresponding Author

*[email protected] Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the U.S. National Science Foundation under grant DMR-1607135. TEM imaging was performed in the Penn State Microscopy and Cytometry facility.



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