When Ligand Exchange Leads to Ion Exchange: Nanocrystal Facets

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When Ligand Exchange Leads to Ion Exchange: Nanocrystal Facets Dictate the Outcome Indika K. Hewavitharana, and Stephanie L. Brock ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b05534 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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When Ligand Exchange Leads to Ion Exchange: Nanocrystal Facets Dictate the Outcome Indika K. Hewavitharana and Stephanie L. Brock* Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States ABSTRACT: This study demonstrates that ligand-exchange of nanocrystals (NCs) is not always an innocuous process, but can lead to facile (room temperature) ion-exchange, depending on the surface crystal faceting. Rock salt PbTe NCs prepared as cubes with neutral facets undergo room-temperature ligand exchange with sulfide ions, whereas cuboctahedron-shaped particles with neutral {100} and polar {111} facets are transformed to PbS, driven by ion-exchange along the direction. Likewise, cation-exchange (with Ag+) occurs rapidly for cuboctahedra, whereas cubes remain inert. This dramatic difference is attributed to the relative surface area of {111} facets that promote rapid ion-exchange, and shows how facet engineering is a powerful knob for the control of reaction pathways in nanoparticles. *email: [email protected]

KEYWORDS: fcc · cube-shape · cuboctahedron-shape · lead telluride · reaction control · surface energy

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Surface chemistry is a key player in nucleation and growth kinetics, chemical and colloidal stability, solubility, and self-assembly of nanocrystals (NCs). As particle size decreases, the surface area increases; thus, an understanding of surface-dictated physicochemical properties is critical for exploiting NCs in catalytic, optoelectronic and biomedical applications. With respect to solid-state device performance, the insulating ligands used in colloidal synthetic routes preclude efficient electrical transport, limiting functionality.1,2 Numerous research approaches have been evaluated to augment inter-particle transport properties, including stripping of the organic surfactants and/ or replacement with small organic ligands, yet thermal instability and relatively low electrical conductivity remain as limitations for their use in optoelectronic applications.3-5 Surface functionalization of NCs using inorganic anions, instead of organic ligands, can address some of these deficits. Such ligand exchange has been demonstrated with chalcogenide ions (S2-, Se2- and Te2-), chalcogenidometallate ions (MCCs - SnS44-, Sn2S64-, Sn2Se64-, In2Se42-, Ge4S104-) and other anions (halometallates, oxometallates, polyoxometallates, halides, OH-, SCN-, HS-, etc.) leading to decreased NC spacing and increased inter-particle interaction.6-11 Chalcogenides and MCCs have proven to be appropriate ligands for generating thin films of metal chalcogenide NCs that serve as active photovoltaic or thermoelectric devices.12-14 The general assumption has been that ligand exchange, conducted at room temperature, is a purely surface-based event where negatively charged ions bind to the undercoordinated metal-rich surfaces of NCs by replacing organic surfactants without affecting the crystal structure or the mean crystallite size.8-10,14-22 However, our appreciation of the complexity of ligand-surface interactions is evolving as we gain a better understanding of the dynamic nature of nanocrystal surfaces.23-25 The present contribution adds to this growing body of work, demonstrating that room temperature ligand


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exchange with chalcogenide or MCC ions can lead to anion exchange and nucleation of a different crystalline phase and this depends upon the surface facets of crystalline domains. This is notable because the ability to perform phase conversion at low temperature, taking advantage of low-energy pathways made available by facet control, is expected to enable targeting of metastable structures. In this work, the role of facets in promoting and limiting reactivity of PbTe NCs is presented. Results and Discussion PbTe nanocrystals capped with oleate were prepared by literature routes.26 It was possible to tune the shape of the PbTe NCs kinetically by adjusting the annealing temperature, leading to cubes at 170°C and cuboctahedra at 150°C, both with an average side length of ~14 nm (Figure 1A, 1C). The resulting PXRD patterns of PbTe nanocrystals match the reference for ICDD-PDF# 38-1435 of Altaite-PbTe, the face-centered cubic (fcc) rock salt structure type (Figure 2). TEM-EDS analysis was carried out to study the chemical composition of PbTe nanocrystals and in the resulting data, the average atomic composition ratios of Pb and Te for cubes and cuboctahedron are Pbcube = 49.6%, Tecube = 50.4% and Pbcuboctahedra = 50.2%, Tecuboctahedra = 49.8% respectively, yielding a ratio (Pb/Te) ~1. In order to exchange the oleate ligands for chalcogenide or chalcogenidometallate ligands, standard phase transfer procedures were used.27 Briefly, oleate-capped PbTe nanoparticles in hexane were reacted with a formamide (FA) solution of Na2S or Na4SnS4. The reaction occurs rapidly (over minutes) as the sterically stabilized oleatecapped nanoparticles move from the hexane phase (top layer) into the FA phase (bottom layer), where electrostatically stabilizing chalcogenide or chalcogenidometallate ligands are favored.28 TEM data suggest the NCs sizes are unchanged after the ligand exchange with inorganic anions


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(as shown for Na2S in Figure 1B, 1D, S1), although the facets become less distinct and the ligand-exchanged samples appear to be more aggregated, perhaps suggesting that the electrostatic repulsion in these samples is not as effective as the steric repulsion provided by oleate. However, TEM-EDS analysis data for cuboctahedron NCs reflected a decrease in the Te atomic % relative to Pb atomic % (Pb/Te = 1.52 – 3.15) whereas the ratio for the cubes remained at ~1 (Figure 2). We hypothesized that the increased Pb: Te ratio was a consequence of Te2replacement by S2- in PbTe cuboctahedra. ICP-MS elemental analysis done on the supernatant from the ligand exchange revealed a significant quantity of solubilized Te in the cuboctahedron supernatant (presumably as Te2-) relative to that of the cubes (by a factor of 3 to 11, Table S1), supporting this assumption. Moreover, the solubilized Te2- was found to increase with the increase of Na2S: PbTe molar ratio, correlating to the decrease in Te observed in EDS data of cuboctahedra (Figure 2) and suggesting that the increasing sulfide drives the equilibrium forward, liberating telluride. In cubes and cuboctahedra, the solubilized Pb2+ concentration is relatively small compared to the Te2- concentration (less than 1/100th), suggesting Pb2+ is retained throughout the ligand-exchange (or ion-exchange) process. Note that 4-14 at% Na+ is also retained in the product, presumably balancing excess negative charge conferred on the particles, either as counterions or p-type dopants29 (Figure 2).


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Figure 1. TEM images of (A) cube-shaped (C) cuboctahedron-shaped oleate-capped PbTe NCs and, (B) cube-shaped (D) cuboctahedron-shaped PbTe NCs ligand exchanged with Na2S (molar ratio of Na2S: PbTe = 5: 1). PXRD data further validate the shape dependent ion-exchange process. The PXRD data for cubes (Figure 2, S2) show that there is no significant change in the crystal structure of the NCs after ligand exchange with Na2S or Na4SnS4, consistent with a simple exchange process. In contrast, PXRD results for cuboctahedron NCs after the same treatment demonstrate the coexistence of both PbS and PbTe crystalline phases. The intensity of PbS peaks (111, 200, 220 and 311) relative to PbTe increases with the increase in molar ratio of Na2S or Na4SnS4 to PbTe.


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Figure 2. TEM-EDS atomic % and PXRD data for (A) cube-shaped and (B) cuboctahedronshaped oleate-capped PbTe NCs before and after ligand exchange with different molar ratios of Na2S. The reference patterns for PbS (JCPD: 05-592) and PbTe (JCPD: 38-1435) are shown.


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While anion-exchange is not common in this system, it is also not unprecedented. For example, reactions of PbS with Te2- or Bi2S3 with Sb2Te3 MCC are shown to result in composite9,29 formation (PbS@PbTe) or fully anion exchanged solid solutions ((Bi,Sb)2Te3), respectively, albeit at high processing temperatures. It is worth noting that the particles in question were all quasi-spherical. While the anion exchange of S2- with Te2- is relatively straightforward to express, the question of how S2- is liberated from SnS44- and the fate of Sn is more complex. Kanatzidis has reported an equilibrium between SnS44- and Sn2S64- in FA: 2 SnS44-→ Sn2S64- + 2S2-.21 Thus, it seems likely that when Na4SnS4 is dissolved in FA, the sulfide ions participate in ion-exchange, while the Sn2S64- ions act as capping ligands. This scenario is reasonably well reflected in the EDS data (see Table S2). Averaging the two data sets for cuboctahedra leads to a formula: PbTe0.75S0.25 + ~0.1Na4Sn2S6. Looking instead at cubes where Pb: Te is about 1:1, the ratio of S: Sn is about 3:1, also consistent with Sn2S64- as the dominant species associated with the particles. One major difference between the two cases is the fact that the cubes have a lot more Sn2S64- (3.5x the amount in cuboctahedra) but very little Na+. This may be due to differences in washing strategies, dopant incorporation, or in the native electrostatic characteristics of the anion-exchanged surface vs. the native surface. We note that we cannot rule out some alloying of Sn into PbTe; however, given the similar reactivity of the thiostannate with PbTe to the sulfide, we think that anion-exchange is the dominant reaction. To understand how the morphology and composition are affected by the exchange treatment, EDS line scanning data were collected on individual cube and cuboctahedron NCs treated with Na2S (Figure 3). The z-contrast line scanning profiles across the cubes show a core-shell structure where Pb, Te and S are distributed uniformly within the core but Pb and Te are deficient at the surface (Figure 3A). The surface bound S2- content contributes to the relatively


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high S counts across the NC. In the cuboctahedron, the presence of Pb and S over the entire width of the particle is clearly shown, whereas Te is concentrated in the center of the particle. This suggests that the Te at the surface has been leached away, resulting in individual particles consisting of a PbTe core and PbS polycrystalline shell structure (Figure 3B). Such phase separation is not unexpected as PbS, while also fcc, nevertheless has little solubility in PbTe.12,13

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Figure 3. Z-contrast line scanning images of (A) cube-shaped and (B) cuboctahedron-shaped PbTe NCs ligand exchanged with Na2S (molar ratio of Na2S: PbTe = 5: 1).

High resolution TEM (HRTEM) images of cube-shaped NCs after ligand exchange with S2reveal the amorphous structure of the shell and the single crystal structure of the cubic PbTe core


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with a lattice d-spacing ~ 0.323 nm corresponding to the {200} lattice plane of PbTe (Figure 4A). The crystalline product from the exchange of cuboctahedron NCs (Figure 4B) shows multiple crystalline interfaces with fringes corresponding to {200} planes of PbTe phase (d lattice spacing = 0.323 nm), {111} planes of PbTe phase (d lattice spacing = 0.373 nm) and the {200} planes of PbS phase (d lattice spacing = 0.297 nm).30 In cuboctahedron NCs, the lattice distance mismatch between PbTe and PbS phases with random relative orientations of superimposed lattices gives rise to Moiré patterns in the HRTEM, as observed in Figure 4B. The line scanning data and the presence of Moiré fringes at the crystalline PbS/PbTe boundaries provide strong evidence for the presence of two distinct crystalline phases.12,30

Figure 4. Atomic resolution HAADF-STEM images showing lattice spacing of (A) cube-shaped and (B) cuboctahedron-shaped PbTe NCs ligand exchanged with Na2S (molar ratio of Na2S: PbTe = 5: 1).


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Table 1. Calculated Scherrer crystallite sizes for cube-shaped and cuboctahedron-shaped PbTe NCs before and after ligand exchange with Na2S. This data is reproduced graphically in Figure S4.

Analysis of Scherrer crystallite size from PXRD data provides insight into the mechanism of exchange (Table 1, Figure S4). As the Na2S: PbTe molar ratio is increased (up to 10) a pronounced decrease in the Scherrer size for the {222} reflections (from 12.6 - 2.2 nm) of cuboctahedra PbTe NCs is observed, while the {200} and {220} reflections are minimally impacted (1-2 nm decrease in crystallite size), implying that the etching is occurring preferentially along the direction. Concomitant with the etching of PbTe, Scherrer size calculations on PbS crystallites formed from PbTe cuboctahedra show an increase from 0 to ca 6 nm as the Na2S: PbTe ratio is increased to 10: 1, and there doesn’t appear to be a preferential growth direction (PbS {111}, {220} and {200} reflections all yield similar Scherrer sizes). Intriguingly, while PbS crystallites are not detected by PXRD from ligand exchange of cubeshaped PbTe NCs, for the highest (10:1) Na2S: PbTe ratio, a decrease is observed in the Scherrer size of the {222} PbTe reflection (from 13.7 to 8.6 nm) whereas the {220} and {200} reflections are (again) minimally effected. This suggests that the edges and faces of the cubes are resistant to


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ion-exchange, and the ion-exchange that does occur happens along the corner of the cubes (the direction).31,32 In comparing a cube and a cuboctahedron with equal facet edge-lengths, the cube has a larger surface-to-volume ratio by a factor of 1.5; thus, based only on surface area, cubes would be expected to be more active than cuboctahedra. The fact that the cube corners have overall a minimal surface area relative to edges and faces means that the etching process is significantly hampered, enabling ligand exchange to dominate. In general, anion-exchange is much less common than cation exchange because the larger size of the ion results in lower mobility, requiring higher activation energies. In order to probe whether the facets of fcc PbTe also control the more facile cation exchange process, PbTe cubes and cuboctahedron were reacted with AgNO3 at room temperature. TEM-EDS (Figure 5A, S5) and PXRD results (Figure 5B, 5C) reveal that only the cuboctahedron-shaped NCs undergo evident ion-exchange reactions at room temperature, validating the assumption of preferential ion diffusion kinetics along the directions. Our data are consistent with previous literature on ion-exchange of fcc NCs. For example, Cd2+ exchange of cubic PbSe nanorods is purported to occur by a temperature-activated layer-by-layer replacement of cations at the {111} facets via a vacancy-assisted cation diffusion mechanism.33 Similarly, a study of CdTe/PbTe core/shell formation by cation exchange also favored the direction, leading to octahedral PbTe core particles embedded within CdTe cubes.20 With respect to anion-exchange, the room-temperature conversion of PbTe to amorphous Pb(OH)2 is observed to initiate from the corners of the PbTe nanocubes, leading to spherical core particles within a hydroxide cube over a period of several weeks.34 The fact that ion-exchange processes in fcc cube-shaped NCs requires high-temperature activation or long reaction times is a function of the low surface area of reactive {111} facets. The kinetic barrier for exchange can be reduced by


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favoring growth of {111} facets in the initial particles, thereby enabling rapid exchange at room temperature.34

Figure 5. (A) TEM-EDS atomic % for cube-shaped and cuboctahedron-shaped PbTe NCs cation exchanged with AgNO3. PXRD data for (B) cube-shaped and (C) cuboctahedron-shaped PbTe NCs before and after cation exchange with different molar ratios of AgNO3. The reference patterns for PbTe (JCPD: 38-1435) and Ag2Te (JCPD: 81-1820) are shown.


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The observed shape-dependent ion diffusion can be explained by considering the atomic stacking order of the facets and referring to available density functional calculations on surface models. As illustrated in Figure 6A, {100} facets comprise a square array of alternating cations and anions that stack in an alternating fashion, making the facet neutral, and each surface atom has a single dangling bond. In contrast, {111} facets are polar, comprising a hexagonal array of either cations or anions (Figure 6B), and surface atoms give rise to three dangling bonds.34-36 The density of surface dangling bonds is directly related to the surface energy, which explains the higher surface energy available on the {111} facet compared to {100} facet.37 In surface ion diffusion, the transport rates directly relate to the available surface energy. Thus, while the {100} square atomic arrangement is more open, it is the {111} facets that have higher diffusion rates. We

surmise

that

surface

Pb2+

cations

act

as

adatoms

to

coordinate

with

chalcogenide/chalcogenidometallate anions and the higher surface energy (higher diffusion rates) available on the {111} facet can easily overcome the potential energy barrier to drive the reaction forward, PbTe + S2-↔ PbS + Te2-. Thus, as shown in Figure 6C, where {111} facets are prominent (i.e., in cuboctahedra) diffusion is facile, even at room temperature. The diffused Te ions are solubilized (Table S2) while Pb2+ combines with the diffused S2- to form PbS polycrystalline structures that minimize the facet energy and stabilize the system thermodynamically.38 Thus, the {111} facets act like a channel for the ion diffusion process whereas the {100} facets act as a barrier.


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B

C

Figure 6. Face centered cubic (fcc) unit cell (A) projection on {100} facets (B) hexagonal close packed arrangement of cations or anions on the {111} planes. (C) Schematic diagram representing ion diffusion along facets of cube-shaped and cuboctahedron-shaped PbTe NCs. Conclusions: Through morphological control, it is possible to drive the reactivity of commonly-employed chalcogenide/chalcometallate ligands to achieve either the expected surface-limited ligandexchange of metal chalcogenide nanoparticles, or bulk-phase ion-exchange. For fcc PbTe, oleatecapped nanoparticles of cube morphology undergo simple ligand exchange when treated with


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Na2S at room temperature due to the low surface energy of the neutral {100} facets, whereas cuboctahedra are transformed into PbTe/PbS composite nanoparticles by a process of ion exchange, facilitated by the presence of high surface-energy {111} facets. Similar reactivity is observed for cation exchange processes (Ag+). This work contributes to our understanding of the role of facets in dictating and limiting reactivity, providing an interesting gateway for targeting metastable phases and controlling facet-dictated catalytic processes. Future work will focus on understanding the mechanism by which ions are transported along the “superhighway” employing computation and atomic-resolution HAADF-STEM. Methods Chemicals: Lead acetate trihydrate (Pb(OAc)2·3H2O, Baker chemicals), tellurium powder (Te, 200 mesh, 99.8%), 1-octadecene (ODE, technical grade, 90%, Aldrich) oleic acid (OA, technical grade, 90%, Aldrich), trioctylphosphine (TOP, technical grade, 90%), sodium sulfide (Na2S·9H2O, technical grade, 98%, Aldrich), tin(IV) chloride (SnCl4·5H2O, technical grade, 98%, Aldrich), ethanol (EtOH, 200 proof), formamide (FA, technical grade, 98%, Aldrich), acetonitrile (anhydrous, technical grade, 99.8%, Aldrich), and hexane (anhydrous, technical grade, 95%, Aldrich). All chemicals were used as received. Synthesis of Na4SnS4 MCC The metal chalcogenide salt, Na4SnS4 was freshly prepared according to a procedure developed by the Kanatzidis group.14,39 Synthesis of oleate-capped PbTe nanocrystals


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PbTe nanocrystals capped with oleate were prepared similarly to a previous report with a slight modification of heating temperatures.26 A mixture of Pb(OAc)2⋅3H2O, OA and ODE were heated at 170°C for 30 min in an inert atmosphere, followed by rapid injection of TOPTe as a Te precursor at 170°C for cubes and 150°C for cuboctahedra and annealing for 10 minutes. Isolation was achieved by centrifugation, adding hexane first as the solvent and acetone as the antisolvent, then drying under vacuum for 24 hours. The samples were stored under a N2 atmosphere. Exchange of oleate ligands with S2- and SnS44Ligand-exchange was performed according to a previously reported protocol.8,9,17 The oleatecapped PbTe crystals were dispersed in hexane and combined with either S2- or SnS44- in formamide and stirred vigorously for 30 min. The molar ratios between Na2S/Na4SnS4 and PbTe were systematically changed to Na2S/Na4SnS4: PbTe = 3:1, 5:1, 7:1 and 10:1. The moles of PbTe were estimated from the measured weight of dried oleate-capped PbTe NCs and the molar mass of bulk PbTe (334.8 g/mol), neglecting the contribution from organic ligands. Cation exchange of PbTe cube- and cuboctahedron-shaped NCs with AgNO3 The oleate-capped PbTe crystals were dispersed in hexane and combined with AgNO3 in methanol at room temperature and stirred vigorously for 30 min. The molar ratios between AgNO3 and PbTe were systematically changed to AgNO3: PbTe = 5:1, 10:1 and 20:1. Moles of PbTe were calculated as described above. Structural characterization The resultant materials were characterized by powder x-ray diffraction (PXRD), transmission electron microscopy/energy dispersive x-ray spectroscopy (TEM/EDS), high-angle annular dark


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field scanning transmission electron microscopy (HAADF-STEM) and inductively coupled plasma mass spectrometry (ICP-MS). PXRD data in the 2-theta range of 20-70 were collected on a Rigaku RU 200B (40 kV, 150 mW, Cu K alpha radiation) or a Bruker Phaser II (LYNXEYETM Detector, 30 kV, 10 mA, Cu K alpha radiation) diffractometer and processed with Jade software, comparing patterns to PDF files in the International Center for Diffraction Data Database (ICDD). TEM images and EDS data were collected on a JOEL 2010 transmission electron microscope (200 kV, 108 µA, LaB6 filament gun) equipped with an energy dispersive x-ray detector (EDAX Inc.) and Amtv600 software (Advanced Microscopy Techniques Corp.). Specimens were prepared by placing a drop of sample solution onto a carbon coated 200 mesh Cu grid followed by air drying. The HAADF STEM and ChemiSTEM EDS line scan/mapping studies were carried out in a FEI Titan 80-300 probe aberration corrected STEM operated at 200 kV. The convergence semi-angle was 20 mrad. The probe current was set at about 60 pA to avoid obvious damage during EDS acquisition. ICP-MS analysis was performed using an Agilent 7700x ICP-MS. The samples were made up in conc. nitric acid and diluted with deionized water to achieve quantification ranges ~ 100 ppb level. The instrument was calibrated using a series of standard reference solutions. The samples were introduced directly into the ICP-MS system using a standard peristaltic pump with Tygon pump tubing (internal diameter of 1.02 mm), and ASX-520 autosampler. Supporting information available: Size histograms; ICP-MS analysis of supernatants; PXRD data for PbTe NC’s ligand exchanged with Na4SnS4; elemental analysis for PbTe NCs ligand


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When Ligand Exchange Leads to Ion Exchange: Nanocrystal Facets

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