Cation Exchange Combined with Kirkendall Effect in the Preparation

Letter pubs.acs.org/JPCL

Cation Exchange Combined with Kirkendall Effect in the Preparation of SnTe/CdTe and CdTe/SnTe Core/Shell Nanocrystals Youngjin Jang,† Diana Yanover,† Richard Karel Č apek,† Arthur Shapiro,† Nathan Grumbach,† Yaron Kauffmann,‡ Aldona Sashchiuk,† and Efrat Lifshitz*,† †

Schulich Faculty of Chemistry, Solid State Institute, Russell Berrie Nanotechnology Institute, Nancy and Stephen Grand Technion Energy Program, and ‡Department of Materials Science and Engineering, Technion−Israel Institute of Technology, Haifa 3200003, Israel S Supporting Information *

ABSTRACT: Controlling the synthesis of narrow band gap semiconductor nanocrystals (NCs) with a high-quality surface is of prime importance for scientific and technological interests. This Letter presents facile solutionphase syntheses of SnTe NCs and their corresponding core/shell heterostructures. Here, we synthesized monodisperse and highly crystalline SnTe NCs by employing an inexpensive, nontoxic precursor, SnCl2, the reactivity of which was enhanced by adding a reducing agent, 1,2hexadecanediol. Moreover, we developed a synthesis procedure for the formation of SnTe-based core/shell NCs by combining the cation exchange and the Kirkendall effect. The cation exchange of Sn2+ by Cd2+ at the surface allowed primarily the formation of SnTe/CdTe core/shell NCs. Further continuation of the reaction promoted an intensive diffusion of the Cd2+ ions, which via the Kirkendall effect led to the formation of the inverted CdTe/SnTe core/shell NCs.

N

thermoelectric performance of SnTe by introducing a dopant, suggesting an alternative material for PbTe-based material.19 Previous reports described a successful preparation of SnS,15,20−24 SnS2,24 SnSe,16,18,20,24−26 SnSe2,26,27 and SnSxSe1−x NCs. 24 However, achievements in the preparation of monodisperse SnTe NCs14,17,28 were deferred by difficulties in controlling their chemical stability. The pioneer reports suggested the use of bis[bis(trimethylsilyl)amino]tin(II) as a highly reactive precursor,14,16,21 although this precursor is considered to be relatively expensive, toxic, and air sensitive. Thus, the replacement of bis[bis(trimethylsilyl)amino]tin(II) precursor is important for practical applications. Furthermore, surface properties of Sn-based NCs have a large influence on the properties of the materials; the development of core/shell NCs should provide a high degree of surface passivation, as well as band gap engineering. The present work describes a facile colloidal synthesis of high-quality SnTe NCs by using an inexpensive, nontoxic, and stable precursor, SnCl2, assisted by reducing agents. Moreover, SnTe-based core/shell heterostructures were developed, using a cation exchange procedure combined with the Kirkendall effect. The Sn2+ to Cd2+ cation exchange at the surface generated primarily SnTe/CdTe core/shell NCs. Further continuation of the reaction promoted intensive diffusion of the Cd2+ ions, which, via the Kirkendall effect, led to the formation of the inverted CdTe/SnTe core/shell NCs. To the best of our

arrow band gap IV−VI colloidal nanocrystals (NCs) attract special scientific and technological interest because of their tunable optoelectronic properties, showing promising implementation in photovoltaic devices, field-effect transistors, thermoelectric systems, near-infrared (NIR) detectors, and biological applications.1−6 Among the IV−VI NCs, the lead chalcogenides have been studied most extensively and show a wide absorption profile from NIR to visible range, large exciton Bohr radius (PbS, 18 nm; PbSe, 46 nm), high dielectric constant, and small effective masses, presenting distinctive physical properties, e.g., multiple exciton generation;7−10 the entire properties are a benefit in various applications.3,11,12 However, in the long-run, the use of heavy metal compounds should be avoided. Recently, Sn chalcogenides (SnX; X = S, Se, and Te) have been considered as promising alternatives for narrow band gap materials with low toxicity and earth-abundance.13 The bulk SnTe is a direct band gap semiconductor (0.18 eV at 300 K) with a cubic crystal structure,14 while SnSe and SnS possess orthorhombic structure and show indirect band gap properties.15,16 Bulk SnTe is also characterized by an exceptionally large exciton Bohr radius (95 nm) and small carrier effective mass (∼0.025 m 0 ).17 Accordingly, SnTe NCs are expected to offer NIR optical activity, strong quantum size effects, and long-range charge hopping in their ensemble form,14,17 thus being most suitable for implementation in technologies such as photovoltaic and thermoelectric applications. For example, Brutchey and coworkers showed the enhanced short-circuit current density and power conversion efficiency values of solar cells using SnSe NCs.18 Recently, the Kanatzidis group demonstrated the high © XXXX American Chemical Society

Received: May 10, 2016 Accepted: June 22, 2016

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The Journal of Physical Chemistry Letters knowledge, this is the first report of the synthesis of these SnTe-based core/shell heterostructures. The colloidal SnTe NCs were prepared by a hot-injection process. The Sn precursor consisted of SnCl2 dissolved in oleylamine (OLA). The Te precursor solution was prepared by dissolving tellurium (Te) powder and trioctylphosphine (TOP) in OLA, forming TOP:Te. Two different strategies were used to enhance the reaction rate of the precursors: (1) 1,2hexadecanediol (HDD)29,30 was added as monomer formation rate-enhancing agent. On the example of lead chalcogenide synthesis, it was discussed that it supports lead chalcogenide formation via the reduction of Pb2+ to Pb0.30 (2) Alternatively, the TOP:Te precursor was activated by the addition of a secondary phosphine, diphenylphosphine (DPP).31−34 It was shown for PbSe and CdSe synthesis that the addition of secondary phosphines to tertiary phosphine chalcogenides leads to the formation of highly reactive secondary phosphine selenides, allowing a strong enhancement of the monomer formation rate.33,34 In both cases, the Te precursor solution was injected into the Sn solution under inert gas conditions, and the reaction took place either at 180 °C when using mild reactive HDD agent, or at 80 °C when using the strongly reactive DPP agent. Temperature quenching to room temperature, viz., the termination of the reaction, was followed by a surface ligand exchange from OLA to oleic acid by mixing the reactants with a toluene/oleic acid. Previous studies showed that alkylamine (e.g., oleylamine) is weakly bound on the surface of nanocrystals, resulting in the coalescence after a long time.14,16 However, oleic acid is known to be a good capping agent when forming metal−oxygen bonds, thus rendering stability of the nanocrystals’ surfaces.14 After centrifugation and the decanting of the supernatant solution, the precipitated NCs were rinsed with toluene/acetone solution and redispersed in hexane, toluene, or CCl4 storage solutions. More specific details regarding the synthesis of core SnTe NCs are given in the Supporting Information. Figure 1A shows a representative transmission electron microscopy (TEM) image of as-synthesized SnTe NCs based on a reaction induced by the presence of HDD. The figure displays NCs with nearly cubic shape and a mean diameter size of 12.8 ± 0.7 nm. The corresponding size distribution histogram is given in Figure S1, suggesting a size dispersion of 5.7%. TEM measurements of SnTe NCs prepared in the presence of DPP (without HDD) under conditions similar to those shown in Figure 1A show an average size of 8.2 ± 0.5 nm (see Figure S2). Figure 1B presents a high-resolution TEM (HRTEM) image of a NC from Figure 1A, resolving atomic lattice fringes that match the (200) lattice planes of rock-salt SnTe, with a mutual spacing of 0.315 nm (see the mark in Figure 1B). Energy-dispersive X-ray (EDX) analysis of the NCs from Figure 1A is shown in Figure S3, indicating a slight Sndeficient Sn:Te stoichiometric ratio (0.94:1). The crystal structure determination was further supported by performing X-ray diffraction (XRD) analysis. The dominant peaks of the X-ray pattern (see Figure 1C) are assigned to lattice planes of a cubic rock-salt crystal structure of bulk SnTe (space group Fm3m, a = 6.304 Å, JCPDS no. 65-0322) related to the SnTe NCs, while the weak peaks are related to TeO2, which more likely located at the exterior surfaces. The crystallite size is determined to be 12.2 nm when using the Scherrer equation, in agreement with the result obtained from the TEM analysis.

Figure 1. Characteristics of the 12.8 nm SnTe NCs. (A) TEM image. (B) HRTEM image. (C) XRD pattern. Vertical lines indicate the position and relative intensity of diffraction peaks for bulk SnTe. (D) Absorption spectrum (Inset: FT-IR spectrum of SnTe NCs).

The absorption spectrum of the SnTe NCs is displayed in Figure 1D, showing a tail in the NIR spectral regime and a deep broad band extending to the short-wave infrared (SWIR) region. The SWIR band also appeared in a Fourier transform infrared spectrum, as shown in the inset. The overlaying sharp bands in the inset were identified with vibrational modes of the surfactant oleic acid molecules. The absorption tail was fitted to a curve proportional to (αhυ)2, specifying the existence of an extremely strong size confinement, based on a large Bohr radius of an electron−hole pair (95 nm) that is photogenerated across a band gap. The origin of the SWIR broad band either may be related to a sub-band gap transition into unintentional doping states or designates a plasmon band. On the basis of the apparent nonstoichiometry, a p-type doping is predicted. Considering the size range under investigation, the cubic structure of the NCs as well as the 6% Sn atoms deficiency, a doping level of ∼9.6 × 1020 cm−3 was evaluated (calculation details are given in the Supporting Information), in agreement with the values of previous reports.19,35−37 Such a level of doping induced by nonstoichiometry was shown before in GeTe NCs,38 copper chalcogenides,39−44 and metal-oxides materials.45−47 The localized surface plasmon resonances (LSPR) frequency (ωsp) was estimated using the following equation:39,48 ωsp =

ωp2 1 + 2εm

− γ2 (1)

Here, ωp is the bulk plasma frequency, read as ωp =

Nhe 2 ε0mh

(2)

where Nh is carrier concentration, ε0 free space permittivity, mh the hole effective mass, εm dielectric constant of the solvent, and γ the damping parameter. Using the calculated Nh value (∼9.6 × 1020 cm−3), the LSPR energy was estimated as ∼0.31 2603

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in the Supporting Information.) At the end of the process, oleic acid was added into the reaction mixture for the exchange of the surface ligands. Figure 2A displays a HRTEM image of SnTe/CdTe core/ shell NCs obtained when the reaction temperature reached 180

eV, being close to band center energy appearing in the Fourier transform infrared (FT-IR) spectrum (Figure 1D). The occurrence of a plasmonic effect in SnTe NCs could not be concluded from the current experimental evidence and thus should be further verified in the future. The SnTe NCs are further characterized by Raman spectroscopy (see Figure 3B). Two distinct peaks in the Raman spectrum of the SnTe NCs are assigned to the longitudinal optical (LO) and transversal optical (TO) modes of SnTe,49,50 which are red-shifted from the ones found in bulk SnTe (LO mode at 131 cm−1 and TO mode at 147 cm−1).51,52 Energy shift can be related to a phonon size confinement.53−55 X-ray photoelectron spectroscopy (XPS) measurements provided the chemical composition of SnTe core NCs. Representative XPS spectra are presented in Figure S4, showing the binding energies of Sn 3d5/2 and Sn 3d3/2, as well as those of Te 3d5/2 and 3d3/2 at two different energies, corresponding to the existence of Te2− and Te4+ (40.5%) more likely at the exterior surfaces. Te4+ may be associated with the formation of TeO2, in agreement with the evidence shown in Figure 1C, as well as with observation shown in a few other cases.56,57 The results expose the main drawback of SnTe core NCs, with a large tendency for oxidation of the Te atoms at the exterior surfaces. Recent observation gained from 119Sn−Mössbauer spectroscopy revealed the oxidation of Sn ions in SnTe NCs, further expressing the propensity for oxidation of the SnTe compound.58 Hence, the current study was motivated by the interest in coating SnTe core NCs with an inorganic shell as a chemical protection59 in order to isolate them from direct contact with the surroundings, assisting in mitigating the oversensitivity of SnTe surfaces to oxidation. Efforts in forming SnTe/shell heterostructures included a few different attempts, starting from utilization of SnS and SnSe as shell constituents deposited directly onto the core surface. The first attempts failed because of the large crystallographic mismatch between the semiconductors, with SnTe being a cubic structure and the candidate shells having orthorhombic unit cells. Alternatively, shell formation via a cation exchange process60,61 was considered, involving substitution of metal cores with new cations penetrating from the outer facets toward the inner area. This method permitted the growth of II−VI semiconductor shells onto IV−VI semiconductor cores in a way that could not be obtained by a direct deposition. Cation exchange is already widely used in the synthesis of various nanocrystals and core/shell heterostructures.60−69 The current project developed the growth of CdTe shell onto SnTe NCs cores. This pair of components was selected because of the similarity of their crystallographic parameters: CdTe and SnTe possess a zinc blende and rock-salt crystal structure, respectively, both having a cubic crystal arrangement; the lattice parameter in CdTe is 6.48 Å69 and in SnTe, 6.30 Å; hence, there is a small lattice mismatch (1 h) at 180 °C, as seen in Figure 2E. A continuation of the reaction eventually generates a complete penetration of the Cd-cations into the core regime, and at the same time, the Sn-cations occupied the shell regime (see marks in Figure 2E). The elemental line profile by HAADF-STEM analysis of an inverted core/shell NC is presented in Figure 2F, showing a Sn-rich shell and a core dominated by Cd atoms. In other words, the reaction was terminated by the formation of the inverted CdTe/SnTe core/shell heterostructure. Further characterization proofs of the core/shell NCs composition, as well as the explanation about a transfer from the straight to the inverted core/shell heterostructure, are elaborated below. The XRD patterns (see Figure 3A) show several peaks corresponding to a rock-salt SnTe structure (also see Figure

⎛ −E ⎞ ⎟ D = D0exp⎜ ⎝ kT ⎠ Figure 3. Optical and structural characterizations of SnTe/CdTe and CdTe/SnTe core/shell NCs. (A) XRD patterns. (B) Raman spectra. (C) Cd 3d XPS spectrum. (D) Absorption spectra.

(4)

D0 is the pre-exponential factor, and E is the activation energy as appearing in refs 81 and 82. Based on the literature values, the diffusion coefficients of Sn and Cd at 180 °C in the NCs studied were calculated to be ∼3.4 × 102 and ∼8.2 × 103 nm2 s−1, respectively. The diffusion distance was estimated by the root-mean-square distance (x) as given by eq 5:83

1C), as well as distinguished peaks associated with a zinc blende CdTe component (see vertical lines in Figure 3A). The Raman spectra (see Figure 3B) are dominated by the LO modes of both SnTe and CdTe, while the TO frequencies basically overlap each other, so that the individual contributions cannot be resolved (see labels and vertical lines in Figure 3B). Interestingly, the mentioned LO and TO modes of SnTe in heterostructures are slightly shifted to higher wavenumbers with respect to those in SnTe NCs,72,73 presumably due to strain induced by the lattice mismatch.72 The XPS spectra (see Figure S7 and Figure 3C) also supplied evidence for the existence of Cd 3d in both the straight and the inverted core/ shell heterostructures. Furthermore, signatures of Te4+ in the XPS spectrum of SnTe/CdTe core/shell NCs nearly disappeared, verifying a surface passivation owing to a CdTe

x=

2Dt

(5)

Accordingly, the anticipated diffusion distances of Sn and Cd ions were found to be ∼2 × 102 and ∼1 × 103 nm after 1 min at the elevated temperature of 180 °C, respectively, distances that are much larger than the typical size of the NCs investigated. As noted, an extensive diffusion process actually was ignited around the reaction temperature of 180 °C, correlated with a temperature dependence dictated by the Arrhenius relation (eq 4).68,84 Numerical evaluation of the diffusion distances further supports the occurrence of extensive diffusion processes which follows the exchange stage. Moreover, qualitative considerations using interface strain, solubility 2605

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elements across the NCs’ volume, i.e., outward migration of Sncation via bridging paths and simultaneous diffusion of Cdcations toward the core center. The final stage of the synthesis produced a well-ordered core/shell structure including inversion of distribution of the cationic elements, viz., formation of CdTe/SnTe core/shell NCs. The chain of events was confirmed by following the HRTEM images and the corresponding HAADF-STEM analysis, as well as use of other conventional tools (e.g., XPS, Raman spectroscopy). The study showed intriguing phenomena originating from a combination of cation exchange and occurrence of the Kirkendall effect. Importantly, the study showed, for the first time, the generation of Sn-based core/shell semiconductor materials with significant importance for applications in devices operating in the NIR and SWIR optical regimes. CdTe shell was used here as a prototype case, while effort to replace it with a nontoxic component is presently under consideration. Also, our observation of the reorganization of the core−shell structure gives new insight about core−shell structures in general, showing that the structures are highly dynamic, allowing a thermodynamically driven reorganization of the core−shell structure, and thus are highly relevant for the synthesis of core−shell structures.

product, and surface energy suggest that forming inverted core/ shell NCs is thermodynamically favored. More detailed discussions are given in the Supporting Information. The drastic crystal transformations shown in Figure 2E,F involved an approximate retention of the exterior NCs’ diameter; nonetheless, internal rearrangement of elemental distribution occurs via a diffusion process of cations with exceptionally large diffusion length compared to the NCs’ size. The inversion of the core and shell elemental composition resembles a process controlled by a Kirkendall effect. A typical Kirkendall process involves outward diffusion of core elements (e.g., cations) that is faster than the inward diffusion of secondary elements (e.g., anions), mostly leading to formation of hollow structures (e.g., Cu2O,85 CoO(S)86,87). A modified effect occurs in the presence of a third element of a large diffusion coefficient that quickly diffuses into the hollow void and inverts the elemental distribution (e.g., Au/InAs83). The case shown in Figure 2 can be associated with a modified Kirkendall effect, where the fast diffusion rate of the Cd-cations permits their penetration into the core region at the temperatures and reaction duration discussed, while the Sncations moved outward in a way that generated a shell component. It is worth noting that the liability of the Te-anions from one region to another is undistinguishable because of their extremely slow diffusion rate (DTe = 5.7 × 10−4 nm2s−1). In addition, the bridge structure seen in Figure 2C displays the Sncation diffusion paths. A similar picture has been reported elsewhere.76,86,87 Figure 4 displays a schematic drawing of the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00995. Experimental details, calculation of p-doping concentration, histogram of SnTe NC size distribution, TEM and HRTEM images, EDX and XPS spectra, band diagram, and thermodynamic discussion (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Figure 4. Schematic illustration of the formation of SnTe-based core/ shell NCs via cation exchange reaction and the Kirkendall effect. A SnTe NC (first circle) is changed to a straight SnTe/CdTe core/shell NC (second circle) by cation exchange. Through an intermediate structure (third circle), an inverted CdTe/SnTe core/shell NC (fourth circle) is formed by the Kirkendall effect.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the Israel Council for High Education - Focal Area Technology (872967), the Volkswagen Stiftung (88116), the Israel of Defense (Project 4440665406), and the NiedersachsenDeutsche Technion Gesellschaft E.V (ZN2916).

stages leading to the formation of an inverted core/shell: the formation of a straight SnTe/CdTe core/shell structure via cation exchange reaction (second circle from left); generation of an intermediate structure composed of bridges for the transport of Sn-cations outward (third circle); and a complete occupation of the core region by Cd-cations (fourth circle), stemming from a Kirkendall effect with efficient cationic diffusion processes. As a whole, the sequence of events shown in the scheme leads to dramatic elemental redistribution across the entire NC volume, culminating in the conversion of straight SnTe/CdTe into inverted CdTe/SnTe core/shell heterostructures. In summary, a facile synthesis of SnTe colloidal NCs with unique use of a nontoxic, stable precursor, SnCl2, is reported. The SnTe NCs produced were used for the formation of core/ shell derivatives. Primarily, the work involved generating SnTe/ CdTe core/shell heterostructures via cation exchange procedure at mild conditions (at 180 °C with a reaction time of 10 min). Application of more rigorous conditions (long duration time up to 1 h) induced intermediate redistribution of the

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REFERENCES

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