Can the Morphology of Biconcave Metal Sulfide Nanoplatelets Be

Jun 28, 2019 - Cation exchange (CE) has become a central strategy for preparing many previously unavailable semiconductor nanocrystals (NCs). However ...
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Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

Can the Morphology of Biconcave Metal Sulfide Nanoplatelets Be Preserved during Cation Exchange? Yang Liu,† Chang-Keun Lim,‡ Zheng Fu,† Deqiang Yin,† and Mark T. Swihart*,†,‡,§ †

Department of Chemical and Biological Engineering, ‡Institute for Lasers, Photonics, and Biophotonics, and §RENEW Institute, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States

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S Supporting Information *

ABSTRACT: Cation exchange (CE) has become a central strategy for preparing many previously unavailable semiconductor nanocrystals (NCs). However, general understanding of mechanisms governing morphology preservation or evolution upon CE reaction remains limited. In this study, we carried out CE starting from biconcave Cu1.94S nanoplatelet (NPl) templates, whose relatively complex structure both provides opportunities to lower overall surface energy by shape evolution and facilitates observation of any shape evolution that occurs. From these templates, we prepared NCs of binary and ternary metal sulfides including CdS, ZnS, CuInS2, MnS, SnS, PbS, and SnS2. This study reveals that preservation of the template morphology depends upon compatibility of the anionic sublattices (hcp vs fcc) of the template and final product and produces biconcave NPls of several compositions for which this morphology has not previously been reported. In addition, we probed the morphology-dependent optical responses of different anisotropic CdS nanostructures produced by this approach, providing insight into the optical properties of a previously unreported morphology of CdS.

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chemical driving forces during the CE process.17−19 However, mechanisms of morphology transformation and retention have not been fully elucidated. For example, quasi-spherical, rodlike, and plate-like NCs have most often been used in CE reactions.12,20 Many prior studies focused on creating heterostructures via partial CE reaction and manufacturing new materials while inheriting the templates’ morphology and aspects of their crystal structure. Because of the anisotropy of the crystal lattice of Cu2−xS phases, rodlike and plate-like structures can be thermodynamically more stable than volumeequivalent spherical structures. Thermodynamically stable and highly symmetric morphologies are good for exploration of the fundamental theory of CE reactions because the effect of shape evolution (i.e., rearrangement of the atom distribution of the whole structure) on CE reactions is minimized. However, their simple isotropic or highly symmetric structures limit the possibility of further shape evolution. Starting from thermodynamically stable structures, some other metal sulfide (e.g., Au2S and PdS)15 domains, which adopt completely different crystal structures from the template copper sulfide phase, can co-exist with the host copper sulfide domain without any change in morphology during the CE process. Here, we perform CE reactions using biconcave plate-like templates due to their relatively complex, anisotropic, and low symmetric structure. Different cations were incorporated into the biconcave Cu2−xS nanoplatelets (NPls), yielding two

INTRODUCTION Cation exchange (CE) has emerged as an important means of producing semiconductor nanocrystals (NCs) with complex and tunable compositions, crystal phases, and morphologies. CE involves replacing cations of a starting NC with heterocations. CE processes often preserve the template morphology and anion sublattice; however, in some cases, they induce a structural reorganization.1−5 Cu2−xS NCs are an ideal template for CE reactions because Cu atoms are highly mobile at elevated temperatures, enabling efficient diffusion and substitution of incoming cations. Synthesis of semiconductor NCs with morphology-dependent optical, electrochemical, and magnetic properties is of great interest.6,7 New morphologies of metal sulfide NCs can expand the potential range of properties and applications of these materials. Cu2−xS nanomaterials, with many possible morphologies and cation compatibility of the sulfur sublattice, can serve as a CE template for metal sulfide NCs of previously unavailable morphologies.8−11 Ion exchange-induced shape evolution of NCs has attracted considerable attention.2,11−14 Within this growing field, several Cu2−xS templates were employed to design and create various novel heterogeneous nanostructures. Growing understanding of the chemistry (e.g., heterointerfaces, reactivity ranking, and ion diffusion) governing the CE reactions has allowed creation of new structures while retaining the morphology of the templates.15−17 Recent advances in understanding of CE reactions involving Cu2−xS NCs have revealed the basic mechanisms of activation of host (Cu+) ions, promotion and control of diffusion, replacement of guest cations, and other © XXXX American Chemical Society

Received: April 29, 2019 Revised: June 27, 2019 Published: June 28, 2019 A

DOI: 10.1021/acs.chemmater.9b01686 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

ambient temperature for 30 min before heating to 240 °C. Then, 8 mL of Cu1.94S NPl dispersion was injected into the mixture, and it was held at this temperature for 60 min. To collect the NPls, 10 mL of ethanol was added, followed by centrifuging at 4000 rpm for 1 min. Characterization. Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2010 at an accelerating voltage of 200 kV. High-resolution TEM (HRTEM) images were obtained using a FEI Tecnai G2 F30 at an accelerating voltage of 300 kV. High-angle annular dark-field scanning TEM (HAADF-STEM) images, HR-STEM images, and STEM energy-dispersive X-ray spectroscopy (STEM-EDS) maps were collected using a JEOL JEM=ARM200F STEM equipped with spherical aberration correctors on the image at an accelerating voltage of 200 kV. TEM grids were prepared by dropping a dilute NC dispersion onto a carbon-coated copper or nickel TEM grid and allowing the sample to dry under air. Size distributions were obtained from TEM images by measuring at least 100 NCs from multiple TEM images from different areas of the grid, using Nanomeasurer v.1.2. Powder X-ray diffraction (XRD) measurements were carried out using a Rigaku Ultima IV diffractometer with a Cu Kα X-ray source. Samples were prepared by drop-casting concentrated NC dispersions onto glass slides. Elemental analysis of NCs by energy-dispersive X-ray spectroscopy (EDS) was obtained using a Hitachi S-4800 within a Zeiss Auriga scanning electron microscope. We note that CE reaction usually yields products with a detectable signal of the host cation (here, Cu). The residual Cu content can be removed by extra washing of the product NCs. However, the extensive washing process can destabilize the colloidal dispersion of the NPls. To fully remove the residual Cu content, the NCs were washed three times by adding hexane and ethanol. Each time, 10 μL OAm and 10 μL TOP were added to maintain the colloidal stability. PL emission spectra were recorded by a Fluorolog-3.11 Jobin Yvon-HORIBA spectrofluorometer. The absolute PL quantum yield (QY) measurements were performed using a FLS980 fluorescence spectrometer at an excitation wavelength of 470 nm. The time-resolved PL experiments were conducted with the EasyLife fluorescence lifetime system (Photon Technology International, Birmingham, NJ) through a 560 nm long pass filter with a 370 nm excitation LED. Synchrotron-based small-angle X-ray scattering (SAXS) measurements were performed at beamline 1W2A of the Beijing Synchrotron Radiation Facility (BSRF) (λ = 1.54 Å) using a concentrated hexane dispersion (∼250 mM cation concentration) in a capillary. A standard silver behenate sample was used for the calibration of diffraction spacings. X-ray scattering intensity patterns were recorded during 30 s of exposure of the sample to the synchrotron beam. The X-ray powder diffraction intensity data were analyzed using the program Fit2D.

groups of contrasting results: morphology preservation (using Cd2+, Zn2+, Mn2+, and In3+) or shape evolution (using Sn2+, Pb2+, and Sn4+). The successful preservation of the biconcave morphology upon CE reaction showed that precise manipulation of NCs’ size, shape, composition, and internal structure is possible. Moreover, the biconcave CdS NCs possess different optical properties [e.g., photoluminescence (PL) emission efficiency and lifetime] compared with flat CdS NPls. These results shed light on mechanisms of solid−solid phase transformation and provide insight into composition-, morphology-, and crystal structure-dependent optical properties of semiconductor NCs.

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EXPERIMENTAL SECTION

Chemicals. All chemicals were used as received. Cadmium chloride (CdCl2, 99.99%), zinc chloride (ZnCl2, ≥97%), indium chloride (InCl3, 98%), tin(II) chloride (SnCl2, reagent grade, 98%), manganese(II) chloride (MnCl2, 99.999%), lead chloride (PbCl2, 98%), tin(IV) chloride (SnCl4, reagent grade, 98%), oleylamine (OAm, 70%), trioctylphosphine (TOP, 90%), and 1-dodecanethiol (DDT, ≥98%) were purchased from Sigma-Aldrich. Cu(NO3)2· 2.5H2O, toluene, and ammonium sulfide (AS, 21.2 wt % in water) were purchased from Fisher Scientific. Preparing CuS Templates. For the synthesis of hexagonal CuS NPls, 1.5 mmol of Cu(NO3)2·2.5H2O was dissolved in a mixture of 10 mL of OAm and 10 mL of toluene and then heated to 70 °C under an argon flow. AS solution (1.5 mL) (21.2% in water) was injected into Cu−OAm to form CuS NPls. After 2 h, 10 mL of ethanol was added to destabilize the NPl dispersion, followed by centrifugation at 4000 rpm for 1 min. To remove excess AS and OAm, the NCs were washed by precipitation upon addition of ethanol followed by redispersion in chloroform. This purification process was applied to all of the NPls described in the following sections. The NPls could be redispersed in organic solvents including chloroform, hexane, and toluene. Preparing CIS Templates. The CuS NPl colloid was prepared by dispersing dry CuS NPls (one batch, typically ∼230 mg) in 9 mL of OAm. A mixture of 0.5 mmol InCl3, 8 mL of OAm, and 2 mL of DDT was held under flowing argon at ambient temperature for 30 min before heating to 160 °C. Then, 3 mL of CuS NPl dispersion was injected into the mixture, and it was held at this temperature for 30 min. To collect the NPls, 10 mL of ethanol was added, followed by centrifuging at 4000 rpm for 1 min. Synthesis of Cu1.94S NPls. The CIS NPl colloid was prepared by dispersing dry CIS NPls (one batch, typically ∼70 mg) in 3 mL of OAm. A mixture of 1 mmol of CuCl and 10 mL of OAm was held under flowing argon at ambient temperature for 30 min followed by heating to 200 °C. Then, the mixture was cooled to 160 °C, followed by injection of 3 mL of the CIS dispersion. To produce biconcave Cu1.94S NPls, it was held at this temperature for 60 min. The thin (∼4.5 nm) and thick (∼15 nm) Cu1.94S NPls with flat surfaces were produced using reaction times of 1 and 120 min, respectively. To collect the NPls, 10 mL of ethanol was added, followed by centrifuging at 4000 rpm for 1 min. CE Reaction Using Biconcave Cu1.94S NPl Templates. The biconcave djurleite NPl colloid was prepared by dispersing dry biconcave NPls (one batch, typically ∼51 mg) in 5 mL of TOP. A mixture of 0.5 mmol CdCl2 (or other cation precursors as indicated) and 10 mL of OAm was held under flowing argon at ambient temperature for 30 min before heating to 160 °C. Then, 5 mL of Cu1.94S NPl dispersion was injected into the mixture, and it was held at this temperature for 30 min (5 min and 120 °C for SnS2). To collect the NPls, 10 mL of ethanol was added, followed by centrifuging at 4000 rpm for 1 min. CE Reaction Using Excess Driving Force. The biconcave djurleite NPl colloid was prepared by dispersing dry biconcave NPls (one batch, typically ∼51 mg) in 8 mL of TOP or DDT. A mixture of 1.0 mmol InCl3 and 10 mL of OAm was held under flowing argon at

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RESULTS AND DISCUSSION We performed CE on biconcave Cu1.94S NPls using metal ions including Cd2+, Zn2+, Sn2+, Pb2+, Mn2+, In3+, and Sn4+. The above cations can each replace the Cu+ ions in Cu1.94S to make copper-free metal sulfide NCs, except for In, which produces CuInS2. Biconcave djurleite Cu1.94S NPls were prepared by reacting Cu+ with CuInS2 NPls at 160 °C for 1 h. The biconcave structure of these NCs arises through generation of defects produced by rapid In3+ out-diffusion. These defects migrate to the center of the NPls, triggering local collapse of the center of the NPls. The XRD pattern (Figure S1) of the NPls matches that of djurleite Cu1.94S. Figure 1a shows a TEM image of the uniform biconcave NPls. Figure 1b,c shows topview and side-view HR-STEM images of biconcave NPls, with lattice spacings of 0.19 nm [(080) planes] and 0.33 nm [(800) planes]. Copper sulfide NCs are an ideal template for CE to produce a variety of metal sulfides such as CdS, ZnS, and PbS,12,21 but most of the Cu2−xS NC templates studied to date have flat surfaces and simple morphologies, or are quasi-spherical, which B

DOI: 10.1021/acs.chemmater.9b01686 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

CdS NPls can be produced by reacting the biconcave Cu1.94S template NCs with Cd2+ and a copper-extracting agent, TOP. TOP can also serve as a reducing agent and has been shown to remove chalcogens from metal chalcogenides via reduction of the metal cation (e.g., converting FeS2 to FeS).22 However, copper is present in its lower oxidation state (Cu+) in Cu1.94S and TOP bonds strongly to Cu+, as expected based on the hard/soft (Lewis) acid/base theory, resulting in preservation of the sulfur sublattice but extraction of Cu+. The XRD pattern of the CdS NPls matches the wurtzite-type CdS phase (greenockite, Figure S2). Figure 1d shows that the biconcave NPl morphology of the Cu1.94S template is retained after CE (Cu+ to Cd2+). The topview HR-STEM images in Figure 1b,e show the hexagonal packing of the atoms before and after transformation. The (110) and (100) planes are visible in Figure 1e, as confirmed by the fast Fourier transform (FFT) shown in its inset. Thus, the concave top and bottom surfaces of the NPls are perpendicular to the c-axis of CdS. Figure 1f shows a side view of the thin center of a CdS biconcave NPl with a minimum thickness of ∼2.5 nm. EDS (Figure S3) confirmed the expected ∼1:1 Cd/S ratio and the presence of only a small amount of residual Cu. This is consistent with STEM-EDS mapping (Figure 1g) that confirms co-localization of Cd and S, with negligible Cu. Figure 1h illustrates the hexagonal arrangement of S atoms in the unit cells of Cu1.94S and CdS when viewed along the a- and c-axes (consistent with the topview TEM images), respectively. The hexagonal arrangement of sulfur atoms in Cu1.94S is slightly disordered. The distance between neighboring S atoms in CdS is 2.39 Å, which is only 5.3% larger than the corresponding distance in Cu1.94S (2.27 Å). Moreover, as shown in Figure S4, the sulfur atoms in the two unit cells both have the ABABAB type of stacking with very similar interlayer distance (3.36 and 3.37 Å) when viewed from the side (90° rotated from the top-view of Figure 1h). This allows transformation with minimal changes in the sulfur framework, which not only facilitates uptake of Cd2+ but also helps to preserve the biconcave structure.

Figure 1. (a) TEM image and (b) top-view and (c) side-view HRSTEM images of biconcave Cu1.94S NPls. (d) TEM image and (e) top-view and (f) side-view HR-STEM images of transformed CdS NPls. The inset in panel (e) is the FFT of the HR-STEM image. (g) HAADF-STEM image and corresponding elemental maps for Cd, Cu, and S. Scale bars in panel (d) are 20 nm. (h) Sulfur atoms of Cu1.94S (viewed along the a-axis) and CdS (viewed along the c-axis). Black lines are unit cell boundaries.

limits the accessible morphologies of the product NCs. Biconcave NCs are particularly interesting templates because they have curved surfaces (like spherical NCs) but are strongly anisotropic (such as NPls). Unlike spheres, they exhibit both convex and concave surface curvatures. We find that biconcave

Figure 2. Top-view and side-view HRTEM images of a single thin CdS NPl [(a) with FFT of the top view in the inset] or a single thick CdS NPl (b). (c) Absorbance (dashed) and PL (solid) spectra and (d) time-resolved PL decay of thin (red), biconcave (blue), and thick (green) CdS NPls. (e) SAXS pattern and (f) radially integrated scattering plot from a dispersion of thin CdS NPls in hexane. C

DOI: 10.1021/acs.chemmater.9b01686 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

scattering. The scattering may arise from self-assembled stacks of thin CdS NPls. Scattering was also evident from the slightly cloudy appearance of the thin NPl dispersion, even at low concentration (Figure S9). Similar light scattering is observed when superstructures are formed via NC self-assembly in solution.28−30 However, we did not observe obvious selfassemblies in TEM images of samples of thin CdS NPls dropcast onto at the TEM grid and dried (Figure S2c). This may reflect different ligand−ligand interactions in different media. In hexane dispersion, the ligands (OAm and TOP) on the surface of the NPls are solvated and the resulting soft ligand shells may interact via van der Waals interactions without becoming completely desolvated. In contrast, during the drying process associated with preparation of samples for TEM imaging, the NPls tend to lay flat on the substrate due to both their anisotropic shape (lying flat maximizes substrate−NPl contact area) and capillary forces during drying. Therefore, a measurement that can directly verify the formation of selfassembled mesostructures in the NPl dispersion was needed. As shown in Figure 2e,f, the SAXS signal of the thin CdS NPls in a concentrated hexane dispersion was radially symmetric with a clear peak at q = 0.69 nm−1 and a much weaker peak and q = 0.20 nm−1 but no higher order diffraction peaks. The two peaks correspond to lattice spacings of d = 9.1 nm and d = 30.6 nm (d = 2π/q), consistent with a strong faceto-face stacking interaction and a weaker side-to-side interaction of CdS NPls. We note that the observed lattice spacing of 9.1 nm is larger than that might be expected based on face-to-face stacking, separated only by the length of the ligands (∼1 nm, from multiple TEM images). Adding the NPl thickness, one would anticipate a spacing of ∼6 nm. The increased spacing can be explained by ligand solvation and swelling in the hexane dispersion to a much more extended ligand conformation than observed under vacuum in the TEM.31,32 Therefore, we conclude that the thin CdS NPls selfassemble into columns by face-to-face stacking, and the resulting mesostructures, while highly scattering, exhibit good colloidal dispersibility. The assemblies exhibit slight precipitation on a time scale of hours but are easily redispersed by mild shaking. The PL spectra of the different CdS NPls are shown in Figure 2c. In these NPls, quantum confinement is limited to one dimension, the thickness of the NPls, as the lateral dimensions exceed 40 nm.27,33 Peak emission wavelengths of thin and thick CdS NPls were nearly identical (745 nm), while that of biconcave NPls was slightly red-shifted (750 nm). The QY of thin, thick, and biconcave NPls were similar at 1.18, 1.31, and 1.08%, respectively. Figure 2d shows PL decays of CdS NPls of different morphologies. Thin and thick NPls both have relatively flat surfaces but exhibit rather different PL lifetimes, which we attribute to nonradiative recombination.34 As demonstrated previously, numerous defects are produced during the formation of thin and biconcave Cu1.94S NPls.2,35,36 Therefore, thin CdS NPls exhibit a faster PL decay (τ = 4.1 ns, compared to thick CdS NPls with τ = 4.6 ns) and a lower PL QY due to nonradiative recombination at trap sites from the defects. Moreover, the red-shifted carrier recombination, attributable to the tunneling luminescence between the trap sites at the concave surface of the biconcave NPls featuring the lowest PL QY and fastest PL decay (τ = 3.7 ns) suggests a higher trap density than that of the other NPls.37,38 We note that although the XRD patterns of CE products suggest that they are copper-free, without careful purification, CdS (or

Flat Cu1.94S NPls of different thicknesses were also converted into CdS NPls with morphology preservation. As shown in Figure S5, Cu1.94S NPls with thicknesses of 4.5 ± 0.4 and 15.2 ± 0.7 nm were prepared (see the Supporting Information for synthetic details).2 Then, the ∼4.5 nm (thin) and ∼15.2 nm (thick) Cu1.94S NPls were converted into CdS NPls with flat surfaces (Figures 2a,b and S5c−f; lattice spacings match those of the biconcave CdS NPls). The CdS NPls retained the thickness of the template NPls, suggesting a similar cation replacement process as proposed for the biconcave CdS NPls. It is well known that CE products can retain properties of the initial template including its crystal phase and morphology.3,23−26 We thus investigated the relationship between Cu1.94S NPls obtained at different reaction times and their CE products. The thin flat NPls obtained by 1 min of CE reaction (from In3+ to Cu+) show relatively lower crystallinity, as demonstrated by their comparatively weak and noisy diffraction patterns (Figure S6, blue pattern). We attribute this low crystallinity to intrinsic crystal defects produced by fast ion diffusion during the initial CE reaction. Upon extending the reaction time, the relative intensity of (080) and (1204) planes changed because of shape evolution from biconcave to thick flat NPls. The phase transition associated with CE from Cu to Cd occurred very quickly (