Structure-Selective Synthesis of Wurtzite and Zincblende ZnS, CdS

Publication Date (Web): December 7, 2018. Copyright © 2018 American Chemical Society. *E-mail: [email protected]. Cite this:Inorg. Chem. XXXX, XXX, XXX- ...
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Structure-Selective Synthesis of Wurtzite and Zincblende ZnS, CdS, and CuInS2 Using Nanoparticle 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, Pennsylvania 16802, United States

Inorg. Chem. Downloaded from pubs.acs.org by YORK UNIV on 12/11/18. For personal use only.

S Supporting Information *

ABSTRACT: For polymorphic solid-state systems containing multiple distinct crystal structures of the same composition, identifying rational pathways to selectively target one particular structure is an important synthetic capability. Cation exchange reactions can transform a growing library of metal chalcogenide nanocrystals into different phases by replacing the cation sublattice, often while retaining morphology and crystal structure. However, only a few examples have been demonstrated where multiple distinct phases in a polymorphic system could be selectively accessed using nanocrystal cation exchange reactions. Here, we show that roxbyite (hexagonal) and digenite (cubic) Cu2−xS nanoparticles transform upon cation exchange with Cd2+, Zn2+, and In3+ to wurtzite (hexagonal) and zincblende (cubic) CdS, ZnS, and CuInS2, respectively. These products retain the anion and cation sublattice features programmed into the copper sulfide template, and each phase forms to the exclusion of other known crystal structures. These results significantly expand the scope of structure-selective cation exchange reactions in polymorphic systems.

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INTRODUCTION Controlling the crystal structures of inorganic solids is important for developing materials with desired functions because the arrangement of atoms, along with composition, coordination environment, and bonding characteristics, together define the band structure and the resulting properties of a material. Subtle differences in the crystal structures of solids having the same composition can lead to significantly different properties. For example, rutile VO2 is metallic, while a lower-symmetry tetragonal VO2 phase that arises from subtle changes in structure and bonding is insulating.1 MoS2 exists in both a metallic (1T) phase and a semiconducting (2H) phase depending on the arrangements of the two-dimensional MoS2 slabs that stack to form the structure.2,3 As such, selectively targeting one particular structure in a polymorphic system that contains multiple accessible structures with the same composition is a foundational goal in solid-state chemistry. This is particularly important for synthesizing phases that are not the most thermodynamically stable in a system, and therefore are not readily accessible using traditional hightemperature solid-state reactions. Methods capable of producing such phases include chemical and physical vapor deposition,4−6 solvothermal synthesis,7,8 flux growth,9 and solid-state metathesis,10,11 but these and other approaches have limitations, including applicability to only certain polymorphs in a given material system or difficulty in generating bulk amounts of the product material. Additionally, design guide© XXXX American Chemical Society

lines for selectively targeting one crystal structure when others may also be accessible are not yet broadly applicable, often rendering such efforts serendipitous rather than rational. Nanocrystal cation exchange has emerged as a useful alternative strategy for rationally and selectively targeting the formation of a desired crystal structure in polymorphic systems.12−15 Such reactions proceed by exchanging mobile cations in a colloidal nanocrystal with cations in solution under conditions that retain key features of the crystal structure, such as anion and/or cation sublattice structure and coordination environments. The material composition is therefore changed while the crystal structure, size, morphology, uniformity, and colloidal dispersibility are preserved.16,17 Nanocrystal cation exchange was first demonstrated in the transformation of CdSe quantum dots to Ag2Se, where solvated Ag+ cations in methanol could rapidly replace the Cd2+ in CdSe at room temperature.18 This reaction is driven by the higher solvation energy of Cd2+ vs Ag+ in methanol, facilitating the exit of Cd2+ from the CdSe nanocrystals to form the cation exchange product Ag2Se. Since then, solution-mediated ion exchange reactions have been used to synthesize nanocrystals of various metal sulfides,13,14,19,20 selenides,12,18,21 tellurides,22 phosphides,15 arsenides,23 lanthanide halides,24,25 and halide Received: October 10, 2018

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DOI: 10.1021/acs.inorgchem.8b02880 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry perovskites,26,27 with morphologies and sizes defined by the precursor nanocrystals. The conditions required for nanocrystal cation exchange to occur are mild, as reactions occur within seconds to minutes at temperatures ranging from below room temperature through several hundred degrees, much lower than the temperatures typically required to generate crystalline nanoparticles or bulk powders. As a result, the anion sublattice structure does not rearrange as cations diffuse through it, leading to products that retain key features of the precursor crystal structure.28 In some cases, this sublattice structure retention can result in the formation of crystal structures that are not the thermodynamically preferred phases, as they are kinetically trapped during the cation exchange process. For example, wurtzite and zincblende ZnSe can be selectively accessed through cation exchange of wurtzite and zincblende CdSe, respectively.12 A previously unreported wurtzite InP phase was obtained by exchanging the Cu+ cations in hexagonal Cu3−xP with In3+.15 In all of these examples, the anion sublattice structure was preserved during the nanocrystal cation exchange reactions. We have been applying the principles of nanocrystal cation exchange to the formation of polymorphic metal sulfides, including CoS and MnS.13,14 These synthetically challenging systems contain phases having multiple accessible crystal structures and compositions, but selectively targeting one individually can be difficult. In contrast, exchanging the Cu+ cations in Cu2−xS nanoparticles with Co2+ and Mn2+ readily yields CoS and MnS. When we use roxbyite Cu2−xS nanoparticles, which have a distorted hexagonal close packed (hcp) anion sublattice and low-coordinate trigonal and tetrahedrally coordinated cations, the CoS and MnS products adopt the wurtzite structure, which also has an hcp anion sublattice and tetrahedrally coordinated cations.13 When we use digenite Cu2−xS nanoparticles, which have a cubic close packed (ccp) anion sublattice and tetrahedrally coordinated cations, the CoS and MnS products instead adopt the zincblende structure, which also has a ccp anion sublattice and tetrahedrally coordinated cations.14 By beginning with two distinct polymorphs of Cu2−xS, nanocrystal cation exchange reactions under identical conditions occur with complete preservation of both anion and cation sublattice features. Each phase forms exclusively, relative to other possible cobalt and manganese sulfide phases, and NiAs-type CoS and rocksalttype MnS, which are the thermodynamically preferred phases with nominal 1:1 metal/sulfur ratios, are not observed. The ability to target and obtain multiple distinct crystal structures across two different material systems, where several other polymorphs are known and accessible, is an important step toward achieving predictive structure targeting in solidstate synthesis. However, it is important to broaden the scope of such reactions so that they are more generally applicable and so that we are better positioned to more fully understand the capabilities and limitations of nanocrystal cation exchange in structure-selective synthesis. Accordingly, here we demonstrate structure-selective cation exchange processes for selectively synthesizing the hcp (wurtzite) and ccp (zincblende) polymorphs of CdS, ZnS, and CuInS2 from roxbyite and digenite Cu2−xS nanoparticles, respectively (Figure 1). These results significantly expand the scope of such reactions to other binary and ternary phases while further validating the hypotheses laid out previously for MnS and CoS with respect to retention of anion and cation sublattice features during nanocrystal cation exchange.13,14

Figure 1. Summary of CdS, ZnS, and CuInS2 crystal structures accessible through structure-templated cation exchange of copper sulfide nanoparticles. Roxbyite and digenite Cu2−xS starting materials yield wurtzite and zincblende CdS, ZnS, and CuInS2, respectively, by preserving the anion and cation sublattice features from the copper sulfide template.

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EXPERIMENTAL SECTION

Chemicals. Copper(I) chloride [CuCl, 97% reagent grade], copper(II) chloride [CuCl2, 97%], zinc(II) chloride [ZnCl2, ≥97% ACS reagent grade, anhydrous], oleylamine [OLAM, 70%, technical grade], octadecene [ODE, 90%, technical grade], oleic acid [OLAC, 90% technical grade], di-tert-butyl disulfide [97%], and tertdodecanethiol [t-DDT, mixture of isomers 98.5%] were purchased from Sigma-Aldrich. Trioctylphosphine [>85%] was purchased from TCI America. Sulfur powder [-325 mesh, 99.5%], cadmium(II) acetate dihydrate [Cd(OAc)2, 99.999% trace metals basis], and indium(III) acetylacetonate [In(acac)3, 98%] were purchased from Alfa Aesar. Benzyl ether [99%] was purchased from Acros Organics. All solvents, including isopropanol, acetone, and hexanes, were of analytical grade. Higher-purity oleylamine [hp-OLAM] was collected by vacuum distillation of technical grade oleylamine. All other chemicals were used as received without further purification. Synthesis of 16 nm Cubic Digenite Cu2−xS Nanospheres. Digenite Cu2−xS nanospheres were prepared according to a previously reported procedure.14 Sulfur powder (320 mg) and ODE (20 mL) were added to a 100 mL three-neck flask equipped with reflux condenser, a thermometer, and a rubber septum and degassed at 120 °C under vacuum for 1 h. Simultaneously, CuCl (990 mg), technicalgrade OLAM (5 mL), and OLAC (4 mL) were added to a separate, similarly equipped 50 mL three-neck flask and degassed at 120 °C under vacuum for 1 h. After degassing, both flasks were placed under a blanket of Ar. The S-ODE solution was then heated to 200 °C, and the CuCl solution was heated to 130 °C. Using a 10 mL glass syringe, all of the hot CuCl solution was rapidly injected into the S-ODE solution at 200 °C. The heating mantle was removed 10 min after the injection, and the solution was allowed to cool to room temperature. The particles were precipitated with isopropanol and collected by centrifugation. The particles were dispersed in hexanes and the precipitation/centrifugation process was repeated twice, until the supernatant was clear and colorless. The resulting precipitate was dispersed in hexanes for further use. Synthesis of 22 nm Roxbyite Cu2−xS Nanospheres. Roxbyite Cu2−xS nanospheres were prepared according to a previously reported procedure.20 CuCl2 (341 mg), hp-OLAM (47.2 mL), and ODE (11.6 B

DOI: 10.1021/acs.inorgchem.8b02880 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry mL) were added to a 100 mL three-neck round-bottom flask equipped with a reflux condenser, thermometer, and rubber septum and degassed under vacuum at 120 °C for 30 min. The solution was placed under flowing Ar and heated to 200 °C, degassing (under Ar flow) for an additional 1 h to remove gaseous side products formed during the dissolution of CuCl2 in oleylamine. After degassing, the transparent yellow solution was placed under an Ar blanket and cooled to 180 °C. At 180 °C, 8 mL of di-tert-butyl-disulfide (degassed under vacuum at room temperature for 1 h) were rapidly injected, turning the solution a turbid brown color. After 40 min at 180 °C, the solution was cooled rapidly by removing the heating mantle and submerging the flask in a room temperature water bath. The particles were precipitated by addition of a 1:1 isopropanol/acetone mixture and collected by centrifugation. The particles were dispersed in hexanes and the centrifugation/precipitation process was repeated twice, until the supernatant appeared clear and colorless. The resulting precipitate was redispersed in hexanes for further use. Cation Exchange of Cu2−xS To Form ZnS. Cation exchange with Zn2+ was performed using a modification of a previously reported procedure.29,30 ZnCl2 (250 mg), hp-OLAM (8 mL), ODE (2 mL), and benzyl ether (15 mL) were added to a 50 mL three-neck flask equipped as described above, followed by degassing at 120 °C under vacuum for 30 min. The solution was placed under flowing Ar and heated to 200 °C, degassing (under Ar flow) for an additional 1 h to remove gaseous side products formed during the dissolution of ZnCl2 to form a metal−oleylamine complex. After 1 h, the solution was cooled to 50 °C, evacuating the reaction atmosphere once more under vacuum after the solution temperature dropped below 130 °C. Meanwhile, 30 mg of dispersed Cu2−xS seeds (roxbyite or digenite) were dried under vacuum to remove the hexanes and redispersed in TOP. The dispersion was degassed under vacuum at room temperature for 10 min and refilled with Ar. The particles in TOP were rapidly injected into the Zn-OLAM solution at 50 °C under Ar. The solution was subsequently heated to 110 °C. After 20 min of reaction at 110 °C, the heating mantle was removed and the solution was allowed to cool to room temperature. The particles were precipitated with isopropanol and collected by centrifugation. The particles were dispersed in hexanes and the precipitation/ centrifugation process was repeated twice, until the supernatant was clear and colorless. The resulting precipitate was dispersed in hexanes for further use. Cation Exchange of Cu2−xS To Form CdS. Cation exchange with Cd2+ was performed using a modification of a previously reported procedure.29,30 Cd(OAc)2 (250 mg), hp-OLAM (8 mL), ODE (2 mL), and benzyl ether (15 mL) were added to a 50 mL three-neck flask equipped as described above, followed by degassing at 110 °C under vacuum for 1 h. After 1 h, the solution was cooled to 50 °C and placed under an Ar blanket. Meanwhile, 30 mg of dispersed Cu2−xS seeds (roxbyite or digenite) were dried under vacuum to remove the hexanes and redispersed in TOP. The dispersion was degassed under vacuum at room temperature for 10 min and refilled with Ar. The particles in TOP were rapidly injected into the CdOLAM solution at 50 °C under Ar. The solution was subsequently heated to 110 °C. After 20 min of reaction at 110 °C, the heating mantle was removed, and the solution was allowed to cool to room temperature. The particles were precipitated with isopropanol and collected by centrifugation. The particles were dispersed in hexanes and the precipitation/centrifugation process was repeated twice, until the supernatant was clear and colorless. The resulting precipitate was dispersed in hexanes for further use. Cation Exchange of Cu2−xS To Form CuInS2. Partial cation exchange with In3+ was performed using a modification of a previously reported procedure.31 In(acac)3 (60 mg), hp-OLAM (1.5 mL), and ODE (7.5 mL) were added to a 50 mL three-neck flask equipped as described above, followed by degassing at 110 °C under vacuum for 45 min. After 1 h, the solution was placed under an Ar blanket and 15 mg of Cu2−xS seeds (roxbyite or digenite, dispersed in hexanes) were injected via syringe. The mixture was degassed under vacuum for an additional 15 min and then placed under an Ar blanket and heated to 140 °C. At 140 °C, 1.5 mL of t-DDT was injected dropwise into the

solution over 1 min. After 20 min of reaction at 140 °C, the heating mantle was removed and the solution was allowed to cool to room temperature. The particles were precipitated with isopropanol and collected by centrifugation. The particles were dispersed in hexanes and the precipitation/centrifugation process was repeated twice, until the supernatant was clear and colorless. The resulting precipitate was dispersed in hexanes for further use. Characterization. Powder X-ray diffraction (XRD) data were collected on a Bruker D-8 Advance X-ray diffractometer using Cu Kα radiation. Transmission electron microscopy (TEM) images were collected on a JEOL 1200 EX II microscope operating at 80 kV. Highresolution TEM (HRTEM), selected area electron diffraction (SAED), and high angle annular dark field scanning TEM (HAADF-STEM) images were collected on an FEI Talos F200X S/ TEM at an accelerating voltage of 200 kV. STEM energy dispersive Xray spectroscopy (STEM-EDS) maps were also collected on the FEI Talos F200X S/TEM using the Super-X EDS quad detector system at a current of ∼0.15 nA. Standardless Cliff-Lorimer quantification was performed on the deconvoluted EDS line intensity data using the Bruker Espirit software. ES vision software (Emispec) was used for EDS data processing.

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RESULTS AND DISCUSSION To access the zincblende and wurtzite polymorphs of CdS and ZnS, spherical nanoparticles of digenite and roxbyite Cu2−xS were first synthesized as morphological and structural templates. Roxbyite Cu2−xS nanoparticles (22 ± 1 nm) (Figure 2a) were obtained by reaction of CuCl2 with di-tert-butyl-

Figure 2. TEM images and powder XRD patterns for (A) roxbyite Cu2−xS and (B) digenite Cu2−xS nanoparticle precursors. Simulated XRD patterns32,35 are shown for reference. In (B), reflections attributed to rhombohedral digenite are indicated with an asterisk (*).

disulfide in oleylamine at 180 °C. A powder X-ray diffraction (XRD) pattern collected from these particles, shown in Figure 2a, is in good agreement with the simulated pattern for roxbyite Cu2−xS. Roxbyite Cu2−xS has a large and complex monoclinic unit cell that contains a distorted hcp sulfur sublattice with tetrahedrally and trigonally coordinated Cu+ cations (Figure 1).32 To synthesize digenite Cu2−xS nanoparticles (16 ± 1) (Figure 2b), a solution of CuCl in oleic acid C

DOI: 10.1021/acs.inorgchem.8b02880 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

signal), as well as the expected 1:1 ratios of Cd/S and Zn/S for all exchange products. (For corresponding EDS spectra, see Figure S1 in the Supporting Information.) TEM images (Figure 4) indicate that the products retain the overall spherical morphology and size of the starting Cu2−xS nanoparticles: 22 ± 1 nm for ZnS and 22 ± 1 nm for CdS derived from roxbyite Cu2−xS, and 16 ± 1 nm CdS and 16 ± 1 nm ZnS derived from digenite Cu2−xS. Powder X-ray diffraction (XRD) and selected-area electron diffraction (SAED) analyses, shown in Figure 4, confirm the disappearance of peaks associated with Cu2−xS phases and the appearance of peaks consistent with the formation of crystalline, single-phase CdS or ZnS. For products obtained through cation exchange with roxbyite, the positions and intensities of the reflections are consistent with those expected for the known wurtzite polymorphs of CdS and ZnS.39 Scherrer analysis of the (002) diffraction peaks reveals an average crystallite size of ∼21 nm for ZnS and ∼23 nm for CdS, approximately consistent with the average diameters of the particles measured from TEM images. For products obtained through cation exchange with digenite, the patterns and intensities of reflections are consistent with those expected for the known zincblende polymorphs of CdS and ZnS,40,41 with average crystallite sizes of ∼17 nm CdS and ∼16 nm ZnS, based on Scherrer analyses of the (111) diffraction peaks. Different crystal structures in the Cu2−xS precursor nanoparticles therefore generate two different crystal structures in the CdS and ZnS products after exchange under otherwise identical exchange conditions. This structure-templating behavior is consistent with previous findings that transform roxbyite and digenite Cu2−xS to wurtzite and zincblende CoS and MnS, respectively.13,14 Similar to roxbyite Cu2−xS, wurtzite metal sulfides contain an hcp sulfur sublattice with cations filling the tetrahedral holes (Figure 1). Zincblende structures contain a ccp sulfur sublattice and tetrahedrally coordinated metal cations, just like the digenite crystal structure (Figure 1). The phases that are thermodynamically preferred are wurtzite CdS and zincblende ZnS, although wurtzite and zincblende ZnS and CdS have both been prepared colloidally and in bulk syntheses. For colloidal ZnS particles, the most commonly synthesized phase is zincblende, but wurtzite ZnS has been observed when a wurtzite ZnO intermediate is formed in a direct synthetic method, or when ZnO is used as a template for postsynthesis anion exchange.34,42,43 While colloidal methods to make both CdS polymorphs exist, the parameters that lead to the preferential formation of zincblende rather than wurtzite CdS nanoparticles are not well understood or able to be rationally modified to predictably target each structure independently. Moreover, zincblende CdS particles are often obtained through low-temperature synthetic methods, which result in products with low crystallinity. In contrast, nanocrystal cation exchange provides rational pathways both to the phases that are not thermodynamically preferred (zincblende CdS and wurtzite ZnS) and to those that are thermodynamically preferred (wurtzite CdS and zincblende ZnS), under identical reaction conditions. In all cases, the product structures retain both the anion and cation sublattice features of the Cu2−xS precursor, and each forms to the exclusion of other possible polymorphs in each system. Complete cation exchange of the Cu+ in Cu2−xS with Cd2+ and Zn2+ yields fully exchanged CdS and ZnS products. In contrast, because Cd2+ and Zn2+ are both immiscible with Cu2−xS, partial cation exchange reactions instead result in

and oleylamine was introduced into a solution of sulfur powder and octadecene at 200 °C. The primary peaks observed by XRD (Figure 2b) match well with the simulated pattern for cubic digenite. The minor peaks correspond to rhombohedral digenite, a commonly observed minority phase that is present in digenite nanoparticles.33,34 Cubic digenite contains a ccp sulfur sublattice and tetrahedrally coordinated Cu+ cations (Figure 1).35 Roxbyite and digenite Cu2−xS nanoparticles were both subjected to identical procedures to form CdS and ZnS products via exchange with Cd2+ and Zn2+, respectively. Briefly, Cu2−xS nanoparticles dispersed in TOP were introduced into a solution containing ZnCl2 or Cd(OOCCH3)2·3H2O, oleylamine, octadecene, and benzyl ether and reacted at 110 °C for 20 min. In this process, TOP, a soft base, binds more favorably to the relatively soft Cu+ metal cation than to the solvated Zn2+ or Cd2+ cations.36 The formation of a favorable Cu+−TOP complex in solution drives Cu+ out of the Cu2−xS nanoparticles and the divalent metal cation into them through a process that occurs rapidly (i.e., within several minutes).16 As the exchange proceeds, the reaction solutions change in color, as both forms of Cu2−xS exhibit a dark brown color as nanoparticles in solution, while the ZnS and CdS products are different. During Zn2+ exchange, the solution color fades as ZnS, a wide band gap semiconductor (Eg > 3.5 eV),37 forms. Exchange with Cd2+ yields CdS (Eg = ∼2.4 eV),38 changing the reaction solution gradually from brown to bright yellow. The completeness of the exchange was confirmed by scanning transmission electron microscopy coupled with energy dispersive spectroscopy (STEM-EDS) mapping, shown in Figure 3. Element mapping reveals Cu signals near background levels (