Ag+-Induced Shape and Composition Evolution of Covellite CuS

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Ag-Induced Shape and Composition Evolution of Covellite CuS Nanoplatelets to Produce Plate-Satellite and Biconcave-Particle Heterostructures Yang Liu, Deqiang Yin, and Mark T. Swihart Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04272 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Chemistry of Materials

Ag+-Induced Shape and Composition Evolution of Covellite CuS Nanoplatelets to Produce Plate-Satellite and Biconcave-Particle Heterostructures Yang Liu, Deqiang Yin and Mark T. Swihart* Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States. ABSTRACT: The creation of novel metal sulfide nanostructures and heterostructures by incorporating heterocations into template copper sulfide nanocrystals (NCs) is an important and intriguing area of colloidal synthesis. In contrast with typical incorporation using divalent, trivalent, and tetravalent cations, the mechanism for incorporation of monovalent Ag+ into copper sulfide NCs is less clear. In this work, we prepared new types of heterostructures by incorporating Ag+ into covellite CuS nanoplatelets (NPls). The incorporation process depended strongly on the initial Ag+ concentration. When a relatively small amount of Ag+ was provided (0.1 mmol, roughly 20% of the Cu present in the template NPls), CuS-Ag2S plate-satellite heterostructures were produced by cation exchange (CE) reactions occurring at the corners of the template hexagonal NPls. In contrast, biconcave-particle Ag2S-Ag heterostructures were obtained via the nanoscale Kirkendall effect when starting with 0.5 mmol of Ag+. Our observations indicate that the Ag+ incorporation process is also template-dependent. A redox process involving disulfide bonds in CuS is essential for producing Ag2S domains, rather than forming a ternary Ag-Cu-S phase, which was the result when djurleite Cu1.94S NPls were treated with Ag+. These results and mechanistic discussions only not provide better understanding of a complex cation incorporation and crystal phase transition process, but also provide a means to design new copper sulfide-based heterostructures.

Introduction Copper sulfide-based heterostructures have attracted extensive attention due to their potential for applications in catalysis, electronics, optics, and photovoltaics.1-5 These heterostructures possess advantages including tunable size, phase, and composition as well as a good compatibility for integration with other metal or semiconductor NCs. The copper sulfide class of materials is known for p-type semiconductor behavior. Integration with other heterodomains may provide a means of influencing charge carrier separation and recombination that is not available in homogeneous structures, and may also produce shifts in localized surface plasmon resonance (LSPR) energy and intensity, or promote other synergistic interactions between domains. Preparation of these heterostructures can be generally divided into one-pot mixing methods and multistep growth methods. Both of the synthetic routes involve nucleation of Cu2−xS template (seed) NCs, followed by incorporation of incoming cations.6, 7 According to our previous reports, the outcome of cation incorporation into Cu2−xS NCs strongly depends on the valence of the incoming cation.8, 9 Cation incorporation into Cu2−xS NCs using trivalent (e.g., In3+ and Sb3+)10, 11 and tetravalent (e.g., Sn4+ and Ge4+)12, 13 cations produces homogenous ternary alloy (e.g., CuInS2 or Cu2SnS3). In contrast, incorporation of divalent cations (e.g. Zn2+ and Cd2+) results in heterogeneous Cu2-xS-MS NCs, or copper-free MS NCs.14, 15 The cation valence selectivity arises from conflicts between charge balance and coordination

between Cu+ and divalent cations.8 However, general rules and mechanisms governing monovalent cation (Au+ and Ag+) incorporation are less clear. The outcomes may depend upon the preferred coordination numbers and cation radius of the incoming cations. Similar to divalent cations, incorporation of tetragonally coordinated Au+ into Cu2−xS NCs leads to a complete replacement of all Cu+ in one Cu2−xS unit cell, producing Cu2−xS-Au2S heterostructures.16-18 However, because ternary Ag-Cu-S alloy and binary Ag2S NCs are both stable and accessible via colloidal synthesis,17, 19-21 the results of the Ag+ incorporation process are expected to depend on the initial template and reaction conditions. Most copper sulfide phases exhibit hcp stacking of their sulfur sublattice (e.g., high chalcocite Cu2S, djurleite Cu1.94S, and roxbyite Cu1.81S).11, 22, 23 Moreover, products of cation incorporation almost always inherit the crystal structure (especially the anionic sublattice) of the template phase.24 As a result, the product NCs after cation incorporation are strongly influenced by the anionic framework of the template. Among many possible copper sulfide crystal phases and compositions, covellite CuS, which is one of the stoichiometric extremes, is unique in several ways. The stacking of sulfur layers in covellite CuS can be described as ABAABA, which is neither the typical hcp-type ABABAB stacking nor the fcc-type ABCABC stacking. Furthermore, rather than simple cation replacement observed in typical CE reactions, the process of cation incorporation into covellite CuS involves additional redox reactions associated with the breaking of disulfide bonds and changes in the oxidation state of sulfur.25, 26 Colloidal synthesis

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of CuS nearly always produces single-crystal nanoplatelets (NPls) with the c-axis of the covellite crystal structure perpendicular to the large flat surfaces of the NPls. As a template for CE reactions, these CuS NPls have advantages of fixed morphology, stable and pure crystal phase, and relatively easy observation of formation of heterodomains, making them ideal templates for studying cation exchange and incorporation processes.8, 22, 27, 28 Thus, we have employed CuS NPls in the present study. We treated covellite CuS NPls with Ag+ to investigate Ag+ incorporation mechanisms. Knowing that Ag+ has relatively high reactivity with sulfur and is easily reduced to form Ag nanoparticles (NPs) upon mild heating, all experiments were carried out at ambient conditions. The Ag+ incorporation process is highly concentration dependent. When the initial amount of Ag+ was relatively low (0.1 mmol, approximately 20% of the total Cu present in the CuS templates), CuS-Ag2S platesatellite heterostructures were produced with Ag2S NPs uniformly grown at the corners of the hexagonal CuS template NPls. Increasing the initial amount of Ag+ led to complete CE reactions from CuS to Ag2S while the plate-satellite morphology was preserved. Biconcave Ag2S NPls were obtained when starting with 0.4 mmol of Ag+, and a Ag NP was found to grow onto the biconcave NPls upon further increasing the initial Ag+ amount (0.5 mmol). In general, the basal planes (top and bottom surfaces of the NPls) are well passivated by ligands, while the edges are relatively active. The CE reactions take place at these edges, producing plate-satellite structures at low initial Ag+ concentrations and biconcave Ag2S NPls at high concentrations, via a nanoscale Kirkendall effect. To the best of our knowledge, the CuS-Ag2S plate-satellite and Ag2S-Ag biconcave-particle nanostructures are reported here for the first time. We believe that our discoveries and discussions can help to guide the design of new types of metal chalcogenide heterostructures.

Experimental Section Chemicals. All chemicals were used as received. Silver nitrate (99.99%, AgNO3), tert-dodecanethiol (t-DDT, 98.5%) and oleylamine (70%, OAm), were purchased from Sigma-Aldrich. Cu(NO3)2·2.5H2O, CuCl2·2H2O, toluene, and ammonium sulfide (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, then heated to 70 °C under an argon flow. 1.5 mL of ammonium sulfide (AS) solution (21.2% in water) was injected into the 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 by 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. Ag+ Incorporation into CuS NPls. The CuS dispersion was prepared by dispersing dry CuS (one batch, typically ~230 mg) in 9 mL of OAm. From 0.1 mmol to 0.5 mmol of AgNO3, as specified for different experiments, was dissolved in a mixture of 10 mL of OAm and 10 mL of toluene via sonication. Then, 3 mL of the CuS dispersion was injected into the Ag-OAm

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complex, and it was held at ambient conditions with magnetic stirring for 10 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 on 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 in 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 for XRD were prepared by drop-casting concentrated NC dispersions onto glass slides. UV−vis−NIR spectra of NC dispersions in toluene (or corresponding OAm and ethanol mixture, as indicated) were taken on a Shimadzu UV-3600. Xray photoelectron spectra (XPS) were acquired on an ESCALAB 250Xi with an Al Kα X-ray source (1486.6 eV). The pressure in the analysis chamber was maintained below 7.5 × 10−10 Torr for data acquisition. Raman spectra were collected using a Renishaw inVia Raman Microscope. All measurements were carried out with laser excitation wavelength of 488 nm. Samples were prepared by drop casting a concentrated NC dispersion onto a glass substrate. Electron paramagnetic resonance (EPR) spectra of diluted CuS NPl hexane dispersions and of supernatant obtained after reaction and separation of NPls by centrifugation were measured at the X-band on a Bruker EMX 390 EPR Spectrometer operating at 9.75 GHz. EPR samples were placed in standard quartz EPR tubes and measured at 77 K.

Results and Discussion Covellite CuS NCs generally adopt a plate-like morphology due to the intrinsic layered structure of covellite. The basal planes of covellite (perpendicular to the c-axis are well passivated by the capping ligands.22 The TEM image in Figure 1a illustrates the hexagonal plate-like morphology of the covellite CuS template NPls. The HR-STEM image and XRD pattern of the template NPls confirm the covellite phase, with an observed lattice spacing of 0.19 nm, corresponding to (110) planes of covellite CuS. (Figure S1 in the Supporting Information). CE reactions of Cu2-xS NCs usually require a copper extracting agent (e.g. trioctylphosphine (TOP) or dodecanethiol (DDT)) to break the Cu-S bonds for subsequent cation replacement.8, 12, 23, 29 However, covellite CuS is an exception because of existing disulfide bonds and average valence of +1 and -1 for Cu and S. This unique structural feature allows simultaneous redox reactions and cation incorporation. Due to the greater thermodynamic stability of Ag2S (relative to its elemental components) compared to CuS, as well as the similar absolute hardness (in the context of hard-soft acid-base interactions) of Ag+ and Cu+, the Ag incorporation process occurs readily, even at ambient conditions and without the use of a copper-extracting agent.


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As shown in Figure 1b, Ag+ incorporation into covellite CuS NPls yields a uniform, plate-satellite structure. The TEM image in Figure 1c reveals that a single plate-satellite structure has a hexagonal plate as a core, with six quasi-spherical NPs at the six corners of the plate. In some of the hexagonal plates, not all six corners have an attached NP, but the location of the NP growth is predominantly at the corners whenever a NP is present. The XRD pattern in Figure S2 (orange curve) suggests that the heterostructure contains covellite CuS (ICDD PDF 04007-1392) and acanthite Ag2S (ICDD PDF 00-014-0072) phases. However, the substantial overlap between the XRD patterns of these two materials, combined with the small size of the satellite domains make definitive assignment of the crystal structure difficult. The pattern does not show any evidence of metallic silver. Thus, the plate-satellite structures are composed of darker-contrast Ag2S domains and lighter contrast CuS domains. The crystalline heterointerface between CuS and Ag2S domains can be clearly seen in Figure 1d, with two similar lattice spacings of 1.9 Å, corresponding to the (110) planes of CuS (plate) and (212) planes of Ag2S (satellite).

Figure 1. Low magnification TEM images of covellite CuS NPls (a) before and (b) after Ag+ incorporation. (c) TEM image of a single CuS-Ag2S plate-satellite nanostructure (d) HRSTEM image of CuS-Ag2S plate-satellite nanostructure. (e) FFT patterns of CuS in blue ([001] zone axis) and Ag2S in red ([221] zone axis) showing the relative orientation and the similarity in crystal lattices (CuS + Ag2S FFT pattern) (f) HAADF-STEM image and corresponding elemental maps of Ag, S, and Cu. The scale bar in panel d is 30 nm.

Figure 1e displays the FFT patterns of the CuS plate, an Ag2S satellite and the whole area of one heterostructure. We note that the [100] zone axis projection of the CuS domains matches the [221] zone axis projection of the Ag2S domains, with only a slight shift between the (110) spots of CuS and (212) of Ag2S, due to very small differences between the respective d-spacings. The unindexed spots are contributed by a neighboring CuS plate visible at the top right of Figure 1d, and other high-index planes. The original FFT patterns are provided in Figure S3. As shown in Figure S4, the lattice similarities were also observed in a different CuS-Ag2S heterostructure, with the same orientational relationship. STEM-EDS elemental maps show the Ag signal is located mainly on the satellite NPs, segregated from the Cu signal, which is mainly located on the plates. EDS spectra and corresponding elemental ratios in Figure S5 also support the above observations. The overall composition from EDS corresponds to CuS-Ag2S heterostructures containing ~77.5% CuS and ~22.5% Ag2S, while the satellite domain is mainly Ag2S, with minimal Cu content. The overlapping Ag and Cu signals are possibly due to the limited thickness of the NPls and the presence of ions absorbed on the surfaces. We note that the Ag+-OAm precursors were prepared by sonicating a mixture of AgNO3 (Ag source), OAm (capping ligand and coordinating solvent) and toluene (non-coordinating solvent in order to dilute the solution). Although heating such a solution could produce Ag NPs, nucleation of metallic Ag does not occur at the temperature and concentrations used here. To further investigate the growth mechanism of CuS-Ag2S plate-satellite heterostructures, we carefully studied top-view and side-view TEM images of these heterostructures at different reaction times. We divided the Ag+ incorporation-induced shape evolution of covellite CuS NPls into three stages. Initially, the CuS NPls have flat surfaces and sharp corners. These sharp corners of the NPls with higher surface energy provide lower reaction barriers for Ag+ incorporation, as discussed further below. This is because the basal planes of the NPls are well passivated by ligands, consistent with previous discussions.11, 30, 31 This stage occurs instantly (< 30 s), producing small Ag S 2 domains on the corners of each hexagon. The side-view TEM image of the heterostructure at this stage confirms the nucleated Ag2S domains have limited size (< 4 nm, the thickness of CuS template NPls) and relatively low crystallinity. Then, from 30 s to 10 min, the size of the Ag2S domains gradually grew larger, with an average diameter of 10.6 ± 1.4 nm. Size- and thicknessdistribution histograms for this and subsequent samples are presented in Figure S6. This is, to the best of our knowledge, the first report of preparation of CuS-Ag2S plate-satellite heterostructures. Heterostructures with larger Ag2S domains can be obtained by further extending the reaction time to 120 min (16.1 ± 1.4 nm). As mentioned above, the (110) planes are observed parallel to the thickness of CuS template NPls. The relative intensity of (110) planes in XRD patterns decreased as the size of the CuS domains shrank and that of the Ag2S domains grew. This is also consistent with the slight increase in relative intensity of peaks near 34° that correspond to the (021), (121) and (002) planes of Ag2S from 10 min to 120 min reaction time. CE reactions are well known to occur in some metal chalcogenide nanostructures under mild conditions, facilitated by high cation mobilities and favorable chemical driving forces in both the NCs and in solution. Here, to avoid other factors that affect the CE reactions, we only use OAm, which was already



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Figure 2. Top panels show the top-view and side-view TEM images of Ag+ incorporation into covellite CuS NPls process at different reaction times. The scale bars in top panels are 20 nm. Bottom panel shows the schematic illustration of Ag+ incorporation-induced shape evolution of covellite CuS NPls. present in the system, as solvent and capping ligand, Ag to replace Cu, and an ambient temperature. As shown in Figure S7, the color of the supernatant of after Ag+ incorporation was dark green, suggesting formation of Cu2+ species. The UV-vis absorbance spectrum of the supernatant also matches well with that of Cu2+-OAm-toluene solution with similar Cu2+ concentration (Figure S8). The formation of Cu2+ species was further confirmed by EPR spectroscopy. As shown in Figure 3a, no EPR signal was observed in the CuS NPl dispersions, confirming that the oxidation state of Cu in the NPls is +1, which is consistent with the XPS results further below. The EPR spectrum of the supernatant after CE reaction clearly shows the presence of Cu2+ species, produced during the conversion from CuS NPls to CuS-Ag2S nanostructures. Covellite CuS can be described as layers of triangular CuS3 units sandwiched between two layers of CuS4 tetrahedra. Each triple layer is then linked to the triple layers above and below it via disulfide bonds (Figure S9).32, 33 Unlike other copper sulfides in which ions are clearly present as Cu+ and S2-, the valency of ions in covellite is still debated.34-37 Many reports agree that the average oxidation states of Cu and S are +1 and -1. The formation of Ag2S then requires the Ag+ to participate in redox reactions (Cu+ to Cu2+ and (S2)2- to 2S2-), and the overall reaction can be expressed as (considering sulfur in CuS to have an average valence of -1): 2Ag + + Cu + S ― →Ag2+ S2 ― + Cu2 + The transformation was further investigated using Raman spectroscopy. As shown in Figure 3b, the Raman spectrum of the template CuS NPls exhibited vibration modes at 263 and 474 cm−1, which are is consistent with the Raman spectra of CuS in the previous reports.25, 38, 39 Compared to the peak at 263 cm−1, which corresponds to a Cu−S vibrational mode, the intensity of the peak at 474 cm−1 (attributed to S-S stretching) decreased as more Ag+ ions were incorporated into covellite CuS. This results from breaking of disulfide bonds in CuS as the participating sulfur atoms were gradually reduced and bound to Ag during the process of formation of Ag2S via the redox reaction. The valencies of Cu and S before and after Ag+ incorporation were further investigated by XPS analysis. As shown in Figure 3c (with detailed fitting and absolute signal intensities shown in Figure S10), the valency of Cu in the template covellite CuS NPls was mainly +1. The sulfur 2p peak consisting of overlapping doublets centered near 161.5 eV is characteristic of the multiple states of S in covellite (e.g. including both disulfide and S2- states). Upon exposure to Ag+, the overall peak intensities corresponding to Cu+ and S-S were decreased, indicating removal of Cu content and partial

elimination of disulfide bonds.25, 26 This is consistent with the proposed redox reaction process in which disulfide bonds were reduced during Ag+ incorporation.40 For the fully converted sample, the sulfur 2p region is characterized by a single doublet characteristic of the uniform oxidation state and similar environment of all S atoms in acanthite.21, 41 We also note that, when the reaction was performed under an inert gas atmosphere (Ar, with other reaction conditions fixed), the reaction outcome was similar to that observed at ambient conditions (Figure S11). This observation reveals that the proposed redox reaction of covellite CuS occurs spontaneously when exposed to Ag+, and oxidation by oxygen from the air does not play a significant role.

Figure 3. (a) EPR spectra of supernatant after CE reaction using low (red) and high (blue) concentration of Ag+, and CuS NPl dispersions (black, in hexane). The dash curves show the controls of the EPR measurements. (b) Raman spectra of covellite CuS NPls before (blue curve) and after (red curve) treatment with 0.1 mmol of Ag+. (c) High-resolution XPS characterization of Cu 2p, S 2p, and Ag 3d for CuS NPls and with different Ag+ treatment. As proposed by Rivest et al. and other prior reports,42-44 the thermodynamic equilibrium of a CE reaction can be analyzed by breaking the overall process into sub-steps for which the Gibbs energy changes are known or can be estimated. In this case, the process of CE reaction can be divided in four ideal


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steps: Cu-S dissociation (3481 kJ/mol), Ag+ desolvation (350 kJ/mol), Ag-S association (-2677 kJ/mol), and Cu2+ solvation (1920 kJ/mol).45-47 Here, the dissociation and association energies include lattice energy and surface energies of the NCs. The solvation energies given here are for an aqueous environment and are thus, at best, crude estimates. The solvation energies in the mixed solvent system used here, in which the cations are complexed with OAm, are unknown and not readily measured. The surface energy component of the dissociation energies is similarly uncertain. Nonetheless, these estimates provide a qualitative basis for understanding the driving force for the reaction. The negative value of the overall Gibbs energy change calculated from the above energies (-416 kJ/mol) implies that the proposed CE reaction tends to be spontaneous, which is consistent with our observations. Though the lattice energies suggest the parent CuS is more stable than product Ag2S crystals, the much greater solvation energy of Cu2+ compared to that of two Ag+ ions drives the reaction toward formation of Ag2S and Cu2+. Because the solvation energies depend strongly on temperature, solvent, ligands, and other factors, the reaction equilibrium might be shifted toward CuS and 2Ag+ under other reaction conditions, but we did not observe any such conditions. This mechanism is also consistent with the previous discussion on Cu+ incorporation into covellite CuS.26, 48 To confirm this assumption, we carried out the reverse reaction by reacting Cu2+ with Ag2S NCs. Ag2S NCs were prepared in a manner similar to that used for CuS preparation, by reacting Ag+-OAm complex with ammonium sulfide solution; synthesis details are provided in the SI. As shown in Figure S12a-b, the size and morphology of Cu2+ treated Ag2S NCs are preserved from the Ag2S template NCs. The HRTEM images and XRD patterns of the Ag2S NCs before and after Cu2+ treatment show that the they are both Ag2S NCs, indicating that the reverse reaction (from Ag2S to CuS) did not occur in the presence of Cu2+ ions (Figure S12c-d). Moreover, the optical evidence with the aid of UV-vis spectra reveals that the Cu2+ content barely changed after treatment with Ag2S NCs, consistent with the above observations (Figure S13). To further confirm that the formation of CuS-Ag2S platesatellite structure is attributed to the local redox reaction, we carried out control experiments in which the template covellite CuS NPls were replaced by NPls of another typical Cu2-xS phase. Djurleite Cu1.94S NPls were prepared, as previously reported by Zhai and Shim,49 by mixing CuCl2, t-DDT, and OAm at elevated temperature (synthesis details are given in the SI). The TEM image in Figure 4a shows that the Cu1.94S NPls have a hexagonal shape. Upon exposure to Ag+ under conditions matching the CE reaction conditions, the NPls maintained the hexagonal NPl morphology of the template, with only a limited variation of their thickness (Figure 4b-g). The XRD patterns in Figure S14 indicate that the NPls treated with 0.05 and 0.1 mmol of Ag+ retained the djurleite crystal phase. However, a phase transition was triggered using an increased amount of Ag+ (0.5 mmol), and jalpaite Ag3CuS2 NPls were obtained. HRTEM images of the 0.05 and 0.1 mmol Ag+ treated NPls show interplanar spacings of 1.9 Å in top view and 3.3 Å in edge-on view, corresponding to (080) and (800) planes of the djurleite phase. Lattice spacings of 1.9 and 3.6 Å are observed in Ag3CuS2 NPls, corresponding to (332) and (103) planes of jalpaite, respectively. The HAADF-STEM images and corresponding elemental maps in Figure S15 confirm the very

small amount of residual Ag+ in the 0.05 and 0.1 mmol Ag+ treated Cu1.94S NPls. In contrast, EDS results indicated that the transformed jalpaite Ag3CuS2 NPls are Ag-deficient (Ag : Cu = ~ 1.7 : 1) but clearly had substantial Ag content. Here, 0.5 mmol of Ag+ was enough to trigger the complete phase transition (rather than formation of heterogeneous domains), even though the product was far from the ideal stoichiometry. This observation is consistent with our previous discussion that complete phase transition usually precedes complete composition transformation.8 Compared to the CE reaction that led to formation of CuS-Ag2S plate-satellite structures, the absence of disulfide bonds allows the cation replacement in djurleite to occur without an accompanying redox reaction or reconstruction of the sulfur sublattice. Therefore, we conclude that the rupture and reorganization of disulfide bonds in covellite CuS NPls is essential for both shape evolution to plate-satellite structure and formation of binary Ag2S. Moreover, the process of Ag+ incorporation in to CuS NPls is similar to the incorporation behavior of divalent cations, in which Cu+ ions are replaced by Ag+, producing heterogeneous domains. In contrast, a homogenous Ag-Cu-S alloy was obtained by using Cu1.94S templates, similar to the behavior of trivalent and tetravalent cations that nearly always incorporate into copper sulfide phases to produce a ternary alloy phase, rather than completely replacing copper.8 Incorporation of trivalent or tetravalent cations produces phases in which all cations are 4-coordinated, unlike the parent copper sulfide phases.50 However, Ag3CuS2 contains both 4-coordinated and 6-coordinated Ag atoms and only 2-coordinated Cu atoms. Therefore, although incorporation of Ag+ into djurleite Cu1.94S yields a homogeneous alloy, we cannot conclude more broadly that incorporation of monovalent cations into copper sulfide produces homogenous alloys. In light of the results obtained using covellite CuS templates, we note that for CE with monovalent cations like Ag+ the final outcome depends on the specific copper sulfide phase used as the template.

Figure 4. (a) TEM image of djurleite Cu1.94S template NPls. Panels (b)-(d) are low magnification TEM images of Cu1.94S treated with 0.05, 0.1, and 0.5 mmol Ag+, respectively. Panel (e)-(g) are top-view HRTEM images of samples in panels (b)-



(d), respectively. The insets are corresponding side-view HRTEM images, sharing the same magnification with the corresponding TEM images, with the same scale bar of 2 nm. The outcome of the CE reaction is not only affected by the nominal oxidation states of the elements in the template, but also by their coordination numbers in the template and product NCs. Table S1 compares the coordination numbers of cations and sulfur atoms before and after CE reaction. Compared to the formation of plate-satellite heterostructures, the formation of ternary Ag-Cu-S alloy only follows a CE process with minor anionic sublattice rearrangement, so the coordination of sulfur atoms tends to be preserved. All sulfur atoms in both Cu1.94S and Ag3CuS2 are 6-coordinated, which allows the CE process to occur with minimal reorganization of the sulfur sublattice. The mixed coordination of copper atoms in Cu1.94S also enables the cations in the product Ag3CuS2 to have mixed coordination. However, in the structure of acanthite Ag2S, which is the complete CE product, the sulfur atoms only have a coordination number of 5. This is a very important feature to prevent Cu+ from being fully replaced by Ag+. On the other hand, although the sulfur atoms in covellite CuS have mixed coordination, the 4-coordinated sulfur atoms, which formed disulfide bonds, are all converted into 5-coordinated ones during the redox reaction that accompanies the CE process. Because all the Cu+ ions were replaced by Ag+ ions via the redox reactions, the mixed coordination of Cu+ in the template CuS does not play a significant role. A typical CE process consists of the replacement of cations of a starting NC with new cations, with preservation of the original anion sublattice. However, Ag2S produced by Ag+ incorporation into covellite CuS requires not only cation replacement of Cu+ with Ag+, but also extensive structural reorganization, due to elimination of disulfide bonds and additional sulfur atom migration. Although the crystal structure of covellite is hexagonal, the sulfur atoms in covellite CuS are not in a typical hcp stacking (ABABAB). The stacking of sulfur atoms in covellite CuS is more similar to ABAABA, with adjacent A anion layers linked by disulfide bonds, and no cation layer between them. On the other hand, the space group of Ag2S is monoclinic (P21/c, with a = 4.229 Å, b = 6.931 Å, c = 8.2822 Å, and β = 110.618 °). The detailed route by which transformation between these two complex structures occurs remains unclear. Direct observation of the very rapid transition is not readily possible. Most cation incorporation processes require minimal anion sublattice migration because the radii of anions are much larger than those of cations, and the cations have relatively high mobility due to applied chemical driving forces and the presence of cation defects. Therefore, rearrangement of the anion lattice is the limiting factor in the transformation, and we proposed a mechanism of phase transformation from CuS to Ag2S in terms of evolution of the sulfur sublattice. Generally, the transformation can be described as in-layer migration of S1 sulfur atoms and out-of-layer migration of S2 sulfur atoms. More details on the proposed structural transformation (Figure S16) are provided in the SI. Of course the cation replacement from Cu+ to Ag+ requires uptake of two Ag+ cations for each Cu+ that is removed, which drives volume expansion along with rearrangement of the anion lattice. We note that there are 6 and 4 sulfur atoms in each CuS and Ag2S unit cell respectively, with unit cell volumes of approximately 205 and 227 Å3, respectively. Thus, upon transformation from

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CuS to Ag2S, the volume per sulfur atom increases by approximately 66%. This substantial volume increase may explain why the template shape is not maintained during CE reactions. Instead, the transformed satellite particles are thicker than the NPls. As seen in Figure 5b and c, as the Ag2S domains grow, they increasingly lie outside the original boundaries of the template NPls. The increase in size of the Ag2S domain, relative to the volume of the template CuS NPl that has been replaced by Ag2S, is significantly greater than can be accounted for by the 66% volume increase expected for the conversion from CuS to Ag2S. Thus, the growth of these domains suggests that S ions are diffusing out of the center of the NPls, along with Cu ions, to contribute to the growth of the Ag2S domains. This is consistent with the overall thinning of the center of the NPls, to produce biconcave structures, when the Ag+ concentration is increased further, to a level that allows complete conversion to Ag2S as discussed further below.

Figure 5. TEM image of NPls obtained by treating covellite CuS NPls with (a) 0.1, (b) 0.2, (c) 0.3, (d) 0.4, and (e) 0.5 mmol of Ag+. All TEM images share the same magnification with a scale bar of 50 nm. (f) Typical HRTEM image of Ag2S (satellite) domain of plate-satellite structure. The shape evolution of plate-satellite structure was highly Ag+-concentration dependent. In a typical synthesis using 0.1 mmol of Ag+, CuS-Ag2S plate-satellite heterostructures were produced. When an increased amount (> 0.1 mmol) of Ag+ was employed, the overall plate-satellite morphology was still observed, but NCs were fully transformed into acanthite Ag2S (the XRD patterns are provided in Figure S17). As shown in Figure 5, the size of the satellite particle increased with increasing initial Ag+ concentration from 0.1 to 0.3 mmol. The increased relative contrast also indicates the increased size of satellite domains, whereas the thickness of the plate remains nearly constant. According to the HRTEM images in Figure 5f, the lattice spacings observed on the Ag2S domain are assigned to the (111) and ( 112 ) planes of Ag2S. Interestingly, when 0.4 mmol of Ag+ was employed, the resulting NPls exhibited a biconcave shape, without satellite particles. Further, treatment of CuS NPls with 0.5 mmol of Ag+ produced a particle attached to the biconcave NPl. Furthermore, compared with the EPR results of the supernatant obtained using a lower concentration of Ag+ (Figure S3a blue curve), CE reaction using a higher concentration of Ag+ yields an increased Cu2+ content. This observation is again consistent with our proposed CE reaction, along with the spontaneous redox reaction. We also measured EPR spectra of control samples prepared by dissolving 0.1 mmol or 0.5 mmol of Cu2+ ions in the mixture of OAm and toluene. The EPR intensity of the supernatant of CuS NPls after


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treatment with 0.1 mmol of Ag+ is comparable to the 0.1 mmol Cu2+ EPR control. The EPR intensity of the supernatant from the CuS NPls treated with 0.5 mmol of Ag+ ions is weaker than that of the 0.5 mmol control, consistent with the fact that the amount of Cu present in the starting NPls is less than the amount of Ag+ added, as reflected in the formation of pure Ag domains after complete conversion of CuS to Ag2S, as discussed further below.

Figure 6. Low magnification of Ag2S-Ag biconcave-particle heterostructures. (b) HR-STEM image of a single heterostructure with Ag NPs grown on the edge. (c) Typical side-view HR-STEM image of the heterostructures. (d) HRSTEM image of a single heterostructure with Ag NPs grown in the center of concave. Panels (e) and (f) are HAADF-STEM image and corresponding elemental maps for Ag, S, and Cu of a single heterostructure from panel (d) and (b), respectively. The scale bars in panel (e) and (f) are both 20 nm. We further investigated the novel biconcave-particle heterostructures using HAADF-STEM imaging. The lowmagnification HAADF-STEM image in Figure 6a reveals that the transformed biconcave NPls do not preserve the hexagonal shape of the CuS templates, possibly due to crystal transformation from the highly symmetric hexagonal CuS to less symmetric monoclinic Ag2S. Compared to the edges, the center of the biconcave NPl has low contrast and less obvious crystallinity, due to limited layers of Ag atoms and less contribution to the 2D projection for imaging. In some cases, the attached particle is located at the center of biconcave NPl (Figure 6e); otherwise, the particle grows at the edge of the biconcave NPl (Figure 6f). As shown in Figure 6e, 6f and Figure S18, both Ag and S signals are distributed throughout the whole NC. The residual Cu content is negligible, consistent with the

XRD patterns (blue curve) in Figure S19, and indicating that Cu was removed completely. This observation is also supported by EDS spectra and corresponding elemental ratios (Figure S20). We note that the Ag signal is more concentrated in the particle domain, which also has higher contrast in the TEM and HAADF-STEM images. However, the S signal is weaker at those locations, indicating that the particle domains are elemental Ag. Moreover, a pentagonally twinned structure was observed in the high contrast Ag domain (Figure 6b). This behavior is consistent with previous reports of the propensity of pure Ag NPs to form penta-twinned structures,51-54 which further confirms that the product is a Ag2S-Ag heterostructure. According to the elemental ratios, the system contains around 36.8% Ag and 63.2% of Ag2S, which is consistent with the observation of the ratio of two distinct domains from STEM imagining. The lattice spacings were found to be 2.4 Å on the Ag domain and 2.2 Å on the Ag2S domain, corresponding to (111) and (031) planes, respectively. Similar lattice spacings were observed in heterostructures with the particle at the center of the biconcave NPl. The side-view HR-STEM image in Figure 6c reveals the typical thin center (4.2 ± 0.9 nm) of biconcave NPls, with interplanar spacings of 2.7 Å and 3.4 Å, consistent with (120) and (012) planes of Ag2S. According to the XPS analysis of the heterostructure (Figure 3c, bottom panel), the Cu content was extensively decreased, while the overall S content remain the same as the plate-satellite heterostructures and CuS template. Moreover, the S 2p regions of the Ag2S-Ag biconcave-particle heterostructure evolve from three peaks to two peaks (elimination of shoulder peak at ~161.5 eV). This observation indicated that the disulfide bonds were completely reduced and all sulfur atoms have the oxidation state of -2, which is consistent with the above results. To investigate the mechanism of formation of biconcaveparticle heterostructures, we then examined the NPls early in the transition process, 30 sec after injection of the CuS NPls into the Ag+ solution. Figure S21 presents a typical TEM image of the NPls extracted at this time. Compared to product Ag2SAg biconcave-particle heterostructures, the NPls obtained after 30 s are less concave, and have smaller Ag domains, which is supported by the increase in relative intensity of the diffraction peak of (111) planes of Ag from 30 s to 10 min (Figure S19). Note that the phase transition from CuS to Ag2S is already complete after 30 s (also supported by EDS spectrum and corresponding elemental ratios in Figure S22). Side-view TEM images of the NPls reveal that, compared to the CuS template NPls, the thickness of the NPls first expands to 8.0 ± 0.7 nm at the start of the shape evolution. Therefore, the formation of biconcave structure is due to removal of the center of NPls instead of simply expansion of the edges. Interestingly, the thinnest center portion of the NPl occasionally breaks to produces a nanoring (a representative HRTEM image is provided in Figure S23), which confirm the proposed shape evolution process, and consistent with our previous observations. The formation of biconcave structures is not unprecedented, however, it is very rare in Ag2S based nanostructures. Mu et al. showed that CuInS2 nanorings (NPls with a central region removed) can be prepared from template Cu1.94S NPls by a CE reaction with In3+.30 In contrast, our group previously reported that Cu1.94S biconcave NPls could be produced by reacting flat CuInS2 NPls with Cu+.11 The formation of a hole or concave region in the center of these NPls was attributed to the nanoscale Kirkendall effect, first reported by Alivisatos and coworkers.55



According to this mechanism of shape evolution, the basal planes of the CuS template NPls are well passivated by the ligands. Therefore, the edge planes are relatively active, which allows Ag+ ions to diffuse into the NPl. This is also supported by that fact that Ag2S NPs tend to grow at the corner of CuS NPls. Thus, CE through the edges apparently has a lower barrier compared to CE on the top and bottom planes. The rate of cation replacement and sulfur sublattice reorganization was modest when starting with lower initial concentration (0.1 mmol) of Ag+, producing controllable growth of Ag2S domains at the corners of the NPls. When Cu+ ions are more rapidly replaced by Ag+ due to a high concentration of Ag+ in the environment, the edges only provide a limited pathway for in- and out-diffusion of cations. Initially, Cu+ rapidly diffuses out from the center to the edges of the NPls, where CE reaction occurs, producing complete Ag2S NPls (stage 1). However, the incoming Ag+ ions migrate toward the center of the NPls much more slowly, due to the narrow edges. Cation deficiency near the center then provides a driving force for transport of S toward the edges as well. Overall, material is transported from the center of the NPl toward the edges, where formation of Ag2S is occurring (stage 2). As discussed above, even during formation of the plate-satellite structures, with insufficient Ag+ for complete conversion of CuS to Ag2S, we saw evidence of transport of sulfur to the growing Ag2S domains. After the biconcave Ag2S NPls were produced, the Ag+ ions had not been completely consumed and the colloidal system contained excess Ag+. Although the system cannot provide enough reducing potential for nucleation of Ag NPs alone, the Ag+ ions can gradually nucleate on the existing Ag2S domains, producing Ag2S-Ag heterostructures. In some cases, the Ag NPs locate at the center of biconcave NPls, which can be explained by the lower crystallinity of these centers and minimization of total surface area. The heterostructure formation was then followed by a ripening process in which the depth of the concave region was increased and the NPs grew larger (stage 3). Though Ag2S-Ag hybrid nanostructures have been explored and reported,19, 56-58 the biconcave-particle morphology is unprecedented. Its formation is attributed the unique crystal structure and morphology of covellite CuS template NPls, which in turn, highlights the novelty of our work. The schematic illustration of overall shape evolution is shown in Figure 7.

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Conclusion In summary, we have incorporated Ag+ into covellite CuS NPls to produce heterostructures with new and interesting morphologies. The morphology of the resulting NCs depends on the initial concentration of Ag+ ions. Generally, the shape evolution and crystal phase transition are based on rates of Cu+ out-diffusion and Ag+ in-diffusion. We showed that the crystal structure and morphology of the template NPls play a decisive role in formation of Ag2S and biconcave structures. The platesatellite CuS-Ag2S and biconcave-particle Ag2S-Ag heterostructures are reported for the first time. More importantly, we have established explicit transformation mechanisms for the complex transformation from covellite CuS to acanthite Ag2S, by comparing the distinct hexagonal CuS structure with monoclinic Ag2S. We believe that our work not only provides new insight into cation incorporation mechanisms but also provides a novel colloidal synthetic route for preparation of previously unavailable nanostructures.

ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge on the ACS Publications website. XRD pattern and HR-STEM image of covellite CuS template NPls; XRD patterns of 0.1 mmol of Ag+ treated CuS NPls at different reaction time; FFT pattern and EDS spectrum of CuS-Ag2S heterostructure; size- and thickness-distribution histograms for samples; photograph and UV-vis absorbance spetrum of precursors, sample dispersions and supernatant of 0.1 mmol of Ag+ treated CuS NPls; structural model of covellite CuS; XPS, EPR and Raman spectra of covellite CuS before and after Ag+ incorporation; TEM, HRTEM images and XRD patterns of Ag2S NCs before and after Cu2+ treatment; additional synthesis details; XRD patterns, HAADF-STEM image and corresponding elemental maps of djurleite NPls treated with different amount of Ag+; illustration of structural transformation from CuS to Ag2S; XRD patterns of covellite CuS NPls after treatment with different amount of Ag+; additional HAADF-STEM image, EDS spectra and corresponding elemental maps of Ag2S-Ag biconcave-particle heterostructure; TEM images of covellite CuS template NPls after treatment with 0.5 mmol of Ag+ for 30 sec; representative HRTEM image of Ag2SAg ring-particle heterostructures.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported in part by the New York State Center of Excellence in Materials Informatics.

REFERENCES

Figure 7. Schematic illustration of overall shape evolution.

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Ag+-Induced Shape and Composition Evolution of Covellite CuS

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