Selective Cation Incorporation into Copper Sulfide Based

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Selective Cation Incorporation into Copper Sulfide Based Nanoheterostructures Yang Liu,† Maixian Liu,‡ Deqiang Yin,† Liang Qiao,† Zheng Fu,† and Mark T. Swihart*,† †

Department of Chemical and Biological Engineering and ‡Department of Pharmaceutical Science, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States

ACS Nano 2018.12:7803-7811. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/28/18. For personal use only.

S Supporting Information *

ABSTRACT: Heterogeneous copper sulfide based nanostructures have attracted intense attention based on their potential to combine the plasmonic properties of copper-deficient copper sulfides with properties of other semiconductors and metals. In general, copper sulfides are versatile platforms for production of other materials by cation incorporation and exchange processes. However, the outcomes of subsequent cation exchange (CE) or incorporation processes involving nanoheterostructure (NH) templates have not been explored. In this work, we incorporate indium and tin into Cu1.81S−ZnS NHs. We demonstrate that the outcomes of cation incorporation are strongly influenced by heterocation identity and valence and by the presence of a Cu-extracting agent. The selectivity of cation incorporation depends upon both the cation itself and the heterodomains in which CE reactions take place. The final nanocrystals (NCs) emerge in many forms including homogeneous NCs, heterodimers, core@shell NHs and NHs with three different domains. This selective cation incorporation not only facilitates the preparation of previously unavailable metal sulfide NHs but also provides insight into mechanisms of CE reactions. KEYWORDS: cation exchange, cation incorporation, nanoheterostructures, metal sulfide nanocrystals, shape evolution

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uncommon and useful combinations of properties. However, further cation incorporation using these NCs to produce quaternary Cu−M−N−S NCs has rarely been considered. Upon subsequent exposure of ternary Cu2−xS−MS NCs to a third type of cation Nn+ under appropriate conditions, the incoming Nn+ may (1) diffuse into the Cu2−xS domain to form a homogeneous Cu−N−S alloy domain; (2) replace Mn+ in the MS domain; (3) enter both domains and promote mixing of Cu and Mn+ ions, producing a homogeneous, single-phase NC; or (4) trigger morphology evolution of the entire NC. The overall process of transformation from Cu2−xS into a ternary heterogeneous system and then further into a different multidomain NH provides a powerful means of accessing different combinations of NH composition, crystal phase, and morphology and can provide valuable insights into mechanisms of NCs’ shape and phase evolution. When designing and attempting to synthesize previously unavailable metal sulfide (MS-NS) NHs, two common synthetic routes are available: (1) one-pot synthesis by directly mixing multiple cation precursors simultaneously;8,18,19 and (2) multistep synthesis by growing a second NS domain onto an existing MS domain.14,20,21 However, when neither of the alternatives can produce desired NHs, an approach using

eterogeneous metal chalcogenide nanostructures are of considerable interest for applications in solutionprocessed solar cells, photocatalysis, and photoacoustic imaging.1−5 Among these nanostructures, copper sulfide based nanoheterostructures (NHs) have attracted extensive attention due to their controllable morphology, tunable bandgaps, and large absorption coefficients.3,6,7 Formation of NHs not only integrates properties of two or more different materials in a single structure, but also provides a means of influencing charge carrier separation and recombination that is not available in homogeneous structures and may promote other synergistic interactions between domains. Thus, the ability to design and synthesize targeted NHs is of great potential value. Fabrication of these NHs generally involves formation of a Cu2−xS nanocrystal (NC) matrix followed by heterocation incorporation at appropriate conditions.8,9 Cu2−xS NCs have emerged as an ideal template for cation incorporation because Cu atoms are highly mobile at elevated temperatures, enabling efficient interstitial diffusion and substitution of incoming cations.10−12 In most cases, cation exchange (CE) reactions preserve the morphology of template NCs. Incomplete CE can produce NHs with a clear boundary between immiscible Cu2−xS and MS domains.13−15 Less commonly, cation incorporation leads to additional shape evolution when heterogeneous NCs are formed.16,17 The synthesis of heterogeneous, ternary Cu2−xS− MS NCs has drawn considerable attention due to their © 2018 American Chemical Society

Received: March 12, 2018 Accepted: July 9, 2018 Published: July 9, 2018 7803

DOI: 10.1021/acsnano.8b01871 ACS Nano 2018, 12, 7803−7811

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ACS Nano Cu2−xS template NCs could be the solution for preparation of previously unavailable NHs. The synthetic route generally involves synthesizing Cu2−xS−MS NHs by M incorporation into Cu2−xS template NCs, followed by N incorporation into the rest of Cu2−xS domain. Thus, a better understanding of cation incorporation mechanisms, especially using Cu2−xS template NCs, is indeed intriguing and significant. According to our previous report, incorporation of divalent cations into Cu2−xS NCs exclusively produces heterogeneous Cu2−xS−MS NCs due to conflicts between valence and coordination number of divalent cations.9,22 Heterogeneous ternary NCs can also be obtained by incomplete trivalent and tetravalent cation incorporation and diffusion, under conditions that would ultimately fully convert the Cu2−xS templates into homogeneous ternary NCs.16,23,24 For example, treatment of Cu2−xS using Zn2+ and Cd2+ consistently produces Cu2−xS−ZnS/CdS heterostructures rather than homogeneous alloy NCs. In contrast, cation incorporation using trivalent In3+ or tetravalent Sn4+ produces Cu−In−S or Cu−Sn−S ternary alloy NCs (possibly with Cu2−xS-CuInS2 or Cu2−xS−Cu2SnS3 as an intermediate product). Thus, the heterostructures produced by divalent cation incorporation provide two clearly defined domains of different composition, one of which (ZnS) has both the preferred cation valence (+2) and coordination satisfied, and one of which (Cu1.81S) still suffers from a conflict between preferred cation valence (+1) and coordination. Observing the evolution of this heterostructure upon exposure to different cations can provide insights into the relative stability of the two domains, and of the interface between them. In addition, incorporation of foreign cations with cation radii much different from Cu+ could lead to lattice expansion- or shrinkage-induced shape evolution of the Cu2−xS template NCs.22,25−27 Therefore, NHs produced by Zn2+ incorporation into Cu2−xS (here we use Cu1.81S-ZnS), are an ideal template to investigate cation incorporation into copper sulfide based NHs. In this work, Cu1.81S−ZnS NHs were first prepared by Zn2+ incorporation into a roxbyite (Cu1.81S) template. Indium and tin are known to readily incorporate into Cu2−xS to produce Cu−In−S and Cu−Sn−S ternary alloys, indicating the overall compatibility of In/Sn with Cu2−xS.6,20,23,28−30 Trioctylphosphine (TOP) is usually applied as a copper-extracting agent during Cu2−xS CE reactions, because TOP is a soft base, which binds strongly to Cu+(a soft acid).31 The final products of different combinations of treatments of Cu1.81S-ZnS NHs with In or Sn with or without TOP exhibit a diverse array of structures including homogeneous ZnS, Cu1.81S, and CuInS2 NCs; CuInS2−ZnS and ZnS−SnS heterodimers; Cu1.81S@SnS core@shell NHs; and triple-domain Cu2−xS−In2S3−ZnS. Thus, our work expands the range of options for design and production of interesting and useful NHs which were previously unavailable.

Figure 1. (a) Low magnification STEM-HAADF and (b) HRSTEM image of Cu1.81S-ZnS NHs. (c) STEM-HAADF image and corresponding elemental maps of a single NC from panel (a). Scale bar in panel c is 5 nm. The red area shows the Cu1.81S domain, and the green area shows the ZnS domain.

grows epitaxially, parallel to the a-axis of roxbyite Cu1.81S (caxis of ZnS) to form caps on both sides of each NH. Their XRD pattern (Figure S3) and elemental mapping (Figure S4) reveal that the NCs consist of both Cu1.81S and wurtzite ZnS phases. The STEM-EDS results suggest that Cu (red) is concentrated in the middle of the NCs, whereas Zn (green) is located in the caps. Sulfur (yellow) is distributed homogeneously throughout the entire NC (Figure S5). Although the thickness of the Cu1.81S interlayer is tunable by controlling the reaction time (Figure S6, detailed reaction conditions and results are provided in the SI), we choose NCs with a Cu1.81S interlayer thickness of ∼7 nm as the template for further cation incorporation experiments because they can not only provide clear evidence of any morphology evolution but also retain enough Cu and Zn content to allow formation of different Cuand Zn-containing crystal phases after additional cation treatment. Next, the Cu1.81S−ZnS NHs were used as a template for cation incorporation. TOP has been widely used as a Cuextracting agent because it has strong affinity for Cu+, compared with the modest stability of the soft copper− chalcogenide bond.32 Thus, the presence of TOP is expected to strongly affect the cation incorporation results. Note that, to ensure effective CE, we used excess TOP (5 mL) whenever TOP was used in experiments reported here. Figure 2 shows the results obtained using In3+ with TOP. The lowmagnification TEM image in Figure 2a reveals that the product NCs preserve the overall shape, size (d = 21.6 ± 0.7) and multidomain structure of the template NHs, but with less obvious boundaries between different domains. Panels b and c of Figure 2 show HR-STEM images of a single NH treated with In3+ and TOP and the corresponding elemental maps, indicating that the resulting NC consists of a Cu−In−S alloy center with two Zn−S caps. The EDS spectra suggest the elemental ratio is as shown in Figure S7, the XRD pattern of the resulting NCs most closely matches that of the wurtzite CuInS2 phase. However, because of overlap in the XRD patterns ( Zn2+ > Sn2+ > Cu+.27 This difference in hardness of cations also strongly affects the affinity of cations toward S2−. Therefore, In3+ and Sn2+, which occupy intermediate positions in this sequence, can bind to S2− more strongly than Sn4+, producing a lower reaction barrier for cation incorporation. Compared to Sn2+ (118 pm), incorporation of In3+ (∼62−80 pm) is more favorable due to its similar cation radius to Cu+ (∼60−77 pm),22,35 partial cation replacement required in a unit cell, and the slightly weaker Cu−S (274.5 ± 14.6 kJ/mol) bond in comparison with In−S (287.9 ± 14.6 kJ/mol) in CuInS2.39 Thus, even without a Cuextracting agent, In3+ can partially replace Cu in the Cu1.81S domain, producing CuInS2. On the other hand, using a Cu-extracting agent such as TOP leads to elimination of the Cu1.81S domain while retaining the ZnS domain. Postsynthesis treatment with Sn4+ or Sn2+ in the presence of TOP leads to complete removal of Cu from the NHs. Sn2+, which is a softer cation than Sn4+ can replace Cu to produce a SnS domain. However, although Sn4+ accelerated the dissolution of the Cu1.81S domain, its incorporation into the NHs was negligible. This is consistent with prior studies in which Sn4+ and some other ions (e.g., Al3+) were employed to modify the morphology during synthesis of Cu2−xS.40−43 In those cases, Sn4+ was not incorporated into the growing NCs, but the formation of Sn(IV)−organo−halide complexes influenced the nucleation and growth rates of specific facets, producing NCs with controlled morphology as well as decreased ensemble polydispersity.41,42 To investigate how the ZnS domains affect the cation incorporation results and to avoid interferences due to overlapping diffraction patterns, we treated the template roxbyite Cu1.81S NCs (without ZnS domains) with In3+/Sn2+ in the presence of TOP. As shown in Figure 10a,b and Figure S34, colocation of Cu, In and Sn signals and HRTEM imaging indicate the formation of CuInS2, The XRD pattern further confirms the formation of wurtzite CuInS2. These results imply that the ZnS domains do not influence the outcome of the In3+ incorporation. Figure 10c−e and Figure S35 show that Sn2+, in the presence of TOP, can replace all the Cu+ from the Cu1.81S NCs, producing homogeneous SnS NCs. The XRD pattern of these NCs (Figure S36) shows only the herzenbergite SnS phase. To further confirm the effect of Sn4+ on the cation incorporation process, we also treated the Cu1.81S−ZnS NHs at similar reaction conditions in the absence of Sn4+, both with (Figure S37a) and without (Figure S37b) TOP. The results indicate that Sn4+ accelerates the dissolution of the ZnS (without TOP) or Cu1.81S (with TOP) domains, even though it is not incorporated into the NCs under our reaction conditions, consistent with the studies using Sn4+ to control NC morphology during growth mentioned above.41,42

observation is supported by the XRD pattern, indicating that the copper sulfide domain is roxbyite Cu1.81S (Figure S31). The HR-STEM image in Figure 8b shows that the Cu1.81S core has a lattice spacing of 0.18 nm, corresponding to (886) planes of Cu1.81S. Though the crystal phases and elemental distribution have been confirmed, we did not obverse the heterointerface between Cu1.81S and SnS on NHs. The EDS line profile and point scan at different positions show that, although Cu and S signals are higher near the center of the NC (due to its spherical morphology), the Sn signal is distributed almost uniformly on the NC and is more concentrated on the shells (Figure S32). As shown in Figure 8c and Figure S33, the SnS shell is as thin as 1−2 nm. We attribute this thin SnS shell to the limited extent of CE that occurs without any copperextracting agent. Similar to the case using Sn4+, the ZnS caps were removed in the absence of TOP. Meanwhile, Sn2+ ions adsorbed onto the surface of Cu1.81S core and started replacing Cu+ ions. However, the CE reactions only occur on the surface of the Cu1.81S core, due to lack of Cu-extracting agent, producing Cu1.81S@SnS core@shell NHs with a shell thickness of only 1−2 nm. Therefore, the blurry heterointerface on NHs may arise from the thin and less-crystalline SnS shell. The formation of core@shell structures can occur via the following steps: The ZnS caps of the template were removed one after another in the absence of TOP, leaving behind the less spherical Cu1.81S cores. Because Sn2+ has a relatively large cation radius and no TOP was present to promote Cu+ extraction, the Sn2+ ions could only adsorb on the surface of the Cu1.81S core and therefore the extent of CE reaction (Cu+ replacement with Sn2+) was very limited. Simultaneously, the S2− ions released from dissolved ZnS caps (no ZnS was detected according to XRD and EDS characterization) can react with the absorbed Sn2+, producing thin SnS shells. This process is also limited by its higher reaction barrier, compared with the CE reaction. Therefore, no individual SnS particles free from the Cu1.81S cores were observed. Combining all of the above results and observations, as summarized schematically in Figure 9, we can propose mechanisms of selective cation incorporation, extraction, and exchange in these NHs. Starting from the Cu1.81S−ZnS NHs, direct exposure to foreign cations, without a copper-extracting agent, leads to rapid dissolution of one of the ZnS caps, finally

Figure 9. Schematic illustration of overall selective cation incorporation into Cu1.81S−ZnS NHs. 7808

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ACS Nano

Synthesis of Cu1.81S−ZnS NHs. The synthesis of Cu1.81S−ZnS NHs was adapted from Ha and co-workers’ report with some modifications.14 The roxbyite Cu2−xS NC dispersion was prepared by dispersing 35 mg dry roxbyite Cu2−xS NCs in 8 mL of TOP. A mixture of 250 mg ZnCl2 and 10 mL of OAm was held under flowing argon at ambient temperature for 30 min before heating to 180 °C. The mixture was held at this temperature for 30 min before cooling to 55 °C. During the cooling process, 20 mL of toluene was added. Eight milliliters of the roxbyite Cu2−xS NC dispersion was injected into the mixture, and the mixture was held at 55 °C for 60 min. To collect the NHs, 10 mL of ethanol was added, followed by centrifuging at 4000 rpm for 1 min. Cation Incorporation into Cu1.81S−ZnS NHs without TOP. The NH dispersion was prepared by dispersing dry NHs (one batch, typically ∼22.5 mg) in 5 mL of OAm. A mixture of 0.15 mmol of SnCl4 (or InCl3 or SnCl2) 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 NH dispersion was injected into the mixture, and it was held at this temperature for 30 min. To collect the NHs, 10 mL of ethanol was added, followed by centrifuging at 4000 rpm for 1 min. Cation Incorporation into Cu1.81S−ZnS NHs Using TOP. The NH dispersion was prepared by dispersing dry NHs (one batch, typically ∼22.5 mg) in 5 mL of TOP. A mixture of 0.15 mmol SnCl4 (or InCl3 or SnCl2) and 10 mL of OAm was held under flowing argon at ambient temperature for 30 min before heating to 160 °C (for InCl3 or SnCl2) or 120 °C (for SnCl4). Then, 5 mL of the NH dispersion was injected into the mixture, and it was held at this temperature for 10 min (30 min for the InCl3 and SnCl2 case). To collect the NHs, 10 mL of ethanol was added, followed by centrifuging at 4000 rpm for 1 min. Cation Incorporation into Roxbyite Cu1.81S NCs Using TOP. The roxbyite Cu2−xS NC dispersion was prepared by dispersing 35 mg of dry roxbyite Cu2−xS NCs in 5 mL of TOP. A mixture of 0.3 mmol of InCl3 (or SnCl2) 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 the Cu2−xS NC dispersion was injected into the mixture, and it was held at this temperature for 30 min. To collect the NHs, 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 JEMARM200F 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 for XRD were prepared by drop-casting concentrated NC dispersions onto glass slides.

Figure 10. (a) Low magnification TEM image and (b) HRTEM image of roxbyite Cu1.81S NCs after treatment with In3+ and TOP. (c) Low magnification TEM image, (d) HR-STEM image, (e) HAADF image and corresponding elemental maps of roxbyite Cu1.81S NCs after treatment with Sn2+ and TOP. The scale bar in panel (e) is 10 nm.

CONCLUSION In summary, we achieved a variety of outcomes for selective cation incorporation into Cu1.81S−ZnS heterostructures by controlling the heterocations and reaction conditions. The results are strongly affected by the heterocation identity and oxidation state and the presence of a copper-extracting agent. We conclude that (1) only certain types of cations can be incorporated into the NHs and (2) incoming cations enter through a particular domain, rather than throughout the entire NCs. The final NCs include a variety of distinct nanostructures such as triple domain NCs, heterodimers, core@shell NCs and homogeneous NCs. These results provide insights into CE reactions of copper sulfide based nanomaterials. Among the resulting NHs, triple domain Cu2−xS−In2S3−ZnS NHs, ZnSSnS heterodimers and Cu1.81S@SnS core@shell NHs appear to be previously unreported structures. We believe that our discoveries and discussions can help to guide the design of even more types of heterostructures. METHODS Chemicals. All chemicals were used as received. Indium chloride (InCl3, 98%), tin(IV) chloride (SnCl4, reagent grade, 98%), tin(II) chloride (SnCl2, reagent grade, 98%), di-tert-butyl disulfide (97%), oleylamine (70%, OAm), and trioctylphosphine (TOP, technical grade, 90%) were purchased from Sigma-Aldrich. CuCl2·2H2O and toluene were purchased from Fisher Scientific. Synthesis of Roxbyite Cu2−xS Template NCs. The method for preparing roxbyite Cu2−xS NCs was adapted from Li and co-workers’ report with some modifications.34 In a typical synthesis, 1 mmol of CuCl2·2H2O was dissolved in 10 mL of OAm, and the solution was held under flowing argon at ambient temperature for 30 min before heating to 180 °C. Then, 4 mL of di-tert-butyl disulfide was injected into the mixture, and the solution was held at this temperature for 15 min. To collect the NCs, 10 mL of ethanol was added, followed by centrifuging at 4000 rpm for 1 min. To remove excess di-tert-butyl disulfide and OAm, the NCs were washed by precipitation with ethanol and centrifugation followed by redispersion in chloroform. This purification process was applied to all of the NCs described in the following sections. The NCs could be redispersed in organic solvents including chloroform, hexane, and toluene.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b01871. TEM, HRTEM image, and XRD pattern of roxbyite Cu1.81S template NCs; size distribution histograms of all samples; XRD pattern, low-magnification HAADF image and corresponding elemental maps of Cu1.81S−ZnS NHs; XRD pattern and additional elemental maps of cation incorporation results using In3+ and TOP; XRD pattern and additional elemental maps of cation 7809

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ACS Nano incorporation results using excess In3+ and TOP; XRD pattern, additional TEM image and elemental maps of cation incorporation results using Sn4+ and TOP; TEM images of roxbyite Cu1.81S synthesized using extended reaction time; XRD pattern and additional elemental maps of cation incorporation results using In3+; structural model of CuInS2; XRD pattern and additional elemental maps of cation incorporation results using Sn4+; XRD pattern, additional elemental maps of cation incorporation results using Sn2+ with or without TOP. XRD pattern and additional elemental maps of In3+/Sn2+ and TOP-treated Cu1.81S NCs; TEM image of TOP- and oleylamine-treated Cu1.81S−ZnS NHs. EDS spectrum and corresponding elemental ratios for all product NCs; EDS point scans and line profiles for all product NHs (PDF)

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AUTHOR INFORMATION Corresponding Author

*E-mail: swihart@buffalo.edu. ORCID

Yang Liu: 0000-0001-5586-623X Maixian Liu: 0000-0003-1179-2376 Deqiang Yin: 0000-0002-5680-9365 Liang Qiao: 0000-0002-6345-3504 Mark T. Swihart: 0000-0002-9652-687X Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acsnano.8b01871 ACS Nano 2018, 12, 7803−7811

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DOI: 10.1021/acsnano.8b01871 ACS Nano 2018, 12, 7803−7811