In Situ Observation of Divergent Phase Transformations in Individual Sulfide Nanocrystals

Transcription

1 Supporting Information for: In Situ Observation of Divergent Phase Transformations in Individual Sulfide Nanocrystals Matthew T. McDowell 1, Zhenda Lu 1, Kristie J. Koski 2, Jung Ho Yu 1, Guangyuan Zheng 3, Yi Cui 1,4 * 1 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA. 2 Department of Chemistry, Brown University, Providence, RI, 02912, USA. 3 Department of Chemical Engineering, Stanford University, Stanford, CA, 94305, USA. 4 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA. *Corresponding author: Y.C. yicui@stanford.edu, phone: , fax: Current address: Postdoctoral Scholar, California Institute of Technology, Division of Chemistry and Chemical Engineering. Keywords: Inorganic nanocrystals, phase transformations, conversion reactions, displacement reactions, in situ TEM.

2 1. Supplementary videos Videos showing the reaction process for Cu 2 S, Co 3 S 4, and FeS 2 nanocrystals are available on the publisher s website. Video S1 shows the reaction of the group of three larger Cu 2 S particles shown in the main text in Fig. 2. This video is presented at twice the actual speed. Video S2 shows the growth of a copper dendrite from a group of larger Cu 2 S particles undergoing reaction. Images from this video are also shown in Fig. 2 in the main text. This video is presented at five times the actual speed. Video S3 shows the reaction of a small Cu 2 S nanocrystal viewed edge-on, as shown in Fig. 3 in the main text. This video is presented at the actual speed. Video S4 shows the reaction of Co 3 S 4 nanocrystals. This video is presented at five times the actual speed. Note that lithiation does not occur until near the end of the video. Video S5 shows the reaction of a FeS 2 nanocrystal. This video is presented at five times the actual speed. 2. Synthesis methods 2.1 Cu 2 S Synthesis (smaller particles) The smaller Cu 2 S particles were synthesized using standard Schlenk line techniques. 1 First, g copper acetylacetonate (Cu(acac) 2, 97%, Sigma Aldrich) was mixed with 50 ml oleylamine (OLA, 90%, Sigma Aldrich) and stirred in an open threeneck round-bottom flask at room temperature. 53 mg of sulfur (99.98%, Sigma Aldrich) was then added, and the solution was stirred for 2 hours. After thorough mixing, the flask was evacuated to a pressure of ~300 mtorr, and the solution was held under vacuum for 30 min. Nitrogen was then introduced into the flask until atmospheric pressure was reached, and then a needle was inserted to allow nitrogen to flow during the reaction. Using a heating mantle, the temperature was increased to ~ C over 40 min and the solution was then held at this temperature for another 40 min. After this process, the heat was turned off and the solution was allowed to cool naturally to room temperature. The products were then purified three times with ethanol and then dispersed in toluene. 2

3 2.2 Cu 2 S Synthesis (larger particles) Larger Cu 2 S disks were synthesized using a similar high-temperature pyrolysis reaction. 2 Typically, 3 mmol Cu(acac) 2, 10 ml dodecanethiol (DDT, 90%, Sigma Aldrich), and 20 ml OLA were mixed in a three-neck round-bottom flask. After degassing with nitrogen for about 30 min, the mixture was heated slowly to 250 C and kept at this temperature for 60 min. The mixture was cooled, and the Cu 2 S crystals were precipitated with ethanol and collected with centrifugation. The disks were then purified with a mixture of toluene/ethanol two more times and then dispersed in toluene. 2.3 Co 3 S 4 Synthesis The hollow Co 3 S 4 nanocrystals were synthesized via an established method in which Co nanoparticles are first formed and then converted to Co 3 S 4 via the Kirkendall effect. 3 Briefly, trioctylphosphine oxide (TOPO, Sigma Aldrich, 98%) was degassed with nitrogen at 60 C for about 20 min, and then a solution of 0.1 ml oleic acid (OA, Sigma Aldrich, 99%) in 15 ml of anhydrous o-dichlorobenzene (DCB, Sigma Aldrich, 99%) was introduced. The OA-DCB solution was prepared in an argon glove box. The mixture was heated to the reflux temperature (about 182 C) while another solution consisting of 0.54 g dicobalt octacarbonyl (Fisher Scientific, contains 5% hexane as a stabilizer) in 3 ml of DCB was prepared in a separate flask in the glove box. The dicobalt octacarbonyl solution was then rapidly injected into the initial mixture with vigorous stirring. 2 min after the Co precursor injection, a second solution of g sulfur in 5 ml DCB was also injected. The solution was heated under stirring for another 3 min and then cooled to room temperature. 2.4 FeS 2 Synthesis The FeS 2 nanocrystals were also synthesized via previously-reported techniques. 4 Briefly, 189 mg FeCl 2 (Sigma Aldrich, 97%), 384 mg 1,2-hexadecanediol (Sigma Aldrich, 90%), 30 ml octadecene (Sigma Aldrich, 90%), and 12 ml oleic acid were mixed. The mixture was degassed at 100 C for 1 hour under nitrogen gas. A second solution of 576 mg of sulfur in 15 ml OLA was then injected into the original solution under stirring, followed 3

4 by heating up to 240 C and holding at that temperature for 1 hour. The solution was then cooled to room temperature and methanol was added to collect the nanocrystal powder. Further purification was performed with ethanol/chloroform and methanol/chloroform solutions. 2.5 Synthesis of Carbon Tubes The carbon tubes were synthesis via depositing a carbon film onto the surface of anodic aluminum oxide (AAO) membranes (Whatman, ~60µm thickness, 200 nm pore diameter) and then removing the AAO templates. 5 For synthesis, 120 mg of AAO was put in an alumina boat in a tube furnace, and the temperature was increased to 750 C at 5 C/min under argon flow. After reaching this temperature, carbon deposition was accomplished by flowing the argon through a stainless steel bubbler filled with hexane for 2 hours; the hexane decomposes to form a carbon coating on the AAO. After cooling, the AAO template was removed by immersing in 2M H 3 PO 4 for 10 hours. 3. TEM methods TEM imaging was carried out at Stanford University with two different instruments: a 200 kv FEI Tecnai G2 F20 and a 300 kv FEI Titan equipped with a spherical aberration corrector. All high-resolution images in the main text were taken with the Titan at 300 kv except for the image in Fig. 3d. The low-magnification images in Fig. 2a-h were taken at 200 kv with the Tecnai. For high-resolution imaging, the beam current density was between 3000 and 5000 electrons per Å 2 per second. As mentioned in the main text, relatively large fluxes such as these have been shown to heat Cu 2 S nanocrystals so that they transform to the high chalcocite (HC) phase; 6 this may have occurred during HR imaging in these experiments. The HC phase is known to have higher ionic diffusivity than the low chalcocite (LC) phase. However, the same Cu 2 S reaction mechanisms were observed at high magnification, low magnification, and away from the beam, so it was concluded that any possible phase changes among the various Cu 2-x S phases do not significantly influence the transformation pathways at the reaction rates studied here. As an example, Fig. S1 shows a transformed Li 2 S/Cu particle that was not imaged during the transformation. This particle shows the same morphology as those 4

5 imaged during the transformation: there is a single Li 2 S crystal with an attached copper cap. Finally, we note that the reaction products (Li 2 S + metal) in all three nanocrystal chemistries were generally morphologically stable during TEM imaging. Figure S1. An example of a transformed Cu 2 S nanocrystal that was not imaged during the transformation. Here, the Li 2 S crystal is attached to the Cu particle. 4. Further characterization of Cu 2 S nanocrystals 4.1 Larger Cu 2 S Crystals An image of a group of larger Cu 2 S crystals is shown in Fig. S2. These crystals are typically ~15 nm thick and ~30-40 nm wide. Figure S2. TEM image of a group of larger Cu 2 S nanocrystals on a flat carbon TEM film grid. Groups of crystals such as these tend to stack together. 5

6 Figure S3 shows two SAED patterns from these larger Cu 2 S crystals. Figure S3. Example SAED patterns from two different regions of a Cu 2 S sample dispersed onto a carbon film TEM grid. a) This pattern closely matches the LC phase, with characteristic rings distinguishing the LC phase from the djurleite phase marked with asterisks. b) This pattern strongly resembles the first, with similar primary diffraction rings. However, the characteristic rings marked with an asterisk in (a) are less intense in (b); the possible reasons for this are discussed in the text. Figure S3a shows a typical SAED pattern from a group of larger Cu 2 S nanocrystals. This pattern corresponds nicely to the LC phase (JCPDS no ), with rings distinguishing the LC phase from djurleite marked with asterisks. 7 This is a similar pattern to that shown in Fig. 1 in the main text for smaller Cu 2 S crystals, although in that case the relative intensities of the diffraction rings are different because the smaller crystals were oriented with the disk face flat on the substrate while the larger crystals here were oriented more randomly. Figure S3b shows a SAED pattern from a different location on the same sample. Here, the rings indicating the presence of either LC or djurleite rather than HC are still present, but the rings marked with an asterisk in (a) are of lower or negligible intensity in (b). As mentioned in the main text, this could indicate the presence of djurleite (JCPDS no ) crystals in the sample. In addition, since the crystals have a tendency to stack together, preferential orientation of the crystals with respect to the beam also could contribute to the varying intensity of different diffraction rings. So, we conclude that most of the crystals are synthesized in the LC 6

7 phase, but we cannot rule out the possible inclusion of djurleite. As noted in the main text, however, the structure of the sulfur sublattice is very similar for all these phases, and the shift of the sulfur sublattice during the transformation to Li 2 S is the most important part of these experiments. 4.2 Cu 2 S Heating Experiments To further understand differences of the diffraction patterns of the Cu 2-x S phases, in situ TEM heating experiments were carried out while the SAED pattern of larger Cu 2 S crystals was monitored, as shown in Fig. S4. A Gatan heating holder was used for these experiments. Recall that both LC and djurleite transform to HC at temperatures > ~104 C. 8 During heating, the less intense rings in the LC pattern in Fig. S4a disappear as the HC phase forms. The HC diffraction pattern (Fig. S4b, JCPDS no ) contains the most intense rings from the LC pattern because of the structural similarities between the two phases. Figure S4. SAED patterns from an in situ heating experiment of larger Cu 2 S nanocrystals. a) The SAED pattern at room temperature is similar to the LC pattern shown in Fig. S3a. b) Heating above the transition temperature causes a transformation to HC, as shown by the indexed rings of this pattern. All the HC diffraction rings are also present in the LC pattern in (a), but the less intense diffraction spots in the LC pattern disappear upon heating. 7

8 5. Fourier-filtered characterization of Cu 2 S reaction products To probe the structure of the products of the reaction of Cu 2 S and Li, highresolution TEM images were analyzed using Fourier filtering to correlate the position and periodicity of lattice fringes, as shown in Fig. S5 below. Figure S5. Fourier transform (FFT) analysis of a reacted Cu 2 S nanoparticle. a) TEM image of Li 2 S/Cu bilayer structure created by the lithiation of a Cu 2 S nanoparticle. b) Fourier-filtered image of the TEM image in (a). The components of the FFT selected to generate the Fourier-filtered image arise due to the crystalline periodicity of Li 2 S, and they are circled with various colors in the inset FFT. Black circles correspond to Li 2 S {111} planes, red circles correspond to Li 2 S {220} planes, and blue circles correspond to Li 2 S {200} planes. From this image, it is clear that the single crystal at the top of the TEM image in (a) is Li 2 S. c) Fourier-filtered image created by selecting the components in the inset FFT that correspond to Cu {111} planes. Periodicity is visible in the bottom half of the Fourier-filtered image, indicating that the darker material in the TEM image in (a) is polycrystalline Cu metal. 6. EELS and EDS characterization of reaction products 6.1 Energy Dispersive Spectroscopy Energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) were carried out to further characterize the reaction products. EDS can detect the presence of heavier elements, such as the transition metals in these experiments, but it cannot detect lithium. Figure S6 shows EDS spectra of a group of Cu 2 S nanocrystals before and after the reaction with lithium. The copper-to-sulfur peak ratio remains fairly 8

9 constant before and after the reaction, indicating that these elements are not lost to heating or other beam effects during the transformation. Figure S6. EDS spectra of a group of Cu 2 S particles before (top) and after (bottom) the reaction with lithium. The Ag and Ti peaks in the spectra are a result of spurious x-rays from the conductive epoxy and the sample holder, respectively. Similarly, Fig. S7 shows EDS spectra for FeS 2 nanocrystals before and after the reaction with lithium. Again, the iron-to-sulfur intensity ratio remains fairly constant. In Fig. 1 in the main text, it was shown that FeS 2 and Co 3 S 4 form very small metallic nanoparticles upon reaction with lithium; these nanoparticles do not diffract strongly and as such are often difficult to detect by electron diffraction. These EDS results unambiguously show the presence of the Fe species after the reaction (data from Co 3 S 4 is not shown, but it exhibits similar EDS spectra for Co instead of Fe). 9

10 Figure S7. EDS spectra of a group of FeS 2 particles before (top) and after (bottom) the reaction with lithium. The Ti and Cu signals in the spectra are a result of spurious x-rays from the sample holder and the current collector wire, respectively. It should be noted that FeS 2 has been reported to undergo a two-step lithiation mechanism in which the first step involves Li insertion and the second step involves formation of Li 2 S and Fe. 9 However, the insertion step takes place above ~1.5 V vs Li + /Li, whereas in the present experiments a large negative overpotential is applied with respect to Li + /Li. As such, we expect that Li insertion is convoluted with the conversion reaction in our experiments Electron Energy Loss Spectroscopy Electron energy loss spectroscopy (EELS) is useful in this type of experiment because lithium can be detected with this technique. Figure S8 is an EELS spectrum of reacted Cu 2 S nanocrystals that shows the Li K-edge, indicating the presence of lithium. It should be noted that the Li signal comes both from the nanocrystals and the underlying lithiated carbon tube; these two materials have different lithium chemical environments. 10

11 Figure S8. Li K-edge from the EELS spectrum of lithiated Cu 2 S nanocrystals. 7. Original images from Figure 4 in the main text Figure 4 in the main text shows a Fourier-filtered image series that details the transformation process in a single Cu 2 S nanocrystal as the lithium reaction sweeps across the crystal. Figure S9 shows the original TEM images from this experiment. Figure S9. Original TEM images from Fig. 4 in the main text. This is an edge-on view of a larger Cu 2 S nanocrystal. The width of each image is 15.7 nm. In these images, the Li 2 S phase can be seen growing from the left to the right, and the Cu 2 S phase exhibits darker contrast because of the heavier copper atoms. The clarity of these images is lacking because in this experiment the nanocrystal was inside a carbon tube, which means that there are two different carbon films at different Z-positions through which the nanocrystal was imaged. Fourier-filtering improves clarity because the amorphous carbon film background is largely removed from the image. 11

12 8. Structural schematics of FeS 2 and Co 3 S 4 Figure S10 shows schematics of the crystal structures of Li 2 S, FeS 2, and Co 3 S 4. All three materials are cubic, and each of these schematics is a projection of the structure along the [110] direction. Figure S10. Schematics showing the structure of a) Li 2 S, b) FeS 2, and c) Co 3 S 4. Each image is a projection of the structure along the [110] direction of the cubic lattice. Li 2 S takes the antifluorite structure, with sulfur atoms positioned on an fcc sublattice and lithium filling all the tetrahedral interstices. FeS 2 crystallizes in the pyrite structure, which features iron atoms on an fcc sublattice and sulfur atoms located at positions offset from the tetrahedral interstitial sites. This results in bonded pairs of sulfur atoms throughout the structure, as evident from the schematic in Fig. S10b. Co 3 S 4 takes the cubic spinel structure, which features sulfur atoms on an fcc sublattice and cobalt atoms arranged on both tetrahedral and octahedral interstices. The FeS 2 and Co 3 S 4 structures are significantly different than the Li 2 S structure. FeS 2 contains sulfur atoms arranged in pairs throughout the iron fcc sublattice, and the conversion to Li 2 S thus involves rearrangement of these pairs. Although the Co 3 S 4 structure features an fcc sulfur sublattice like the Li 2 S structure, the {111} plane spacing is 20% smaller in Co 3 S 4, which necessitates significant volume changes during the transformation from Co 3 S 4 to Li 2 S. As discussed in the main text, the combination of these structural differences and the lower mobility of cobalt and iron compared to copper in these sulfide materials leads to the different observed reaction pathways. 12

13 References 1. Zhang, H.-T., Wu, G. & Chen, X.-H. Large-Scale Synthesis and Self-Assembly of Monodisperse Hexagon Cu2S Nanoplates. Langmuir 21, (2005). 2. Aiwei, T. et al. One-pot synthesis and self-assembly of colloidal copper(i) sulfide nanocrystals. Nanotechnology 21, (2010). 3. Yin, Y., Erdonmez, C.K., Cabot, A., Hughes, S. & Alivisatos, A.P. Colloidal Synthesis of Hollow Cobalt Sulfide Nanocrystals. Adv. Funct. Mater. 16, (2006). 4. Wang, D.-Y. et al. Solution-Processable Pyrite FeS2 Nanocrystals for the Fabrication of Heterojunction Photodiodes with Visible to NIR Photodetection. Adv. Mater. (Weinheim, Ger.) 24, (2012). 5. Zheng, G., Yang, Y., Cha, J.J., Hong, S.S. & Cui, Y. Hollow Carbon Nanofiber- Encapsulated Sulfur Cathodes for High Specific Capacity Rechargeable Lithium Batteries. Nano Lett. 11, (2011). 6. Zheng, H. et al. Observation of Transient Structural-Transformation Dynamics in a Cu2S Nanorod. Science 333, (2011). 7. Lotfipour, M., Machani, T., Rossi, D.P. & Plass, K.E. α-chalcocite Nanoparticle Synthesis and Stability. Chem. Mater. 23, (2011). 8. Putnis, A. Electron diffraction study of phase transformations in copper sulfides. Am. Mineral. 62, (1977). 9. Shao-Horn, Y., Osmialowski, S. & Horn, Q.C. Reinvestigation of Lithium Reaction Mechanisms in FeS2 Pyrite at Ambient Temperature. J. Electrochem. Soc. 149, A1547-A1555 (2002). 13