Supporting information for Metallic Few-layered VS 2 Ultrathin Nanosheets: High Two-Dimensional Conductivity for In-Plane Supercapacitors Jun Feng, Xu Sun, Changzheng Wu*, Lele Peng, Chenwen Lin, Shuanglin Hu, Jinlong Yang and Yi Xie* [ ] Hefei National Laboratory for Physical Sciences at Microscale, University of Science & Technology of China, Hefei, Anhui, 230026, P.R. China. [ ] Shanghai Supercomputer Center, Shanghai, 201203, P.R. China. Email: yxie@ustc.edu.cn; czwu@ustc.edu.cn. Table of contents S1. The crystallographic information and density of states (DOS) calculations of VS 2 structures...2 S2. Characterization of the as-prepared NH 3 VS 2 bulk sample...3 S3. Characterization of VS 2...5 S4. Formation mechanism analysis of NH 3 VS 2...6 S5. Transfer VS 2 film onto arbitrary substrates...6 S6. Exfoliation efficiency comparison between NH 3 VS 2 and VS 2 in water...7 S1
S1. The crystallographic information and density of states (DOS) calculations of VS 2 structures Vanadium disulfide (VS 2 ) is a hexagonal crystal with layered structure stacked by S-V-S alternating layers, of which the cell parameters a and c of 3.2210 Å and 5.7550 Å, respectively. 1 Weak van der Waals interlayer interactions are present among the layers, bringing the exfoliative characteristic of this compound. The electronic structures of both bulk VS 2 (referred as the bulk state) and single-layered VS 2 structures (referred as the sheet state) were investigated by augmented-wave (PAW) method calculations. Intriguingly, both of the bulk and sheet states show well-defined metallic characteristics, since considerable state dispersions can be observed at the Fermi level of both DOS diagrams shown in Figure S1 (C) and (D). This finding revealed to us that after being thinned downward to single-layered structure, VS 2 could still keep the electronic structure without the degradation of metallic nature, proving the theoretical feasibility for the realization of metallic VS 2 nanosheets with high two-dimensional conductivity. Figure S1. (A) Side-viewed atomic structure of bulk VS 2 crystal. (B) Atomic structure of bulk VS 2 crystal projected along c axis, showing the notable quasi-two-dimensional structural characteristics, in that all vanadium atoms are in the same plane. (C) Calculated DOS diagram based on the bulk VS 2 structure shown in (A). (D) Calculated DOS diagram based on the single-layered VS 2 structure shown in (B). The calculation results indicate that after being exfoliated into two-dimensionally confined structure, VS 2 still remains well-defined metallic behavior, which can be revealed by the fact that the DOS reside across the Fermi level with considerable state dispersions. S2
S2. Characterization of the as-prepared VS 2 NH 3 bulk sample 1. X-ray diffraction analysis Relative intensity(a.u.) 120 100 80 60 40 20 (003) (006) (009) (012) (015) (018) (10 10) (10 13) (110) (113) 0 10 20 30 40 50 60 70 2-Theta (degree) Figure S2. XRD pattern of the VS 2 NH 3 product, in which all the reflection peaks can be well indexed to VS 2 NH 3 phase (JCPDS card No. 41-0642). No other characteristic peaks were observed, indicating that no noticeable impurities exist in the product. It has been reported that NH 3 is easily intercalated in the van der Waals gap of the layered transition metal dichalcogenides. Thus, the structure of the ammonia intercalates of the transition metal disulfides is that NH 3 molecules reside in the sites with trigonal prismatic coordination by sulfur atoms of neighboring sandwiched MS 2 (M=V, Mo, Ti etc.). 2 S3
Table 1. The summary and comparison of d (Å) values of each crystallographic plane index of VS 2 between the experimental and the reported, which shows a good consistence. Lattice d values (Å) plane Experimental JCPDS 41-0642 0 0 3 9.0009 9.1233 0 0 6 4.5069 4.5617 0 09 2.9902 3.0411 0 1 2 2.7649 2.7569 0 1 5 2.4938 2.5031 0 1 8 2.1766 2.1736 1 0 10 1.9640 1.9622 1 0 13 1.6866 1.6859 1 1 0 1.6252 1.6250 1 1 3 1.5983 1.6057 2. Electron microscopy morphological characterization (A) (B) Figure S3. Characterization of the as-synthesized VS 2 NH 3 precursor. (A) SEM image of VS 2 NH 3. As is indicated in the image, all the products are uniform flakes with the thickness of about 110nm. (B) TEM image of VS 2 NH 3, while the dark part indicates the large thickness of the as-prepared sample, which was in good consistence with the SEM image. S4
S3. Characterization of VS 2 nanosheet (A) (B) Figure S4. Characterization of the as-synthesized VS 2 nanosheets. (A) SEM image of the as-prepared VS 2, indicated that the flakes after ultrasonic treatment are of large area with the width of about 4 µm. (B) TEM image of the VS 2 suspension, in which the ultrathin morphological characteristic of VS 2 with well dispersion can be clearly seen. S4. Surface morphology of the assembled VS 2 film Figure S5. (A) Low-magnification SEM image of the surface morphology for the thin film assembled from VS 2 nanosheets. (B) High-magnification SEM image of the surface morphology for the thin film assembled from VS 2 nanosheets. (C) Schematic illustration of the layer-by-layer overlapping structure of assembled VS 2 ultrathin nanosheets. S5
S5. Formation mechanism analysis of VS 2 NH 3 To carry out the ammonia-assisted exfoliation strategy, NH 3 -intercalated precursor is the foremost prerequisite. In order to synthesize NH 3 VS 2, we suggest thioacetamide (TAA) be a favorable choice for the construction of a reaction environment with high ammonia and sulfide concentrations. As is known, TAA is an ambient chemical source of both ammonia and sulfide, of which the aqueous solution is weakly alkaline (ph~9). After hydrolysis, TAA would generate HS - and NH 3 as follows: CH 3 CSNH 2 + 2OH - CH 3 COO - + HS - + NH 3 (1) Specifically, HS - ions not only act as the sulfide source, but also as a ambient reductant, because V(V) would be reduced by HS - into V(IV) and brings the formation of VS 2 : 6VO 4 3- + 17HS - 6VS 2 + SO 3 2- +13H 2 O + 8OH - (2) Then, due to the intercalative characteristics of VS 2, NH 3 molecules hydrolyzed from TAA can be inserted into the interlayer spacing of VS 2 layers, forming the target product of NH 3 VS 2. S6. Transfer VS 2 film onto arbitrary substrates Figure S6. The above photographs show the VS 2 films obtained after filtering over a cellulose membrane with 0.22 µm pore size and the transfer of VS 2 film onto different substrates. It can be clearly seen from the above photograph, the VS 2 film on a cellulose membrane was with black luster, which was brought by the metallic characteristic of VS 2. Various materials were applicable as the substrates, including glass tube, silicon wafer, plastic film and quartz slide, indicating that the VS 2 film can be readily transferred onto arbitrary surfaces. Of note, the transfer of the obtained film onto the curved glass tube indicates the high flexibility of the VS 2 thin film. S6
S7. Exfoliation efficiency comparison between VS 2 NH 3 and VS 2 in water VS 2 VS 2 NH 3 Figure S7. The digital photograph of the comparison between VS 2 NH 3 and bulk VS 2. In order to demonstrate the high exfoliation efficiency of VS 2 NH 3, the sample of bulk VS 2 flakes obtained by a previously reported method 3 was also studied under the same conditions. 40 mg of VS 2 NH 3 and VS 2 were added into 30ml water (bubbled with Argon beforehand) and ultrasonicated in iced water for 1 hour. Then both dispersions were allowed to stand still for about 30 min. As can be seen from the result shown in the photograph, the exfoliation efficiency of VS 2 NH 3 was much better than that of the VS 2 sample. References in the Supporting Information (1) Murphy, D. W.; Cros, C.; Di Salvo, F. J.; Waszczak, J. V. Inorg. Chem. 1977, 16, 3027. (2) Bouwmeester, H. J. M.; Wiegers, G. A.; van Bruggen, C. F. J. Solid State Chem. 1987, 70, 57. (3) Vadivel Murugan, A.; Quintin, M.; Delville, M. H.; Campet, G.; Vijayamohanan, K. J. Mater. Chem. 2005, 15, 902. S7