Supporting Information Reagent-Free Electrophoretic Synthesis of Few-Atom- Thick Metal Oxide Nanosheets Chengyi Hou,*,, Minwei Zhang, Lili Zhang, Yingying Tang, Hongzhi Wang, and Qijin Chi*, State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, People s Republic of China Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark Corresponding Authors * hcy@dhu.edu.cn (C.H.) * cq@kemi.dtu.dk (Q.C.) S-1
Contents 1. Supporting figures and discussions 2. Supporting references S-2
1. Supporting figures Figure S1. XPS spectra of ZnO nanosheets. Nanosheets were ultrasonicated wildly and spin-coated on Si/SiO 2 substrates. The positions of all peaks were aligned by being referred to the C1s peak. Since the substrates were not fully covered by ZnO nanosheets, SiO 2 signals were also detected. All other peaks suggest the existence of high purity ZnO. In O 1s spectrum, the peak at 530 ev is assigned to ZnO, while the peak at 532 ev is assigned to SiO 2. Figure S2. (a) UV-vis spectrum of the ZnO nanosheet suspension. (b) Corresponding Tauc plot for direct transition of the ZnO nanosheet. The figure exhibits the optical band gap (E g ), obtained by extrapolation (see red lines). For direct transition of ZnO, the E g is found to be ~4.1 ev. According to our knowledges, this is the highest value for ZnO nanostructures (including doped and modified ones) ever reported. The E g of our ZnO nanosheet is also wider than that of most semiconductors. The remarkable increase in the E g suggests the significant decrease in nanostructure size (nanosheet thickness in this case), according to the quantum confinement effect. (c and d) Valence-band XPS spectra of ZnO nanosheets. The band gap energy deduced from XPS results is consistent with UV-vis spectroscopy analyses. S-3
Figure S3. PL spectrum of ZnO nanosheets. A sharp PL peak at 379 nm indicates almost nonchemical-absorption on ZnO surface. Two common PL peaks at around 540 and 600 nm, assigned to oxygen vacancies and oxygen interstitials respectively (Ref. S1), are not observed in our case. This suggests that the as-synthesized ZnO nanosheets are ultrapure and have perfect incorporation of oxygen in the lattice. Figure S4. A SEM image (a) and optical microscope image (b) showing a large amount of ZnO nanosheets. S-4
Figure S5. A high-magnification TEM image showing both wrinkle and flat monolayer regions of a suspended ZnO nanosheet. It is clear that the nanosheet is made up of nanocrystals. Figure S6. A TEM image of monolayer ZnO nanosheets supported on carbon films. S-5
Figure S7. AFM images of polycrystalline ZnO nanosheets (a), graphene oxide nanosheets (b) and Cu 2 O nanowires (c) synthesized in our lab. ZnO and Cu 2 O nanostructures are synthesized through reagent-free electrophoretic assembly. Graphene oxide nanosheets are synthesized based on the wellknown approach (Ref. S2). Typical R q values were obtained from the marked areas. S-6
Figure S8. (a) A TEM image of ZnO nanosheets. (b and c) IFFT images (processing with ImageJ analysis software) of the selected area, corresponding to Figure 2e and f, respectively. Figure S8b shows totally misaligned planes with obvious lattice dislocations as well as grain discontinuity. In contrast, Figure S8c displays only slightly disaligned planes. Grain boundaries are visible. Figure S9. (a) Schematic illustration for sample collection: Suspension was collected from different locations, representing different assembly steps. (b) Three bottom-up steps and corresponding morphology development are sketched in coordination with dimension parameters. The ratios of different main morphologies at different distances were estimated to be 100% (d=0.1), 97±1% (d=0.5), and 95±1% (d=0.9), based on their coverage area. S-7
Figure S10. (a) A TEM image of ZnO QDs. (b) Schematic view of individual QD showing size paratemers. (c) Optimized atomic cinfiguration of the QD. Side view from the thickness direction. The thickness matchs the value twice that of unit-cell parameter in c-axis of ZnO (0.52 nm). This suggests that the QDs, nanoribbons, and nanosheets are only nine-atom thick, namely 2D atomic materials. Figure S11. Upper panel: Comparison of surface property of ideal crystal structure fragments. Left to right: Cu 2 O, Fe 2 O 3, NiO, and ZnO. Top views are shown. Polar surfaces terminated by a layer of only metal or only oxygen are indicated with green marks. Nonpolar surfaces that expose both metal and oxygen atoms are indicated with grey marks. Lower panel: Corresponding TEM images of the metal oxides synthesized through reagent-free electrophoresis reported in this work. Only ideal crystal structures are illustrated here to represent the different polarities of certain facets. The difference in S-8
surface polarity between the facet along c axis and those along a and b axes can be seen in Fe 2 O 3 (trigonal), NiO (trigonal), and ZnO (hexagonal). All these metal oxides synthesized through our strategy have a nanosheet morphology. This could be explained by the surface-polarity dominated mechanism proposed in this work. In comparison, low-anisotropy cubic Cu 2 O will only randomly assemble into polycrystalline nanowires, which don t have crystal orientation, as we described previously (Ref. 29). This will impede further formation of Cu 2 O nanosheets. In conclusion, all results fit in the surface-polarity dominated electrophoretic mechanism, though it is unusual and need further explanation. For example, it is widely accepted that polar surfaces are less stable than nonpolar ones. We proposed a principle that leads to a large polar-to-nonpolar surface area ratio, but further studies are still ongoing. Considering that the reaction enviorment is extremely green, the only aboundant chemicals water molecules that absorbed on polar surfaces of crystals may play a key role in stabilizing the atomic nanosheet structure. Figure S12. (a) A photograph of a thin layer of transparent ZnO hydrogel covering on electrode. Inset shows a zoomed in picture, in which the ZnO hydrogel layer is visible at the edge of the electrode. (b) A SEM image of freeze-dried ZnO hydrogels. Inset shows a photograph of the sample. (c) A SEM image of ZnO aerogels naturally dried in open air. Inset shows a photograph of the sample (still covering on electrode). (d) A STEM-HAADF image of ZnO aerogels naturally dried in open air. Pores ranging from 20 nm to 2 μm in diameter are seen in the images. S-9
Figure S13. XPS spectra of native ZnO layer on Zn foils. The positions of all peaks were aligned by being referred to the C1s peak. The remaining content of impurities were absorbed from atmosphere. The O 1s and Zn 2p spectra suggest the existence of ZnO layer. S-10
2. Supporting references (S1) Vempati, S.; Shetty, A.; Dawson, P.; Nanda, K. K.; Krupanidhi, S. B. Solution-Based Synthesis of Cobalt-Doped ZnO Thin Films. Thin Solid Films 2012, 524, 137-143. (S2) Tung, V. C.; Allen, M. J.; Yang, Y.; Kaner, R. B. High-Throughput Solution Processing of Large-Scale Graphene. Nat. Nanotech. 2009, 4, 25-29. S-11