Reagent-Free Electrophoretic Synthesis of Few-Atom-Thick Metal

Jan 19, 2017 - Engineering traditional materials into the new form of atomic and free-standing two-dimensional structures is of both fundamental inter...
0 downloads 11 Views 8MB Size
Download PDF
Article pubs.acs.org/cm

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 S Supporting Information *

ABSTRACT: Engineering traditional materials into the new form of atomic and free-standing two-dimensional structures is of both fundamental interest and practical significance, but it is in general facing challenges especially for metal oxide semiconductors. We herein report an ultragreen method for the cost-effective and fast preparation of atomic metal oxide nanosheets that can be further transformed into nanofilms. The method combines top-down building block synthesis and bottom-up electrophoretic assembly in water under ambient conditions, using only bulk metal and Milli-Q water without involving any additional reagents. The focus is on free-standing polycrystalline ZnO nanosheets that can be produced with a lateral dimension as large as 10 μm and a thickness of 1 nm (the thinnest free-standing metal oxide nanosheet ever reported). A new electrophoretic assembly mechanism dominated by intrinsic surface polarity was revealed. We also demonstrate potential applications of this approach for wet electronic systems as exemplified by facile and in situ fabrication of dielectric layers and cellular electrets.

■

INTRODUCTION Two-dimensional (2D) nanomaterials have exhibited unique electronic and optical properties because of their quantum well band structure and large surface exposure.1−3 The recent rise of 2D nanomaterials, represented by graphene,4 phosphorene,5 MXene,6 hexagonal boron nitride (h-BN),7 and MoS2,8 has offered emerging opportunities for the development of newgeneration electronic and photonic technologies. The recent explosive development of these 2D nanomaterials has become feasible because of some facile and high-quality synthesis methods employed such as mechanical and liquid exfoliation, which is facilitated by the intrinsic 2D layered structures of starting materials.9−11 Other synthesis methods, including hightemperature (chemical vapor deposition12 and thermal expansion13) and wet chemical (redox, solvothermal, or colloidal growth reactions14−16) processes, have also been developed because of their capability of scaling up production, but they have suffered from some disadvantages. For example, in most cases, (1) strict reaction conditions and environmentally unfriendly reagents are needed for synthesis, (2) processes are rather complicated and costly, and (3) byproducts are usually generated in addition to the target product. In spite of the drawbacks mentioned above, these existing methods would continue to play important roles in the synthesis of a novel class of 2D atomic nanomaterials that are intrinsically non-2D in their conventional crystal structure. The definition of 2D in these cases is that the materials can be isolated as free-standing few-atom-thick layers. This non-natural free-standing 2D form of traditional materials has attracted a © 2017 American Chemical Society

growing amount of attention because such a structure could exhibit unexpected and/or unpredictable new physicochemical properties. For instance, 2D polymers (also known as monolayers of molecular plywood), organic frameworks, and metal−organic frameworks have shown promising advantages for their application in the fields of molecular electronics, photoelectronics, and electrocatalysis.17−20 Free-standing 2D metals, including Pd,21 Ru,22 Au,23 and Fe24 nanosheets, have also been reported to be highly chemically active. Atomic metal oxide nanosheets have exhibited unusual energy band structure and outstanding electrochemical performance.25−28 However, critical reaction conditions or chemical templates are often required to break materials’ three-dimensional (3D) symmetry and foster anisotropy in 2D growth. In general, the facile and green synthesis of 2D atomic nanomaterials from nonlayered starting materials remains a challenge. The challenges are especially notable for metal oxide (MO) semiconductors that are functionally versatile and particularly important for electronics and photonics, because the lateral expansion of MO atomic nanosheets is highly restricted because of unfavorable energetic kinetics. The development of novel synthetic strategies toward efficient, simple, ecofriendly, and large-scale synthesis of such non-natural structures is highly desirable. On the basis of alternative working principles, in this work, we have demonstrated an effective approach to the synthesis of Received: January 15, 2017 Published: January 19, 2017 1439

DOI: 10.1021/acs.chemmater.7b00188 Chem. Mater. 2017, 29, 1439−1446

Article

Chemistry of Materials Scheme 1. Apparatus for Reagent-Free Electrophoretic Synthesis for the Formation of Metal Oxide Nanosheetsa

a Target metals are used as anodes, while different kinds of conducting material can be used as cathodes, although here it is exempted by only Pt. The gap between the two electrodes varies from micrometers to centimeters (resulting in a dc field intensity of 3−26 V cm−1, but not limited to this range).

xenon lamp as the excitation source. All characterizations were conducted at room temperature. Nanoprobe Fabrication and Modification. Metal nanoprobes (Ag nanotrees) were prepared using reagent-free electrolytic synthesis. Typically, a Au/Ag alloy anode was used in reagent-free synthesis. Ag was anodically dissolved and deposited on the cathode to form Ag nanotrees. Then, the Au/Ag alloy anode was replaced with a Zn anode to produce ZnO dielectric nanosheets.

free-standing 2D atomic metal oxides. The synthesis is performed using only bulk metal and Milli-Q water without involving any additional chemical reagents. As an example, monolayer polycrystalline ZnO nanosheets that are as large as 10 μm in their lateral dimensions but with a typical thickness of only 1 nm can be produced within 1 h at room temperature and with a safe range of applied voltages. We discuss the formation mechanisms of this non-natural 2D atomic structure. In addition, we have shown that this procedure can potentially be scaled up and allows further formation of electret aerogels as well as thin film semiconductor-coated heterostructures. These flexible and ultrathin metal oxide structures might fulfill crucial requirements of flexible electronics and are therefore considered as promising candidate materials for the fabrication of wearable electronic devices.

■

■

RESULTS AND DISCUSSION Reagent-Free Electrophoretic Synthesis. Our reagentfree synthesis principle combines the well-known anodic

EXPERIMENTAL SECTION

Reagent-Free Synthesis. A metal anode (Zn was used in this work) and a cathode (metal, ITO, graphene paper, etc.) were immersed in Milli-Q water (18.2 MΩ cm, total organic carbon value of 1 cm. It can also be reduced to a few micrometers or extended to >10 cm. The direct current (dc) voltage (3−26 V, but not limited to this range) was applied to the electrodes at room temperature. Products were collected from the solutions. Characterizations. We performed transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED), and scanning transmission electron microscopy− high-angle annular dark field (STEM-HAADF) on Tecnai G2 T20 and Titan 80-300ST transmission electron microscopes from FEI Co. Cu grids with holey carbon support films were used as support materials for TEM imaging. Scanning electron microscopy (SEM) and energydispersive spectrometry (EDS) characterizations were performed on an FEI Quanta FEG 200 scanning electron microscope. Samples were tested without further coating. A model 5500 atomic force microscopy (AFM) system (Agilent Technologies) was used for all AFM imaging. X-ray photoelectron spectroscopy (XPS) (Thermo Scientific) was performed to analyze the compositions of samples, with Al Kα radiation (1486 eV) as the excitation X-ray source. The pressure of the analysis chamber was maintained at ≤2 × 10−10 mbar during measurements. UV−vis spectra of the materials were recorded using a model 8453 spectrophotometer from Agilent Technologies (Santa Clara, CA). Photoluminescence (PL) spectra were recorded using an Edinburgh Instrument FLS920 spectrophotometer with a 150 W

Figure 1. (a) Digital photograph with strong Tyndall light scattering displayed. (b) PL spectrum of ZnO colloidal suspensions. A sharp PL peak at 379 nm indicates that there is no detectable adsorption of chemical species on the surface of ZnO nanostructures.

oxidation with electric field-induced assembly. Scheme 1 illustrates our strategy. Metal oxide grains/quantum dots (QDs) are generated from the anode upon electrolysis of water. They are charged and also endowed with built-in dipoles by the applied dc electric field (3−26 V cm−1, but not limited to this range). Given the fact that the applied electric fields are oriented perpendicular to the electrode plane, the metal oxide QDs assemble and align while moving between two electrodes. Assembled nanostructures would appear around the cathode in ∼30 min. In our previous work,29 we showed that the as-synthesized Cu2O (Pn3m) QDs can assemble into aligned polycrystalline nanowires. However, we noticed that the crystals in nanowires 1440

DOI: 10.1021/acs.chemmater.7b00188 Chem. Mater. 2017, 29, 1439−1446

Article

Chemistry of Materials

Figure 2. Structural characterization of 2D atomic ZnO nanosheets. (a) SEM image and corresponding EDS elemental mapping (inset) of a ZnO nanosheet suspended on the holey carbon support. The nanosheet is indicated by an arrow. The selected area is marked. (b) TEM image and corresponding SAED pattern (inset) of a ZnO nanosheet. The selected area is marked. Lattice spacings (d) are listed. (c) AFM image of an isolated ZnO nanosheet. (d) Height profiles of the sample along the red and blue lines in panel c. (e) Experimentally measured Rq values for the three types of polycrystalline nanostructures synthesized in our lab.

carbon support substrates (15−25 nm thickness) for SEM and TEM imaging. Structural and Morphologic Characteristics. The typical SEM image (Figure 2a) shows a free-standing ZnO nanosheet, typically several micrometers across. SEM−EDS element mapping (inset) allows us to distinguish ZnO nanosheets from the carbon support film. It is clear that the ZnO nanosheet is even much thinner than the carbon film, as it appears to be more transparent. The low-magnification TEM image (Figure 2b) shows that ZnO nanosheets are freely suspended on the porous carbon support film, and individual nanosheets typically appear to be folded and overlapped, because of the absence of substrates.30 Both SAED patterns (inset) and the HRTEM image (Figure S5) reveal the polycrystalline nature of ZnO nanosheets. The SAED patterns correspond to the planes of the hexagonal wurtzite structure of ZnO (PDF Card 36-1451). With a decrease in its lateral dimension by strong ultrasonication, ZnO nanosheets could be broken into smaller pieces, which are flat as imaged by TEM (Figure S6). Such ZnO nanosheets were transferred onto mica substrates for AFM measurements. A tapping-mode AFM image (Figure 2c) of the sample reveals irregular 2D morphology. The height profile indicates that the monolayer ZnO nanosheets have an apparent thickness of 1 nm (Figure 2d). A bilayer folding area possesses a thickness of 2 nm. Surprisingly, these 2D ZnO nanomaterials have a geometric feature very similar to that of graphene oxide (which is produced by oxidation of graphite). It is also interesting to reveal that the measured root-mean-square (RMS) roughness (Rq, as summarized in Figure 2e) of ZnO nanosheets [typically 0.113 nm (Figure S7a)] is very close to

have no long-range order but an irregular orientation. This observation suggests that the electric field-induced dipole moment in Cu2O QDs, which was believed to be the main driving force for assembly, does not determine the crystal lattice direction of nanowires. It also indicates that the ionic screening and surface charge effects are negligible in the formation of Cu2O nanowires. Although it was believed that the dipole− dipole interaction plays an important role in the electrophoretic assembly, as it has a long range and is surprisingly strong, the detailed formation mechanism of nanostructure assembly involved in this reagent-free synthesis is still unclear. Interestingly, in the case of ZnO (P63mc) demonstrated in this work, ZnO QDs assemble into 2D atomic nanosheets with an extremely high lateral dimension to thickness ratio and a few-atom thickness [as shown below (see also spectroscopic analyses in Figures S1−S3)], which is a new structurally stable form of metal oxides. This observation is somehow unexpected and is certainly not predicted by the related theories. The novel phenomenon could also offer an opportunity to improve our understanding of the detailed mechanism in reagent-free electrophoretic synthesis, in addition to the preparation of 2D atomic metal oxide nanostructures. Figure 1a and Figure S4 show potential large quantities of production. Though the resulting colloidal suspension obtained is transparent, strong Tyndall light scattering can be observed as seen in the photograph (Figure 1a). The photoluminescience (PL) spectrum (Figure 1b) and XPS spectra (Figure S1) show that the suspension contains high-purity ZnO nanomaterials (see a detailed discussion in the Supporting Information). ZnO nanosheets were transferred from the suspension onto porous 1441

DOI: 10.1021/acs.chemmater.7b00188 Chem. Mater. 2017, 29, 1439−1446

Article

Chemistry of Materials

Figure 3. Crystal structural analyses of the nanosheets. (a) Low-magnification TEM image of a suspended nanosheet. Monolayer regions are marked. (b) High-magnification TEM image of a monolayer nanosheet. The magnified image (inset) shows crystal lattices of ZnO. (c) FFT patterns and electron diffraction spectrum corresponding to panel b. (d) HRTEM image of nanosheets. (e) IFFT image from (0 0 2) diffraction spots showing barely aligned (0 0 2) planes, which are emphasized with yellow dotted lines in the inset. (f) IFFT image from (1 0 0) diffraction spots showing wellaligned (1 0 0) planes and arranged (0 0 2) facets. The IFFT images correspond to panel d.

Figure 4. Assembly of the nanosheets. AFM images showing (a) QDs, (b) nanoribbons, and (c) nanosheets with a 1 nm thickness. Free QDs and nanoribbons prior to assembly can also be observed in panels b and c, respectively. Three different morphologies represent three stages of the assembly processes. (d) TEM image of nanoribbons. (e) HRTEM image and corresponding FFT patterns of a single nanoribbon. In-process QDs are marked and magnified in panels f and g.

1442

DOI: 10.1021/acs.chemmater.7b00188 Chem. Mater. 2017, 29, 1439−1446

Article

Chemistry of Materials

also Figure S8). On the contrary, the well-defined continuous arrangement of its (0 0 2) facet is clearly revealed in Figure 3f. The image shows only slightly misaligned planes that could be considered as the possible boundary location (Figure S8). These results indicate that the surface of ZnO nanosheets is dominated by the polar (0 0 2) facets. An orientation of the caxis of the ZnO crystallites perpendicular to the sheet plane can also be deduced. This is consistent with the fact that the nanosheet thickness is approximately twice the unit cell parameter on the c-axis of ZnO (0.52 nm). Given that the lateral size:thickness ratio of the nanosheet exceeds 10000, this facet-selective 2D ZnO nanostructure must have an extremely high polar:nonpolar facet ratio that is a favorable structural feature of semiconductors used for photocatalytic reactions.32 Formation Mechanisms. To reveal the detailed mechanism of the facet-selective assembly that leads to the unique 2D atomic structures, we observed the assembly process through microscopy analyses. The three assembly steps can be clearly revealed by AFM images (Figure 4a−c). First, ZnO QDs with a 1 nm thickness and an ∼4 nm diameter were produced from the Zn anode (Figure 4a and Figures S9 and S10). The anode oxidation and hydrolysis reactions account for the process.29 These reactions also lowered the local solution pH in the area around the anode. In this area, protons from the environment are likely transferred to the ZnO QDs surface, leading to surface Zn-OH2+ groups. The applied electric field thus provided the driving force to direct the positively charged QDs migrating toward the cathode. Second, the swimming ZnO QDs assembled into 1 nm-thick 2D nanoribbons (Figure 4b). This suggests that the ZnO QDs minimized the total area of nonpolar facets during the assembly. As a result of the facetselective assembly, the polar facets are found to dominate the nanostructure surface as shown above. Therefore, the surface polarity is believed to play a key role in building up the planar structures. We might expect the electrically induced dipole in ZnO QDs to disturb the orientation of their polar axes, but such an effect appears to be very limited as confirmed by the fact that high electric fields did not result in any more profound effect on the crystal orientation. Third, the ZnO nanoribbons further assembled into 1 nm-thick nanosheets during electrophoresis (Figure 4c). The ZnO nanosheets were finally located in the solution area around the cathode. Furthermore, the extent of electrostatic repulsion between nanosheets due to the charged surfaces would increase significantly with an increase in the nanosheet lateral dimension, resulting in a colloidal suspension (Figure 1a). Overall, the whole synthetic procedure combines top-down reagent-free electrolytic synthesis of nano building blocks (QDs) and bottom-up electrophoretic assembly into nanosheets. The surface polarity-dominated assembly are further supported by TEM observations. Figure 4d shows a typical TEM image of ZnO nanoribbons, corresponding to the second assembly step described above. The HRTEM image and corresponding FFT pattern (Figure 4e) indicate that the nanoribbons have the same oriented surface exposure as that nanosheets do. Individual ZnO QDs that are attached to each other via nonpolar facets to form nanoribbons are visible in HRTEM images. The IFFT images in panels f and g of Figure 4 magnify ZnO QDs and give a clear view of their crystal structure. Well-oriented polar (0 0 2) facets on these QDs are identified. These analyses strongly suggest that the intrinsic surface polarity dominates crystal assembly in the reagent-free

Figure 5. Estimated band gap for ZnO nanosheets. (a) Tauc plot based on the UV−vis data. (b) Plot of relative intensity vs binding energy based on the XPS spectrum. The result obtained from UV−vis and XPS spectra is consistent, with the band gap estimated to be 4.1 eV.

that of graphene oxide nanosheets [typically 0.083 nm (Figure S7b)] but is significantly lower than the value for electrophoretically assembled Cu2O nanowires [typically 0.97 nm (Figure S7c)]. It is known that graphene oxide nanosheets have long-range ordered carbon lattices surrounded by the boundary regions of disorder.31 This structural feature leads to a low value of Rq. In contrast, we have reported that the Cu2O nanowires synthesized through similar reagent-free electrophoretic assembly have neither short-range nor long-range order in their crystal orientations (resulting in high Rq). Combining these results, we can anticipate that the synthesized ZnO nanosheets should have a long-range order in their crystal orientation. To prove this prediction, we investigated the crystalline structures of ZnO nanosheets by HRTEM. The wrinkles on ZnO nanosheets might provide misleading information about TEM and SAED results that mainly come from the folded edges; therefore, HRTEM images were taken from the unfolded areas where there are still some small ripples (Figure 3a). The polycrystalline structure of a monolayer ZnO nanosheet is verified by HRTEM images together with a fast Fourier transform (FFT) pattern (Figure 3b, inset, and Figure 3c). The FFT pattern indicates the existence of both (1 0 0) and (0 0 2) planes of wurtzite ZnO crystals. These results could be consistently repeated by imaging various individual ZnO nanosheets. Panels d−f of Figure 3 present HRTEM and the corresponding inverse FFT (IFFT) images taken from a different nanosheet (Figure S8). As shown in Figure 3e, the (1 0 0) facet of ZnO is visible but barely arranged. The image also shows random planes as well as obvious lattice dislocations (see 1443

DOI: 10.1021/acs.chemmater.7b00188 Chem. Mater. 2017, 29, 1439−1446

Article

Chemistry of Materials

Figure 6. Top panels show in situ dielectric layer formation. (a) Sketch representing how the dielectric film is formed and selectively integrated on nanoprobes in water under ambient conditions. (b) TEM image of the two nanoprobes with one coated by a dielectric layer and the other remaining uncoated. Bottom panels show cellular electrets. (c) SEM image showing the 3D porous structure of the ZnO aerogel. (d) XPS spectra of the ZnO aerogel. Satellites are found ∼8 eV above each line and are labeled S.

under ambient conditions in water environments. Flexible dielectric layers with a nanoscale thickness play critical roles in cutting-edge wearable electronics.39−41 However, it is still difficult to directly assemble atomic dielectric layers in water at room temperature, creating wet electronics that can find application in biological−electronic interfaces. Examples of how such layers can be precisely integrated onto nanoprobes in water are shown in panels a and b of Figure 6. Two conducting nanoprobes (∼200 nm in diameter) with one being insulated from the circuit were used as the collecting substrate in the reagent-free electrophoretic production of ZnO nanosheets. The distance between the nanoprobes was only a few hundred nanometers. The as-synthesized nanosheets coated on the nanoprobe connected in the circuit to form a flexible atomic dielectric layer. In comparison, the isolated one nearby remained uncoated (Figure 6b). This experiment demonstrates the selectivity and accuracy of this potential wet nanoelectric fabrication strategy. Another promise of this research is that the synthetic approach allows one to produce cellular electrets simply in a one-step procedure. For example, the as-synthesized ZnO nanosheets in pure water could undergo further assembly on the cathode. The gel-like assembly covers as much as the whole electrode surface area (size in centimeters for real-world applications). To further examine the hydrogel, it was dried into powders. To our surprise, after on-site natural drying (overnight), the hydrogel retained a three-dimensional cellular structure (Figure 6c and Figure S12). This behavior is in contrast with that of well-known polymer or graphene hydrogels, with the latter normally undergoing deswelling to

electrophoretic synthesis of 2D atomic ZnO nanosheets. We also analyzed other binary transition metal oxides with different surface properties, and the results are consistent with our prediction (Figure S11). This mechanism is clearly distinct from those previously proposed mechanisms for electrophoretic assembly of metal (oxide) nanowires and microfibers,29,33 or colloidal crystals/clusters.34,35 In those previous studies, the electrically induced dipole−dipole interaction was regarded as a major driving force. Therefore, our new findings offer an alternative approach to reconfiguring the structures of nanomaterials. This strategy of constructing selective oriented 2D structures could be generalized rationally to other polar materials. Band Gap Characteristics. Figure 5 shows the estimated band gap (Eg) for ZnO nanosheets. The Tauc plot for the direct transistion of ZnO was obtained from the UV−vis spectrum (Figure S2). The valence band XPS spectrum (see also Figure S2) was also used to analyze the band structure of ZnO nanosheets. The monolayer thickness is far shorter than its Bohr radius (∼2.3 nm); therefore, the ZnO nanosheet has an unusual wide band gap of ∼4.1 eV compared with normal values (3.2−3.7 eV). To the best of our knowledge, this represents the largest value of the band gap for ZnO nanostructures (including doped and modified ones) ever reported. The Eg of our ZnO nanosheets is also wider than that of most metal oxide semiconductors.36−38 Fabrication of Dielectric Layers and Cellular Electrets. There are several additional promising aspects of this research. For instance, it offers the possibility of quickly and simply integrating flexible atomic dielectric layers into electronics 1444

DOI: 10.1021/acs.chemmater.7b00188 Chem. Mater. 2017, 29, 1439−1446

Chemistry of Materials form a shrinked structure. Lyophilization, supercritical drying, or similar processes are necessary for transforming those hydrogels into dehydrated aerogels,42,43 but in our experiments, ZnO aerogels can be naturally obtained without the need for any special treatment. ZnO aerogels and control samples [pregelatinized ZnO nanosheets and a native oxide layer on Zn films (see Figures S1 and S13, respectively)] were measured by XPS. XPS spectra of the as-synthesized nanosheets and aerogels show signatures of ZnO (O 1s at ∼530 eV, Zn 2p3/2 at ∼1025 eV, and Zn 2p1/2 at ∼1047 eV), taking the ZnO/Zn film’s data as the standard. Interestingly, both O 1s and Zn 2p lines for aerogel samples exhibit satellites, as shown in Figure 6d. All satellites are found at an ∼8 eV higher binding energy. Considering also the purity of the sample as well as the control results given by nanosheet samples, the satellites must arise from uncompensated charges in aerogels. These results indicate that (1) ZnO nanomaterials synthesized through our strategy store electrical charges and (2) the cellular structure of aerogels could prevent ZnO nanosheets from discharging while the surface charges provide a repulsion field that prevents the aerogel from restacking. Therefore, the stable cellular electret is retained.

ACKNOWLEDGMENTS

■

REFERENCES

(1) Tae, E. J.; Lee, K. E.; Jeong, S.; Yoon, K. B. Synthesis of Diamond-Shape Titanate Molecular Sheets with Different Sizes and Realization of Quantum Confinement Effect During Dimensionality Reduction from Two to Zero. J. Am. Chem. Soc. 2008, 130, 6534− 6543. (2) Lu, Z.; Zhu, J.; Sim, D.; Zhou, W.; Shi, W.; Hng, H. H.; Yan, Q. Synthesis of Ultrathin Silicon Nanosheets by Using Graphene Oxide as Template. Chem. Mater. 2011, 23, 5293−5295. (3) Ren, Y.; Qiao, Z.; Niu, Q. Topological Phases in TwoDimensional Materials: A Brief Review. Rep. Prog. Phys. 2016, 79, 066501. (4) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (5) Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tománek, D.; Ye, P. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8, 4033−4041. (6) Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y. MXenes: A New Family of Two-Dimensional Materials. Adv. Mater. 2014, 26, 992−1005. (7) Song, L.; Ci, L.; Lu, H.; Sorokin, P. B.; Jin, C.; Ni, J.; Kvashnin, A. G.; Kvashnin, D. G.; Lou, J.; Yakobson, B. I.; Ajayan, P. M. Large Scale Growth and Characterization of Atomic Hexagonal Boron Nitride Layers. Nano Lett. 2010, 10, 3209−3215. (8) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (9) Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Liquid Exfoliation of Layered Materials. Science 2013, 340, 1226419. (10) Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; Shvets, I. V.; Arora, S. K.; Stanton, G.; Kim, H.-Y.; Lee, K.; Kim, G. T.; Duesberg, G. S.; Hallam, T.; Boland, J. J.; Wang, J. J.; Donegan, J. F.; Grunlan, J. C.; Moriarty, G.; Shmeliov, A.; Nicholls, R. J.; Perkins, J. M.; Grieveson, E. M.; Theuwissen, K.; McComb, D. W. P.; Nellist, D.; Nicolosi, V. TwoDimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568−571. (11) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451− 10453. (12) Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition. Nano Lett. 2009, 9, 30−35. (13) McAllister, M. J.; Li, J.-L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Single Sheet Functionalized Graphene by Oxidation and Thermal Expansion of Graphite. Chem. Mater. 2007, 19, 4396−4404. (14) Pei, S.; Cheng, H. M. The Reduction of Graphene Oxide. Carbon 2012, 50, 3210−3228. (15) Peng, Y.; Meng, Z.; Zhong, C.; Lu, J.; Yu, W.; Jia, Y.; Qian, Y. Carbon-Doped Anatase TiO2 Powders as a Visible-Light Sensitive Photocatalyst. Chem. Lett. 2001, 30, 772−773. (16) Son, J. S.; Wen, X.-D.; Joo, J.; Chae, J.; Baek, S.-I.; Park, K.; Kim, J. H.; An, K.; Yu, J. H.; Kwon, S. G.; Choi, S.-H.; Wang, Z.; Kim, Y.-

CONCLUSIONS In summary, 2D atomic ZnO nanosheets can be generated simply and fast in a pure water environment under ambient conditions by exploiting reagent-free electrophoresis. This 2D material has an atomic thickness with predominantly exposed polar facets and a large lateral size:thickness ratio (>10000), and the nanosheet is free-standing and flexible. The thickness is far shorter than its Bohr radius (∼2.3 nm); therefore, the ZnO nanosheet has an unusually wide band gap of ∼4.1 eV (Figure S2) compared with normal values (3.2−3.7 eV). The formation of this unique structure reported here is believed to be driven by intrinsic polarity. This mechanism is distinctive from the widely reported electric field-driven dipole−dipole interaction. This new assembly principle of constructing selectively oriented 2D structures could be adopted to other polar materials. Finally, this strategy can provide further possibilities, as already demonstrated at this exploratory stage, for example, simply and in situ fabrication of dielectric layers and cellular electrets in wet electronic systems. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00188. General experimental considerations and additional supporting data (PDF)

■

■

We gratefully acknowledge the financial support from the National Science Foundation of China (51603037 and 51672043), the Shanghai ChenGuang Program (15CG33), the Shanghai Natural Science Foundation (16ZR1401500), and the Shanghai Sailing Program (16YF1400400). Q.C. thanks the Danish Research Council for Technology and Product Science (Project 12-127447). C.H. is grateful for the Ørsted-MarieCurie fellowship.

■

■

Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Telephone: +45 45252032. Fax: +45 4588 3136. ORCID

Minwei Zhang: 0000-0001-5588-6169 Qijin Chi: 0000-0003-4523-2609 Notes

The authors declare no competing financial interest. 1445

DOI: 10.1021/acs.chemmater.7b00188 Chem. Mater. 2017, 29, 1439−1446

Article

Chemistry of Materials W.; Kuk, Y.; Hoffmann, R.; Hyeon, T. Large-Scale Soft Colloidal Template Synthesis of 1.4 nm Thick CdSe Nanosheets. Angew. Chem. 2009, 121, 6993−6996. (17) Pfeffermann, M.; Dong, R.; Graf, R.; Zajaczkowski, W.; Gorelik, T.; Pisula, W.; Narita, A.; Müllen, K.; Feng, X. Free-Standing Monolayer Two-Dimensional Supramolecular Organic Framework with Good Internal Order. J. Am. Chem. Soc. 2015, 137, 14525−14532. (18) Baek, K.; Hwang, I.; Roy, I.; Shetty, D.; Kim, K. Self-Assembly of Nanostructured Materials Through Irreversible Covalent Bond Formation. Acc. Chem. Res. 2015, 48, 2221−2229. (19) Kambe, T.; Sakamoto, R.; Hoshiko, K.; Takada, K.; Miyachi, M.; Ryu, J.-H.; Sasaki, S.; Kim, J.; Nakazato, K.; Takata, M.; Nishihara, H. π-Conjugated Nickel Bis(dithiolene) Complex Nanosheet. J. Am. Chem. Soc. 2013, 135, 2462−2465. (20) Nam, K. T.; Shelby, S. A.; Choi, P. H.; Marciel, A. B.; Chen, R.; Tan, L.; Chu, T. K.; Mesch, R. A.; Lee, B.-C.; Connolly, M. D.; Kisielowski, C.; Zuckermann, R. N. Free-Floating Ultrathin TwoDimensional Crystals from Sequence-Specific Peptoid Polymers. Nat. Mater. 2010, 9, 454−460. (21) Huang, X.; Tang, S.; Mu, X.; Dai, Y.; Chen, G.; Zhou, Z.; Ruan, F.; Yang, Z.; Zheng, N. Freestanding Palladium Nanosheets with Plasmonic and Catalytic Properties. Nat. Nanotechnol. 2011, 6, 28−32. (22) Kong, X.; Xu, K.; Zhang, C.; Dai, J.; Norooz Oliaee, S.; Li, L.; Zeng, X.; Wu, C.; Peng, Z. Free-Standing Two-Dimensional Ru Nanosheets with High Activity Toward Water Splitting. ACS Catal. 2016, 6, 1487−1492. (23) Niu, J.; Wang, D.; Qin, H.; Xiong, X.; Tan, P.; Li, Y.; Liu, R.; Lu, X.; Wu, J.; Zhang, T.; Ni, W.; Jin, J. Novel Polymer-Free Iridescent Lamellar Hydrogel for Two-Dimensional Confined Growth of Ultrathin Gold Membranes. Nat. Commun. 2014, 5, 3313. (24) Zhao, J.; Deng, Q.; Bachmatiuk, A.; Sandeep, G.; Popov, A.; Eckert, J.; Rümmeli, M. H. Free-Standing Single-Atom-Thick Iron Membranes Suspended in Graphene Pores. Science 2014, 343, 1228− 1232. (25) Sun, Z.; Liao, T.; Dou, Y.; Hwang, S. M.; Park, M.-S.; Jiang, L.; Kim, J. H.; Dou, S. X. Generalized Self-Assembly of Scalable TwoDimensional Transition Metal Oxide Nanosheets. Nat. Commun. 2014, 5, 3813. (26) Wang, F.; Seo, J.-H.; Luo, G.; Starr, M. B.; Li, Z.; Geng, D.; Yin, X.; Wang, S.; Fraser, D. G.; Morgan, D.; Ma, Z.; Wang, X. NanometreThick Single-Crystalline Nanosheets Grown at the Water-Air Interface. Nat. Commun. 2016, 7, 10444. (27) Gao, S.; Jiao, X.; Sun, Z.; Zhang, W.; Sun, Y.; Wang, C.; Hu, Q.; Zu, X.; Yang, F.; Yang, S.; Liang, L.; Wu, J.; Xie, Y. Ultrathin Co3O4 Layers Realizing Optimized CO2 Electroreduction to Formate. Angew. Chem., Int. Ed. 2016, 55, 698−702. (28) Yu, T.; Lim, B.; Xia, Y. Aqueous-Phase Synthesis of SingleCrystal Ceria Nanosheets. Angew. Chem., Int. Ed. 2010, 49, 4484− 4487. (29) Hou, C.; Zhang, M.; Kasama, T.; Engelbrekt, C.; Zhang, L.; Wang, H.; Chi, Q. Reagent-Free Synthesis and Plasmonic Antioxidation of Unique Nanostructured Metal-Metal Oxide Core-Shell Microfibers. Adv. Mater. 2016, 28, 4097−4104. (30) Deng, S.; Berry, V. Wrinkled, Rippled and Crumpled Graphene: An Overview of Formation Mechanism, Electronic Properties, and Applications. Mater. Today 2016, 19, 197−212. (31) Dave, S. H.; Gong, C.; Robertson, A. W.; Warner, J. H.; Grossman, J. C. Chemistry and Structure of Graphene Oxide via Direct Imaging. ACS Nano 2016, 10, 7515−7522. (32) Boppella, R.; Anjaneyulu, K.; Basak, P.; Manorama, S. V. Facile Synthesis of Face Oriented ZnO Crystals: Tunable Polar Facets and Shape Induced Enhanced Photocatalytic Performance. J. Phys. Chem. C 2013, 117, 4597−4605. (33) Hermanson, K. D.; Lumsdon, S. O.; Williams, J. P.; Kaler, E. W.; Velev, O. D. Dielectrophoretic Assembly of Electrically Functional Microwires from Nanoparticle Suspensions. Science 2001, 294, 1082− 1086.

(34) Hayward, R. C.; Saville, D. A.; Aksay, I. A. Electrophoretic Assembly of Colloidal Crystals with Optically Tunable Micropatterns. Nature 2000, 404, 56−59. (35) Yan, J.; Han, M.; Zhang, J.; Xu, C.; Luijten, E.; Granick, S. Reconfiguring Active Particles by Electrostatic Imbalance. Nat. Mater. 2016, 15, 1095−1099. (36) Tan, S. T.; Chen, B. J.; Sun, X. W.; Fan, W. J.; Kwok, H. S.; Zhang, X. H.; Chua, S. J. Blueshift of Optical Band Gap in ZnO Thin Films Grown by Metal-Organic Chemcial-Vapor Deposition. J. Appl. Phys. 2005, 98, 013505. (37) Xiu, F.; Xu, J.; Joshi, P. C.; Bridges, C. A.; Paranthaman, M. P. In Semiconductor Materials for Solar Photovoltaic Cells; Springer International Publishing: Cham, Switzerland, 2016; Chapter 4. (38) Copéret, C.; Estes, D. P.; Larmier, K.; Searles, K. Isolated Surface Hydrides: Formation, Structure, and Reactivity. Chem. Rev. 2016, 116, 8463−8505. (39) Wong, W. S.; Salleo, A. Flexible Electronics: Materials and Applications; Springer Science & Business Media: Berlin, 2009. (40) Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Flexible Organic Transistors and Circuits with Extreme Bending Stability. Nat. Mater. 2010, 9, 1015−1022. (41) Mannsfeld, S. C. B.; Tee, B. C.-K.; Stoltenberg, R. M.; Chen, C. V. H.-H.; Barman, S.; Muir, B. V. O.; Sokolov, A. N.; Reese, C.; Bao, Z. Highly Sensitive Flexible Pressure Sensors with Microstructured Rubber Dielectric Layers. Nat. Mater. 2010, 9, 859−864. (42) Hu, H.; Zhao, Z.; Wan, W.; Gogotsi, Y.; Qiu, J. Ultralight and Highly Compressible Graphene Aerogels. Adv. Mater. 2013, 25, 2219− 2223. (43) Baldino, L.; Cardea, S.; Reverchon, E. Natural Aerogels Production by Supercritical Gel Drying. Chemical Engineering Transactions 2015, 43, 739−744.

1446

DOI: 10.1021/acs.chemmater.7b00188 Chem. Mater. 2017, 29, 1439−1446