Exfoliation of Layered Topological Insulators ... - ACS Publications

Exfoliation of Layered Topological Insulators Bi2Se3 and Bi2Te3 via Electrochemistry Adriano Ambrosi,† Zdeněk Sofer,‡ Jan Luxa,‡ and Martin Pumera*,† †

Downloaded via KAOHSIUNG MEDICAL UNIV on September 29, 2018 at 22:17:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 ‡ Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: Among layered materials, topological insulators such as Bi2Se3 and Bi2Te3 are lately attracting much attention due to particular electronic properties and, especially with Bi2Te3, excellent thermoelectric properties. Methods of preparation of few-layered nanosheets of Bi2Se3 and Bi2Te3 range from the bottom-up chemical vapor deposition or hydrothermal synthesis from oxide precursors to the top-down mechanical exfoliation and liquid-based exfoliation supported by sonication from the natural bulk crystals. Here, we propose a simple and rapid electrochemical approach to exfoliate natural Bi2Se3 and Bi2Te3 crystals in aqueous media to single/few-layer sheets. The exfoliated materials have been characterized by scanning transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray powder diffraction, high-resolution transmission electron microscopy, and Raman spectroscopy in addition to evaluation of their electrochemical properties. This electrochemical procedure represents a simple, reagent-free, and scalable method for the fabrication of single/fewlayer sheets of these materials. KEYWORDS: topological insulator, exfoliation, layered compound, electrochemistry, hydrogen evolution reaction

L

exfoliation has also been achieved in polyvinyl pyrrolidone (PVP) with the assistance of ultrasonication18 and also via a preliminary Li intercalation process at 200 °C in ethylene glycol (EG) followed by spontaneous exfoliation in water.19 Bottomup approaches consisted of the growth of few-layer nanoplates using a vapor−solid20 or chemical vapor deposition (CVD)21 method onto oxidized silicon wafers or by solvothermal synthesis using oxide precursors.22 Electrochemical exfoliation methods are being considered lately with enormous interest due to the easy procedure, high controllability, mild conditions, and good efficiencies. They have already been applied with good results to exfoliate graphene23−25 and MoS2.26,27 We propose here a convenient top-down electrochemical exfoliation approach in aqueous solutions to obtain few-layered sheets of Bi2Se3 and Bi2Te3 crystals grown from melt. The materials have been characterized by scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction, high-resolution transmission

ayered materials have recently gained much interest due to the interesting properties emerging when single- or few-layer thick sheets are produced and which are different from the bulk multilayered material.1,2 Graphene represents the most recognized example of this phenomenon, which triggered an explosion of scientific research toward several other 2D materials such as transition metal dichalcogenides (TMDs),3,4 black phosphorus,5−8 antimonene,9,10 boron nitride,11,12 etc. Another class of materials possessing interesting thermal, optical, and electrical properties when down-sized to single- or few-layer sheets is represented by the topological insulators (TIs).13,14 These materials have anisotropic electrical properties which include an insulator along the c-axis in the bulk state but with the metallic phase along the layer surface.15 Antimony and bismuth chalcogenides (with sulfur and tellurium chalcogen) with general structure A2B3 (where A = Bi, Sb; B = Se, Te) represent the most studied materials among the TIs because they are among the best thermoelectric materials.16 Different preparation methods have been proposed to fabricate single sheets of this class of materials in order to study their properties. Similarly to graphene, mechanical exfoliation has been proposed to split apart the crystal layers of Bi2Te3 and to study the thermal and electrical properties of single- and few-layer sheets.17 Liquid © 2016 American Chemical Society

Received: October 20, 2016 Accepted: December 7, 2016 Published: December 12, 2016 11442

DOI: 10.1021/acsnano.6b07096 ACS Nano 2016, 10, 11442−11448

Article

www.acsnano.org

Article

ACS Nano

layered materials, which revealed that the energy of vdW forces of Bi2Se3 and Bi2Te3 are, on the basis of method used, in the range of 13−26 meV/Å2.28 Raman spectroscopy was employed to characterize the Bi2Se3 and Bi2Te3 crystals (Figure 2). Typical Raman-active modes 1 A1g, 2Eg, and 2A1g were recorded for Bi2Se3 at about 72, 132, and 174 cm−1, respectively, and about 62, 102, and 135 cm−1 for Bi2Te3. Similarly to the graphene experience, different exfoliation procedures were proposed for Bi2Se3 and Bi2Te3 such as Li intercalation,19 mechanical exfoliation,17 and liquidphase exfoliation supported by sonication.18 To the best of our knowledge, no attempt has been done to exfoliate crystals of Bi2Se3 and Bi2Te3 using electrochemical methods. We previously developed a controllable electrochemical exfoliation of graphite foils to obtain graphene sheets with different structural and chemical composition depending on the electrolyte employed.25 Here, we propose a modified procedure to exfoliate natural crystals of Bi2Se3 and Bi2Te3. The phase purity of Bi2Se3 and Bi2Te3 crystals was proven by X-ray diffraction. Both samples show single-phase composition of rhombohedral phases (symmetry R3̅m) with preferential orientation of the crystallites due to its layered structure. The X-ray diffractograms with diffraction patterns of corresponding PDF card 01-089-2009 for Bi2Se3 and 01-089-2008 for Bi2Te3 are shown in Figure 3. Using small flakes of Bi2Se3 and Bi2Te3 crystals (see Figure 4G,H) we set up a two-electrode system in combination with a Pt foil as the counter electrode, which is separated from the solution through a dialysis membrane in order to avoid direct contact of exfoliated material with the electrode surface while ensuring electrical and ionic current to flow (Figure 4A,D). We employed aqueous Na2SO4 electrolyte for the experiment. Applying to the working electrode (crystal) positive DC voltages up to +10 V resulted with no exfoliation process being observed. This is different from the exfoliation of graphite because exfoliation was obtained in only anodic conditions.25 We noticed, however, that by inverting the electrode polarity and applying negative voltages to the working electrode, materials were released in solution (Figure 4B,E) in about 2 min. This phenomenon repeated only when a combination of preliminary anodic potential followed by a cathodic potential was applied. We therefore optimized the procedure alternating anodic voltages of +10 V to cathodic voltages of −10 V, both

electron microscopy (HR-TEM), and Raman spectroscopy methods in addition to electrochemical studies involving the inherent redox behavior and catalysis toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).

RESULTS AND DISCUSSION Bi2Se3 and Bi2Te3 are layered materials with rhombohedral crystal structure of the space group D3d (R3m). Each layer is formed by five monatomic sheets called quintuple layer (QL) in which atoms are covalently bonded in the sequence X−Bi−X− Bi−X where X = Se or Te (Figure 1). Weak van der Waals

Figure 1. Layered crystal structure of Bi2Se3 and Bi2Te3 showing quintuple layers with the Te(Se)−Bi−Te(Se)−Bi−Te(Se) sequence along the c-axis. Weak van der Waals interactions hold together the quintuple layers.

(vdW) interactions held together the QLs, and therefore, single- or few-quintuple layers can be obtained upon exfoliation of the bulk crystal. The energy of vdW forces is not experimentally known for most of the compounds; however, theoretical calculations were recently performed for several

Figure 2. Raman spectra of bulk (A) Bi2Se3 and (B) Bi2Te3. 11443

DOI: 10.1021/acsnano.6b07096 ACS Nano 2016, 10, 11442−11448

Article

ACS Nano

Figure 3. X-ray diffractograms of Bi2Se3 and Bi2Te3 and corresponding PDF cards. Unmatched diffraction patterns originate from Kβ lines.

Figure 5. Scanning transmission electron microscopy images of (A) electrochemical exfoliated Bi2Se3 and (B) electrochemical exfoliated Bi2Te3. Scale bar corresponds to 1 μm.

Figure 4. Process of electrochemical exfoliation of Bi2Se3 (A−C) and Bi2Te3 (D−F). (G) Bi2Se3 and Bi2Te3 crystals used for exfoliation. (H) Electrical connection of the crystal using copper tape. Colloidal suspension of Bi2Se3 (I) and Bi2Te3 (J) obtained after the electrochemical exfoliation.

for the duration of 2 min. When this two-step procedure was repeated, abundant material release was observed for both Bi2Se3 and Bi2Te3 (Figure 4C,F). After a series of washing steps to eliminate the electrolyte, both exfoliated materials were resuspended in distilled water and sonicated for 1 h, obtaining a stable dispersion (Figure 4I,J). Representative STEM images of the exfoliated materials can be seen in Figure 5. It can be noticed that exfoliated sheets of materials were obtained, although material debris was also visible. The morphology was further characterized by TEM. The exfoliated sheet of Bi2Se3 is shown on Figure 6A together with the HR-TEM image (Figure 6B), and the exfoliated sheet of Bi2Te3 is shown on Figure 6C with corresponding HR-TEM

Figure 6. (A) Low-resolution TEM and (B) HR-TEM images of Bi2Se3. (C) Low-resolution TEM and (D) HR-TEM images of Bi2Te3.

image (Figure 6D). EDS analysis of sheets shows the composition of sheets from bismuth, selenium, and tellurium (Figure S1 of Supporting Information). The EDS mapping shows the main origin of debris from elemental tellurium in the case of Bi2Te3 and elemental selenium in the case of Bi2Se3. These particles are formed by oxidation of selenides and 11444

DOI: 10.1021/acsnano.6b07096 ACS Nano 2016, 10, 11442−11448

Article

ACS Nano

Figure 7. XPS spectra of Bi2Se3 crystal (upper) and electrochemically exfoliated Bi2Se3 nanosheets (lower). (A) Survey spectra. High resolution spectra of (B) Bi 4f signal and (C) Se 3d signal.

which is close to the 1:3 ratio (Figure 8A). The formation of the oxide species of Bi and Te altering the natural Bi2Te3 composition could be the reason, as also demonstrated by the high-resolution XPS spectra of Bi 4f and Te 3d (Figure 8B,C). Clearly, both Bi and Te present oxidation states higher than that of natural Bi2Te3, producing signals at higher binding energy. An electrochemical study was performed in order to investigate inherent electrochemical behavior of the materials and possible catalytic properties toward the HER and OER. Figure 9A shows the cyclic voltammogram recorded in buffered solution for exfoliated Bi2Se3 drop-casted onto glassy carbon (GC) electrodes. It can be noticed that oxidative and reductive signals resulted at about 0 V and −0.75 V, respectively, suggesting a common redox process. Since both signals persisted for several scans, the redox process can be considered pseudoreversible. Bi2Te3 presented a similar electrochemical behavior indicated by oxidative signals at about −0.1 and 0.1 V and reduction signals at about −0.8 and −1.2 V. Again, as can be seen from Figure 9B, the redox process persisted over several voltammetric scans, suggesting reversible behavior. The redox signals to the oxidation and reduction potential of Bi oxide species are consistent with oxidation of Bi0 films and reduction of Bi3+.29,30

tellurides during electrochemical exfoliation. HR-TEM images are also provided with simulated figures showing orientation of observed planes (Figure S2 of Supporting Information). XPS is a useful characterization tool able to analyze the surface composition of solid materials as well as the atomic oxidation state of the elements. Figure 7A summarizes the XPS survey spectra of the bulk Bi2Se3 crystal and the exfoliated material. The presence of intense oxygen and Si signals for the exfoliated material is due to the SiO2 wafer onto which the exfoliated dispersed material was deposited. It can be noticed that a more intense C 1s signal resulted for the exfoliated material, which should be due to contamination. Survey spectra are particularly useful to determine the relative atomic composition of the material analyzed. The Bi/Se atomic ratio for the bulk crystal material was 0.69, close to the theoretical 0.66, confirming the composition of Bi2Se3. The exfoliated material resulted in a Bi/Se ratio of 0.49, which suggests a ratio 1:2 and is likely due to the formation of oxides. The highresolution XPS spectra of Bi 4f and Se 3d signals (Figure 7B,C) confirm such a hypothesis because both signals are shifted to higher binding energies. Similarly, the survey XPS spectrum of the bulk Bi2Te3 enables the evaluation of the relative atomic composition. A Bi/Te ratio of 0.68 resulted, which confirms the composition of Bi2Te3. After the exfoliation, the Bi/Te ratio changed to 0.37, 11445

DOI: 10.1021/acsnano.6b07096 ACS Nano 2016, 10, 11442−11448

Article

ACS Nano

Figure 8. XPS spectra of Bi2Te3 crystal (upper) and electrochemically exfoliated Bi2Te3 nanosheets (lower). (A) Survey spectra. High resolution spectra of (B) Bi 4f signal and (C) Te 3d signal. carbon (GC) with a diameter of 3 mm was obtained from CH Instruments, USA. Synthesis. For the synthesis of Bi2Se3 crystals, a stoichiometric amount of bismuth and selenium forming 10 g of Bi2Se3 was placed in a quartz glass ampule (15 × 100 mm) and evacuated at pressure below 1 × 10−3 Pa. The evacuated ampule was melt-sealed by an oxygen− hydrogen torch. The ampule was heated at 750 °C at a heating rate of 5 °C/min. After 1 h dwell time at 750 °C, the ampule was cooled to 650 °C at a cooling rate of 0.3 °C/min and finally cooled to room temperature at a cooling rate of 1 °C/min. Similarly, Bi2Te3 crystals were prepared by introducing a stoichiometric amount of bismuth and tellurium forming 10 g of Bi2Te3 into a quartz glass ampule (15 × 100 mm) evacuated at pressure below 1 × 10−3 Pa. This was then melt-sealed by an oxygen− hydrogen torch at 620 °C at a heating rate of 5 °C/min. After 1 h dwell at 620 °C, the ampule was first cooled to 500 °C at a cooling rate of 0.3 °C/min and finally cooled to room temperature at cooling rate of 1 °C/min. Apparatus. STEM images were obtained by using JEOL 7600F SEM (JEOL, Japan) operating at 30 kV. XPS was performed with a Phoibos 100 spectrometer and a monochromatic Mg X-ray radiation source (SPECS, Germany). Raman spectroscopy analysis was performed using a confocal micro-Raman LabRam HR instrument (Horiba Scientific, Japan) in backscattering geometry with a CCD detector, a 514.5 nm Ar laser, and a 100× objective mounted on a Olympus optical microscope. X-ray diffraction (XRD) was done with a Bruker D8 Discoverer diffractometer in Bragg−Brentano parafocusing geometry. A Cu Kα radiation was used. Diffraction patterns were

Probing the catalytic properties of both exfoliated Bi2Se3 and Bi2Te3, we noticed that only Bi2Te3 was catalytically active for HER, showing a reduced overpotential of about 150 mV (at current density of −10 mA cm−2) compared to that of the bare GC electrode. Bi2Se3 presented an almost overlapping polarization curve with that of GC (Figure 10). Both materials were catalytically inactive toward OER, presenting overlapping curves with that of GC (Figure S3 of Supporting Information).

CONCLUSIONS A fast, reagent-less, glovebox-free, and controllable exfoliation method is presented here to prepare single- and few-layer thick Bi2Se3 and Bi2Te3 nanosheets starting from natural crystals. The exfoliated materials present interesting electrochemical properties, particularly a pseudoreversible inherent redox behavior which needs to be taken into account when electrochemical sensing or energy-related applications are the intended applications for these materials. This method has great potential for exfoliation of Bi2Se3 and Bi2Te3. EXPERIMENTAL SECTION Materials. Bismuth (99.999%), selenium (99.999%), and tellurium (99.999%) were obtained from MaTecK, Germany. N,N-Dimethylformamide, potassium sulfate, potassium ferricyanide, and potassium hydroxide were purchased from Sigma-Aldrich, Singapore. Glassy 11446

DOI: 10.1021/acsnano.6b07096 ACS Nano 2016, 10, 11442−11448

Article

ACS Nano

employed to avoid contamination from copper, leaving only the crystals exposed to the solution. A Pt foil was used as the counter electrode. Exfoliation proceeded by applying a preliminary DC voltage of +2 V to the anode for 2 min. This can facilitate a proper wetting of the crystal with the intercalation of the electrolyte anions. This was followed by the application of +10 V for a further 2 min. Exfoliation occurred by alternating anodic (+10 V) and cathodic (−10 V) voltages to the crystal. Exfoliation was carried out in 0.5 M Na2SO4 electrolyte. Upon completion of the electrochemical exfoliation, the exfoliated materials were washed several times with ultrapure water using centrifugation and dried in a vacuum oven for 2 days at 40 °C. Electrochemical Characterizations. Electrochemical experiments were performed at room temperature by using a three-electrode configuration on a Autolab PGSTAT101 electrochemical analyzer (Methrom Autolab B.V., The Netherlands) connected to a personal computer and controlled by NOVA software, version 1.9 (Methrom Autolab B.V.). A platinum electrode (Autolab) served as an auxiliary electrode, and a Ag/AgCl electrode (CH Instruments, USA) served as a reference electrode. The exfoliated materials were drop-casted onto a previously polished working electrode (GC) from 1 mg/mL water dispersions. Cyclic voltammograms were recorded in 0.1 M phosphate buffer solution (pH 7.2) with scan rate of 0.1 V/s. Polarization curves for HER were recorded in 0.5 M H2SO4 solution at scan rate of 5 mV/ s. All potentials are considered versus the relative hydrogen electrode (RHE) using the conversion relation: E(RHE) = E(Ag/AgCl) + 0.059pH + ° . Polarization curves for OER were recorded in 0.1 M KOH at E(Ag/AgCl) a scan rate of 5 mV/s.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07096. EDS mapping, HR-TEM images with simulated orientation planes and OER measurements (PDF)

Figure 9. Cyclic voltammograms of (A) exfoliated Bi2Se3 and (B) exfoliated Bi2Te3. The scan rate is 100 mV/s; 0.05 M PBS; pH = 7.2.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Martin Pumera: 0000-0001-5846-2951 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS M.P. acknowledges a Tier 2 grant (MOE2013-T2-1-056; ARC 35/13) from the Ministry of Education, Singapore. Z.S. and J.L. were supported by Czech Science Foundation (GACR No. 1605167S) and by specific university research (MSMT No. 20SVV/2016).

Figure 10. Hydrogen evolution reaction in 0.5 M sulfuric acid for exfoliated Bi2Se3 (blue line), exfoliated Bi2Te3 (red line), and glassy carbon (GC) electrode (black line). Scan rate is 5 mV/s.

REFERENCES

collected between 10° and 80° of 2θ. The obtained data were evaluated using HighScore Plus 3.0e software. High resolution transmission electron microscopy (HR-TEM) was performed using an EFTEM Jeol 2200 FS microscope (Jeol, Japan). A 200 keV acceleration voltage was used for measurement. Sample preparation was attained by drop casting the suspension (1 mg mL−1 in water) on a TEM grid (Cu; 200 mesh; Formvar/carbon) and drying at 60 °C for 12 h. The simulation of HR-TEM images was performed with JEMS software (version 4.4631U2016) using Blochwave method. The noise level used for the simulation was 1%, astigmatism of 21.51 nm, and sample drift of 0.075 nm.s−1. Procedures. Electrochemical Exfoliation. Small flakes from the natural crystal of Bi2Se3 and Bi2Te3 were electrically connected to the high-voltage device through a copper tape. Parafilm protection was

(1) Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. 2D Materials and van der Waals Heterostructures. Science 2016, 353, aac9439. (2) Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutierrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; et al. Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene. ACS Nano 2013, 7, 2898−2926. (3) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (4) Pumera, M.; Sofer, Z.; Ambrosi, A. Layered Transition Metal Dichalcogenides for Electrochemical Energy Generation and Storage. J. Mater. Chem. A 2014, 2, 8981−8987. 11447

DOI: 10.1021/acsnano.6b07096 ACS Nano 2016, 10, 11442−11448

Article

ACS Nano (5) Eswaraiah, V.; Zeng, Q. S.; Long, Y.; Liu, Z. Black Phosphorus Nanosheets: Synthesis, Characterization and Applications. Small 2016, 12, 3480−3502. (6) Ryder, C. R.; Wood, J. D.; Wells, S. A.; Hersam, M. C. Chemically Tailoring Semiconducting Two-Dimensional Transition Metal Dichalcogenides and Black Phosphorus. ACS Nano 2016, 10, 3900−3917. (7) Bagheri, S.; Mansouri, N.; Aghaie, E. Phosphorene: A New Competitor for Graphene. Int. J. Hydrogen Energy 2016, 41, 4085− 4095. (8) Sofer, Z.; Sedmidubsky, D.; Huber, S.; Luxa, J.; Bousa, D.; Boothroyd, C.; Pumera, M. Layered Black Phosphorus: Strongly Anisotropic Magnetic, Electronic, and Electron-Transfer Properties. Angew. Chem., Int. Ed. 2016, 55, 3382−3386. (9) Singh, D.; Gupta, S. K.; Sonvane, Y.; Lukacevic, I. Antimonene: A Monolayer Material for Ultraviolet Optical Nanodevices. J. Mater. Chem. C 2016, 4, 6386−6390. (10) Zhao, M. W.; Zhang, X. M.; Li, L. Y. Strain-Driven Band Inversion and Topological Aspects in Antimonene. Sci. Rep. 2015, 5, 16108. (11) Galvani, T.; Paleari, F.; Miranda, H. P. C.; Molina-Sanchez, A.; Wirtz, L.; Latil, S.; Amara, H.; Ducastelle, F. Excitons in Boron Nitride Single Layer. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94, 125303. (12) Sandoval, E. D.; Hajinazar, S.; Kolmogorov, A. N. Stability of Two-Dimensional BN- Si Structures. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94, 094105. (13) Peng, H.; Lai, K.; Kong, D.; Meister, S.; Chen, Y.; Qi, X.-L.; Zhang, S.-C.; Shen, Z.-X.; Cui, Y. Aharonov-Bohm Interference in Topological Insulator Nanoribbons. Nat. Mater. 2010, 9, 225−229. (14) Fu, L.; Kane, C. L.; Mele, E. J. Topological Insulators in Three Dimensions. Phys. Rev. Lett. 2007, 98, 106803. (15) Brumfiel, G. Topological Insulators: Star Material. Nature 2010, 466, 310−311. (16) Zhou, G.; Wang, D. Few-quintuple Bi2Te3 Nanofilms As Potential Thermoelectric Materials. Sci. Rep. 2015, 5, 8099. (17) Teweldebrhan, D.; Goyal, V.; Balandin, A. A. Exfoliation and Characterization of Bismuth Telluride Atomic Quintuples and QuasiTwo-Dimensional Crystals. Nano Lett. 2010, 10, 1209−1218. (18) Sun, L.; Lin, Z.; Peng, J.; Weng, J.; Huang, Y.; Luo, Z. Preparation of Few-Layer Bismuth Selenide by Liquid-PhaseExfoliation and Its Optical Absorption Properties. Sci. Rep. 2014, 4, 4794. (19) Ren, L.; Qi, X.; Liu, Y.; Hao, G.; Huang, Z.; Zou, X.; Yang, L.; Li, J.; Zhong, J. Large-Scale Production of Ultrathin Topological Insulator Bismuth Telluride Nanosheets by a Hydrothermal Intercalation and Exfoliation Route. J. Mater. Chem. 2012, 22, 4921−4926. (20) Kong, D.; Dang, W.; Cha, J. J.; Li, H.; Meister, S.; Peng, H.; Liu, Z.; Cui, Y. Few-Layer Nanoplates of Bi2Se3 and Bi2Te3 with Highly Tunable Chemical Potential. Nano Lett. 2010, 10, 2245−2250. (21) Jiang, Y.; Zhang, X.; Wang, Y.; Wang, N.; West, D.; Zhang, S.; Zhang, Z. Vertical/Planar Growth and Surface Orientation of Bi2Te3 and Bi2Se3 Topological Insulator Nanoplates. Nano Lett. 2015, 15, 3147−3152. (22) Fei, F.; Wei, Z.; Wang, Q.; Lu, P.; Wang, S.; Qin, Y.; Pan, D.; Zhao, B.; Wang, X.; Sun, J.; et al. Solvothermal Synthesis of Lateral Heterojunction Sb2Te3/Bi2Te3 Nanoplates. Nano Lett. 2015, 15, 5905−5911. (23) Parvez, K.; Li, R.; Puniredd, S. R.; Hernandez, Y.; Hinkel, F.; Wang, S.; Feng, X.; Müllen, K. Electrochemically Exfoliated Graphene as Solution-Processable, Highly Conductive Electrodes for Organic Electronics. ACS Nano 2013, 7, 3598−3606. (24) Parvez, K.; Wu, Z.-S.; Li, R.; Liu, X.; Graf, R.; Feng, X.; Müllen, K. Exfoliation of Graphite into Graphene in Aqueous Solutions of Inorganic Salts. J. Am. Chem. Soc. 2014, 136, 6083−6091. (25) Ambrosi, A.; Pumera, M. Electrochemically Exfoliated Graphene and Graphene Oxide for Energy Storage and Electrochemistry Applications. Chem. - Eur. J. 2016, 22, 153−159.

(26) You, X.; Liu, N.; Lee, C. J.; Pak, J. J. An Electrochemical Route to MoS2 Nanosheets for Device Applications. Mater. Lett. 2014, 121, 31−35. (27) Liu, N.; Kim, P.; Kim, J. H.; Ye, J. H.; Kim, S.; Lee, C. J. LargeArea Atomically Thin MoS2 Nanosheets Prepared Using Electrochemical Exfoliation. ACS Nano 2014, 8, 6902−6910. (28) Björkman, T.; Gulans, A.; Krasheninnikov, A. V.; Nieminen, R. M. van der Waals Bonding in Layered Compounds from Advanced Density-Functional First-Principles Calculations. Phys. Rev. Lett. 2012, 108, 235502. (29) Wang, J.; Lu, J.; Setiadji, R. Adsorptive Stripping Measurements of Bismuth. Electroanalysis 1993, 5, 319−324. (30) Wang, J.; Tian, B.; Rogers, K. R. Thick-Film Electrochemical Immunosensor Based on Stripping Potentiometric Detection of a Metal Ion Label. Anal. Chem. 1998, 70, 1682−1685.

11448

DOI: 10.1021/acsnano.6b07096 ACS Nano 2016, 10, 11442−11448