Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
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Solvent-Dependent Intercalation and Molecular Configurations in Metallocene-Layered Crystal Superlattices Yue Zhu,† Yumin Qian,† Zhengyu Ju, Lele Peng, and Guihua Yu* Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States
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S Supporting Information *
ABSTRACT: Organic−inorganic superlattices are a class of artificial structures of significant scientific and technological importance. Forming these hybrid materials can be achieved via controlled intercalation of organic molecules into inorganic layered hosts, which is a complex course involving multiple physicochemical processes. In solution phase, it is further complicated by interaction of solvent molecules with the intercalant and/or host. Here we describe an intercalation system exhibiting strong solvent-dependent kinetics and phase evolution. In revisiting intercalation of ferrocene into layered VOPO4·2H2O material by taking into account the influence of solvent, we are able to unravel molecular configurations of ferrocene molecules. An exclusive orientation of ferrocene but different arrangements among the layers are concluded in two model solvents. Resolving this complicated structure is possible thanks to a combined experimental and theoretical approach. Our study provides new insights into understanding molecular configurations and controlling intercalation kinetics in creating organic−inorganic superlattices, which may offer unprecedented properties beyond conventional materials. KEYWORDS: Solvent effect, intercalation, metallocene, layered crystal, organic−inorganic superlattice
D
charge transfer mechanism.14 Many aspects of the intercalation chemistry concerning these relatively simple organic intercalants are still puzzling, for example, the orientation of these near-spherical molecules. In this case, incomplete intercalation and poor crystallinity of the final products also impose significant challenges in analyzing the intercalated structures, leading to only a few explicit answers for TMD hosts.15,16 In addition, intercalation kinetics in different media and associated structural evolutions are poorly understood. Motivated by a lack of convincing results in this system involving ferrocene, whose high ionization potential renders it only a few redox intercalation hosts outside TMDs,17−19 we rationally select layered VOPO4·2H2O as host, which has recently demonstrated fruitful applications in intercalationbased energy storage devices.20−24 More importantly, previous studies in this specific intercalant-host pair drew no cogent or even contradictory conclusions. For instance, an intriguing observation in the intercalation product was two basal spacings, which were attributed to two possible orientations of interlayer residing ferrocene.25 In another study, ferrocene with principal axis perpendicular to VOPO4 lattice planes was concluded.26
espite being a topic with almost a century of history, intercalation chemistry has been constantly revisited due to its profound scientific and technological importance1,2 and is further promoted by the recent surge of interests in tuning properties of layered materials.3−6 A wide spectrum of intercalants is available to generate various layered intercalation compounds, however, predicting and further tuning their material properties are challenging without clear comprehension of associated intercalation processes. Great success has been achieved in understanding the intercalation chemistry of graphite with small inorganic intercalants,7 whereas research in other layered hosts with extension to large organic intercalants was previously limited to transition metal dichalcogenides (TMDs).8 Recently, organic intercalation as a general structural engineering strategy for layered materials has been demonstrated in many functional device applications.9−11 In particular, artificial superlattices consisting of alternating organic−inorganic layers offer unparalleled opportunities beyond the reach of existing materials.12,13 Despite these exciting results, there still is a scarcity of systematic studies to address fundamental aspects, such as configuration of the intercalant, structural alteration of the host, thermodynamics and kinetics, effect of medium, and so forth, that are crucially related to these intercalation processes. In this study, we tackle a classical system involving redox intercalation of metallocenes in layered crystals driven by © XXXX American Chemical Society
Received: July 24, 2018 Revised: August 17, 2018 Published: August 22, 2018 A
DOI: 10.1021/acs.nanolett.8b03030 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. (a) Solvent-dependent energy diagram for intercalation of ferrocene into layered VOPO4·2H2O. (b) EDS elemental mapping and (c) cross-sectional TEM imaging of the intercalation product. (d) Time-dependent XRD patterns of the products when intercalated in 2-propanol and acetone. Intercalation-induced peaks are highlighted in dashed rectangle.
compared to a single peak in 2-propanol. Assuming the appearance of low angle peak is a result of enlarged interlayer distance, peak position of the single peak in 2-propanol (9.04°) gives a value of 9.77 Å according to Bragg’s law, matching well with the one directly measured from TEM. Note that intercalation in 2-propanol is more rapid and is considered to attain completion between 12 to 24 h, judging by the disappearance of the shifted (001) peak, whereas in acetone this point lies beyond 48 h. To quantitively analyze reaction rate, intercalation kinetics is studied by plotting extent of reaction (α) versus time.27 Given the complexity in XRD and thermogravimetric analysis (TGA) measurements (Figure S5), the amount of intercalated ferrocene was evaluated directly in EDS as the atomic ratio of Fe to V (denoted as fc/VOPO4). Then α was calculated as the ratio of experimental intercalated ferrocene to its maximum theoretical value, which is 0.5 ferrocene per VOPO4 unit (the reason will be clear later in structural analysis). From the plot of intercalated ferrocene versus time (Figure 2a), it is immediately obvious that intercalation kinetics is much faster in 2-propanol. The apparent deceleratory shape of the two curves excludes the Avrami−Erofeyev solid-state kinetics model, which features a sigmoidal shape as shown by previous studies in TMDs.28 This indicates that ferrocene intake at the edge of VOPO4·2H2O (nucleation) is rather rapid compared to the diffusion of ferrocene inside the VOPO4 layers (nuclei growth). Therefore, the overall intercalation process is diffusion controlled. In this context, together with the twodimensional (2D) structure of VOPO4·2H2O, the wellestablished 2D diffusion model with integral form (1 − α) ln(1 − α) + α = kt is adopted to describe the intercalation kinetics (Figure 2b).27 The first few points from both cases are able to produce a straight line, validating the applied model. Slopes of the two lines, corresponding to the rate constant k, manifest a 3.7 times faster intercalation rate in 2-propanol, at least in the initial stage. The deviations from the model after 1 h in 2-propanol and 6 h in acetone may imply that the intercalation is no longer diffusion controlled. Interestingly, in
Herein, we revisit this model system from the angle of reaction energy diagram, inspired by insightful observation in solvent-dependent intercalation kinetics and intercalated structures (Figure 1a). From a thermodynamic point of view, the overall intercalation process involves a net release of energy resulted from the summation of the ionization potential of ferrocene, the electron affinity of VOPO4 host, and the electrostatic energy of the charge-separated intercalant-host pair. Kinetically, different solvents may drive this process along distinct pathways, as demonstrated by 2-propanol and acetone employed in this work. This solvent effect, though attracted little attention in previous studies, is hypothesized to play a critical role in several kinetically important steps, such as desolvation, layer sliding and electron transfer, during the intercalation. The observed solvent-dependence also leads to a verdict on the orientation of intercalated ferrocene molecules. The synthesis of VOPO4·2H2O followed previous literature and subsequent intercalation was carried out by adding the host into solutions of ferrocene (see details in Supporting Information). The intercalation process induces no significant morphology change of the host (Figures S1 and S2), as confirmed by scanning electron microscopy (SEM). The incorporation of ferrocene into VOPO4 layers is evident by the detection of Fe element (Figure 1b) and characteristic stretching bands of ferrocene (Figure S3) using energydispersive X-ray spectroscopy (EDS) and Fourier-transform infrared spectroscopy (FTIR), respectively. Another convincing evidence comes from cross-sectional view of the intercalated sample (Figure S4) imaged by transmission electron microscopy (TEM), from which an interlayer distance of 9.9 Å is determined (Figure 1c), larger than that of VOPO4· 2H2O (7.4 Å). Differences in phase evolution between the two solvents are revealed by ex situ X-ray diffraction (XRD) measurements. The patterns of intercalation products obtained at various time intervals (Figure 1d) share a similar shift of the (001) peak of VOPO4·2H2O (2θ = 11.9°) to higher angle with gradual decrease of intensity. However, two low-angle peaks observed in acetone manifest the most striking difference B
DOI: 10.1021/acs.nanolett.8b03030 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 3. (a) Schematic illustration of ferrocene intercalating into VOPO4·2H2O (layer translation is highlighted in dashed rectangle; interlayer water molecules are highlighted in dashed oval). (b) XRD patterns of experimental product and predicted structure by DFT calculation.
Figure 2. (a) Amount of intercalated ferrocene vs time, where fc/ VOPO4 is the Fe/V ratio determined by EDS. (b) Kinetic modeling using 2D diffusion model, as plotted by (1 − α) ln(1 − α) + α versus fc / VOPO4 . time, where α is defined by
The evidently slower intercalation kinetics in acetone suggests possible intermediate products, which can be described by staging phenomenon.7 A zoom-in plot of the time-dependent XRD patterns in acetone (Figure 4a) displays a stationary peak, which is now understood as the fully intercalated structure (stage 1 compound), and a broad peak that shifts gradually toward higher angle with noticeable peak narrowing. The latter is recognized as a result of stage disorder, whose evolution can be quantitatively analyzed by the Hendricks-Teller (HT) theory.30 Taking the XRD pattern at 12 h as an example (Figure 4b), the broad peak sandwiched between peak 1 (stage 1 compound, intercalated layers) and peak 2 (partially dehydrated VOPO4, nonintercalated layers) is as result of a random arrangement of these two layers (see Supporting Information for details). A reasonably good match between the HT-modeled pattern and the experimental one is found by assigning fractions of intercalated layers (f1) and nonintercalated layers (f2) with a ratio of 0.55:0.45. Peaks at other time intervals can be calculated likewise. As intercalation proceeds, such disordered layers would undergo a disorder− order transition, forming an ordered intermediate structure, which turns out to be stage 2 compound with alternative intercalated and nonintercalated layers (Figure 4c). At 48 h, the experimental XRD pattern is already resolvable mainly to be the sum of two patterns belonging to stage 1 and stage 2 compounds (Figure 4d). Therefore, the presence of two intercalation-induced peaks in acetone has been successfully explained. It is intriguing to observe the coexistence of stage 1 and stage 2 compounds in acetone throughout the whole intercalation. This is even true when intercalation time was extended to one month (Figure S11). More interestingly, though quite stable at room temperature the intercalation product in acetone can be converted to pure stage 2 compound when mildly heated at 50 °C (Figure S12). A straightforward interpretation is that the stage 2 compound is thermodynami-
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both cases deviations begin at very close fc/VOPO4 values, indicating a common cause for the retardation. To understand the configuration of intercalated ferrocene, we first survey the interplay of its location and associated steric hindrance. An intuitive reasoning taking into account of the size of ferrocene, lattice parameter of VOPO4·2H2O and its crystallographic symmetry concludes a rational layer sliding by half unit (Figure S6 and S7). Next, we proceed to determine the orientation of ferrocene using first-principles calculations.29 As indicated by the calculation results of several selected configurations with above-mentioned layer translation included (Figure S8), vertical orientation with principal axis of ferrocene perpendicular to VOPO4 lattice planes is ruled out based on the large mismatch in c-axis between modeling and experimental results (Table S1). In contrast, parallel orientation induces smaller interlayer expansion, and also promotes a π−π interaction among the cyclopentadienyl planes.12 The most stable configuration with parallel orientation is concluded by comparing lattice parameters between calculated values and experimental ones determined by XRD, together with the lowest overall energy (Figures S9 and S10). The final resolved ferrocene-VOPO4 structure requires a (1/2, 1/2, 0) layer translation and removal of total interlayer water molecules (Figure 3a) in the original VOPO4·2H2O, leading to a chemical formula fc0.5VOPO4 assuming complete intercalation. A perfect match can be seen between the XRD pattern simulated from this structure and that of 24 h product in 2-propanol (Figure 3b). Indeed, the intercalation product after 48 h in 2-propanol is determined to be fc0.42VOPO4, close to the theoretical formula. At this point, the intercalation in 2-propanol can be clearly understood as a single-phase-to-single-phase transformation. C
DOI: 10.1021/acs.nanolett.8b03030 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 4. (a) Time-dependent XRD patterns zoom-in at low-angle region. (b) Simulated XRD pattern of HT layers at 12 h. (c) Structure of stage 2 intercalated compound (hydrogen atoms are omitted for clarity). (d) XRD patterns of experimental product and calculated structure.
electrochemical properties of ferrocene, which itself alone is known to be active, as intercalant in this work remain unknown. These may be quite relevant features in various energy-related applications.11 Inspired by above ideas, we proceed to briefly examine the electrochemical activity of this organic−inogranic superlattice with different α-values (Figure S14). When tested in lithium cells, cyclic voltammetric (CV) curves (Figure 5) of selected intercalation products reveal
cally more stable. As increase in the basal spacing of the layered host lattice imposes one of the most significant energy penalties during intercalation, the stage 2 compound with less extent of interlayer expansion is thus energetically favorable compared with the stage 1 compound (Figure 1a). The energy difference between the two compounds may be further increased when taking into consideration of the expulsion of positively charged ferrocenium ions in adjacent layers, which is present in the stage 1 compound but absent in the stage 2 compound. However, X-ray photoelectron spectroscopy (XPS) measurements unexpectedly reveal that a significant amount of intercalated ferrocene in 2-propanol has not been oxidized (Figure S13), indicating partially relief of such electrostatic interaction. The origin for the observed faster intercalation kinetics in 2propanol remains elusive. Both 2-propanol and acetone cannot be incorporated into VOPO4·2H2O alone under current intercalation conditions, excluding the possible pre-expansion of host interlayers by solvent molecules.31 In considering solvent-host interaction, 2-propanol is known to effectively exfoliate VOPO4·2H2O under sonication,32 thus it may lower the energy needed for layer sliding, which plays a role in exfoliation of layered materials,33 as well as in the formation of intercalated products. The solvent−host interaction may also be electronic in nature. For example, a recent study reveals that relative difference in electronegativity between solvents and TMDs is able to drive the transfer of electrons from or to the materials affecting their intrinsic properties.34 This may be highly relevant in current ferrocene−VOPO4 system, which features a redox intercalation mechanism that involves electron transfer. Another possible angle to consider is solvent− intercalant interaction, such as desolvation of ferrocene as the prerequisite before intercalating into VOPO4·2H2O host. In this regard, a higher solvation energy leads to a slower intercalation rate, which is exactly the case in acetone according to our calculations (Table S2). Finally, for the preexisting water molecules in the host, though unlikely to interact with ferrocene, their removal to allow the proceeding of intercalation may be affected by solvent. A more complete picture revealing the role of solvent requires understanding of intercalation product in the initial stage, where in situ characterizaiton techniques will be most suitable.35 The ferrocene-intercalated VOPO4 can be considered as an extension of conventional VOPO4 material with unique structral modifications. The nearly 34% increase of interlayer distance (from 7.4 to 9.9 Å) is able to facilite more facile mass transport inside this layered structure. More interestingly,
Figure 5. CV curves of selected intercalation products with various α values.
several intriguing observations. First, a reduced peak separation of vanadium redox reactions can be seen in intercalated VOPO4 when compared with nonintercalated one. This is likely due to a decreased polarization as a result of increased interlayer distance, which was also found in our previous study involving different intercalated products in sodium cells.23 Second, as the extent of intercalation (or the value of α) increases, a new pair of redox peaks emerge while redox peaks of VOPO4 diminish significantly. We attribute the new redox peaks to oxidation and reduction of interlayer residing ferrocene, which is unexpectedly electrochemical active in such confined structure. The deactivation of VOPO4 different from our previous study can be understood for the following reasons. In ferrocene-intercalated VOPO4, due to the redox intercalation mechanism the ferrocene molecule exists in the form of ferrocenium ion, which is positively charged and repels further intake of the alkali-ion. Despite the similar extent of D
DOI: 10.1021/acs.nanolett.8b03030 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
(5) Bonaccorso, F.; Colombo, L.; Yu, G.; Stoller, M.; Tozzini, V.; Ferrari, A. C.; Ruoff, R. S.; Pellegrini, V. Science 2015, 347, 1246501. (6) Peng, L.; Zhu, Y.; Chen, D.; Ruoff, R. S.; Yu, G. Adv. Energy Mater. 2016, 6, 1600025. (7) Dresselhaus, M. S.; Dresselhaus, G. Adv. Phys. 1981, 30, 139− 326. (8) Friend, R. H.; Yoffe, A. D. Adv. Phys. 1987, 36, 1−94. (9) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Nat. Mater. 2014, 13, 897−903. (10) Wan, C.; Gu, X.; Dang, F.; Itoh, T.; Wang, Y.; Sasaki, H.; Kondo, M.; Koga, K.; Yabuki, K.; Snyder, G. J.; Yang, R.; Koumoto, K. Nat. Mater. 2015, 14, 622−627. (11) Zhu, Y.; Peng, L.; Fang, Z.; Yan, C.; Zhang, X.; Yu, G. Adv. Mater. 2018, 30, 1706347. (12) Li, Z.; Zhao, Y.; Mu, K.; Shan, H.; Guo, Y.; Wu, J.; Su, Y.; Wu, Q.; Sun, Z.; Zhao, A.; Cui, X.; Wu, C.; Xie, Y. J. Am. Chem. Soc. 2017, 139, 16398−16404. (13) Wang, C.; He, Q.; Halim, U.; Liu, Y.; Zhu, E.; Lin, Z.; Xiao, H.; Duan, X.; Feng, Z.; Cheng, R.; Weiss, N. O.; Ye, G.; Huang, Y.-C.; Wu, H.; Cheng, H.-C.; Shakir, I.; Liao, L.; Chen, X.; Goddard Iii, W. A.; Huang, Y.; Duan, X. Nature 2018, 555, 231−236. (14) Dines, M. B. Science 1975, 188, 1210−1211. (15) O’Hare, D.; Evans, J. S. O.; Wiseman, P. J.; Prout, K. Angew. Chem., Int. Ed. Engl. 1991, 30, 1156−1158. (16) Wong, H.-V.; Evans, J. S. O.; Barlow, S.; Mason, S. J.; O’Hare, D. Inorg. Chem. 1994, 33, 5515−5521. (17) Schäfer-Stahl, H.; Abele, R. Angew. Chem., Int. Ed. Engl. 1980, 19, 477−478. (18) Rodríguez-Castellón, E.; Jiménez-Ĺ ópez, A.; Martínez-Lara, M.; Moreno-Real, L. J. Inclusion Phenom. 1987, 5, 335−342. (19) Santiago, M. E. B.; Declet-Flores, C.; Díaz, A.; Vélez, M. M.; Bosques, M. Z.; Sanakis, Y.; Colón, J. L. Langmuir 2007, 23, 7810− 7817. (20) Zhu, Y.; Peng, L.; Chen, D.; Yu, G. Nano Lett. 2016, 16, 742− 747. (21) Li, H.; Peng, L.; Zhu, Y.; Chen, D.; Zhang, X.; Yu, G. Energy Environ. Sci. 2016, 9, 3399−3405. (22) Li, H.; Ding, Y.; Ha, H.; Shi, Y.; Peng, L.; Zhang, X.; Ellison, C. J.; Yu, G. Adv. Mater. 2017, 29, 1700898. (23) Peng, L.; Zhu, Y.; Peng, X.; Fang, Z.; Chu, W.; Wang, Y.; Xie, Y.; Li, Y.; Cha, J. J.; Yu, G. Nano Lett. 2017, 17, 6273−6279. (24) He, G.; Kan, W. H.; Manthiram, A. Chem. Mater. 2016, 28, 682−688. (25) Matsubayashi, G.; Ohta, S. Chem. Lett. 1990, 19, 787−790. (26) Davidson, A.; Villeneuve, G.; Fournes, L.; Smith, H. Mater. Res. Bull. 1992, 27, 357−366. (27) Khawam, A.; Flanagan, D. R. J. Phys. Chem. B 2006, 110, 17315−17328. (28) Evans, J. S. O.; Price, S. J.; Wong, H.-V.; O’Hare, D. J. Am. Chem. Soc. 1998, 120, 10837−10846. (29) Draxl, C.; Nabok, D.; Hannewald, K. Acc. Chem. Res. 2014, 47, 3225−3232. (30) Hendricks, S.; Teller, E. J. Chem. Phys. 1942, 10, 147−167. (31) Č apková, P.; Trchová, M.; Zima, V.; Schenk, H. J. Solid State Chem. 2000, 150, 356−362. (32) Wu, C.; Lu, X.; Peng, L.; Xu, K.; Peng, X.; Huang, J.; Yu, G.; Xie, Y. Nat. Commun. 2013, 4, 2431. (33) Ludwig, T.; Guo, L.; McCrary, P.; Zhang, Z.; Gordon, H.; Quan, H.; Stanton, M.; Frazier, R. M.; Rogers, R. D.; Wang, H.-T.; Turner, C. H. Langmuir 2015, 31, 3644−3652. (34) Choi, J.; Zhang, H.; Du, H.; Choi, J. H. ACS Appl. Mater. Interfaces 2016, 8, 8864−8869. (35) Wu, Y.; Liu, N. Chem. 2018, 4, 438−465.
interlayer expansion, the highly ordered superlattice structure, different from previous intercalated products, may not favor the access of charge carrier to the vanadium centers. Nevertheless, above observations manifest a type of material with dual electrochemical active centers, whose activities can be varied in an inversely correlated but controllable manner. In summary, we have investigated unique solvent-dependent intercalation in an organic−inorganic hybrid superlattice consisting of alternating layers of ferrocene molecules and 2D VOPO4. Besides an intriguing observation in intercalation kinetics, detailed molecular configuration of intercalants has been probed via a combined experimental and theoretical approach. In particular, the success of DFT calculation in predicting the orientation of ferrocene is corroborated by powder X-ray diffraction, which also reveals distinct phase evolution pathways and staging phenomena in two solvents. Our study brings a deeper understanding of the intercalation processes and associated intercalated structures, which were either uncovered or wrongly interpreted in previous studies. Offering features like tunable composition/structure and dual electrochemical activity, this model organic−inorganic superlattice represents a huge family of artificial materials that can benefit from wide choices of organic intercalants, meriting their feasibilities in many technological applications. Fundamentally, the observed solvent-influenced interplay between kinetic and thermodynamic processes could potentially attract broad research interests into materials engineering enabled by controlled intercalation.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b03030. Experimental details, SEM, FTIR, TEM, TGA, XPS, DFT calculations, and other additional information (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Guihua Yu: 0000-0002-3253-0749 Author Contributions †
Y.Z. and Y.Q. equally contributed to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS G.Y. acknowledges the financial support from Welch Foundation Grant F-1861, Sloan Research Fellowship, and Camille Dreyfus Teacher-Scholar Award. The authors thank Dr. Josefina Arellano-Jimenez for the TEM characterization.
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REFERENCES
(1) Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Science 2013, 340, 1226419. (2) Goodenough, J. B.; Park, K.-S. J. Am. Chem. Soc. 2013, 135, 1167−1176. (3) Wang, H.; Yuan, H.; Sae Hong, S.; Li, Y.; Cui, Y. Chem. Soc. Rev. 2015, 44, 2664−2680. (4) Shi, Y.; Zhou, X.; Yu, G. Acc. Chem. Res. 2017, 50, 2642−2652. E
DOI: 10.1021/acs.nanolett.8b03030 Nano Lett. XXXX, XXX, XXX−XXX
