Article Cite This: Chem. Mater. 2018, 30, 5333−5338
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Liquid-Phase Exfoliation of Germanane Based on Hansen Solubility Parameters Daisuke Nakamura* and Hideyuki Nakano* Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan
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S Supporting Information *
ABSTRACT: Two-dimensional (2D) materials combine the collective advantages of individual building blocks and synergistic properties and have spurred great interest as a new paradigm in materials science. For the mass production of these materials, large-scale exfoliations are the main production route; but, recent approaches in the liquid-phase exfoliation are based on empirical trial and error strategies. This study aims to propose a method to determine the suitable solvents for efficiently exfoliating germanane (GeH) and the optimal solvent removal conditions at moderate temperatures using Hansen solubility parameters. We confirm the presence of GeH nanosheets using atomic force microscopy. The proposed method revealed that GeH can be efficiently dispersed in 1,3-dioxolane and can be deposited as individual sheets, which have the thickness of 3−4 nm and the lateral size in the range of a few micrometers.
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INTRODUCTION
After the mechanical exfoliation of graphite was reported, many studies based on the exfoliation of other two-dimensional (2D) materials such as transition metal dichalcogenides1,2 and transition metal oxides3 have been reported. The correspondence on the novel electronic or mechanical properties of the exfoliated sheets has also led to a great progress in terms of the device applications. Furthermore, these 2D materials can be used for both nanoelectronics and as a topologically nontrivial materials platform. Recently, Coleman et al. reported the liquid exfoliation of various 2D materials, and their processes are expected to be applicable to the low-cost production of the next-generation large-area electronics devices.4 Although the leading material for 2D sheets is graphene, the only drawback is the gapless property of graphene, which impedes its application in electronics and photonics. Considering the applications to electronic devices and reviewing the previously reported 2D materials, layered germanane (GeH) is a strong candidate. GeH is a hydrogenated single layer of elemental germanium (Figure 1b). It is a 2D graphene analogue material with a direct band gap (1.53 eV). Although GeH does not possess a Dirac cone, it can be calculated to have a very high mobility (18 200 cm2/V·s); however, the only available experimental data for GeH indicate a much lower mobility value (70 cm2/V·s).5 The ideal mobility of GeH is 5 times higher than that of bulk germanium (3900 cm2/V·s) owing to the anisotropic 2D structure of GeH.6 The search for very high mobilities has strongly motivated studies in the field of 2D nanomaterials. © 2018 American Chemical Society
Figure 1. Schematic illustration of (a) CaGe2, (b) layered germanane (GeH), and (c) germanane dispersion. Orange, Ge; green, Ca; white, H.
To date, GeH has been successfully exfoliated into individual sheets by mechanical means (utilizing Scotch tape), but this procedure would require transferring the sample to specific substrates.7 Using another approach, CaGe2 has directly been converted into methyl-terminated germanane (GeCH3).8 In this work, appropriate solvents for the GeH exfoliation were identified, simultaneously satisfying both the requirements, i.e., good GeH dispersibility and low solvent boiling point. However, the selection method of the solvents for liquid exfoliation has been based on trial and error; therefore, there are few rigorous reports that address the concrete solvent characteristics. The proposed study employs the Hansen solubility parameters (HSP)9−12 for evaluating the dispersibility of GeH. Received: May 21, 2018 Revised: July 15, 2018 Published: July 16, 2018 5333
DOI: 10.1021/acs.chemmater.8b02153 Chem. Mater. 2018, 30, 5333−5338
Article
Chemistry of Materials
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When Ra is small, the compatibility of the two different materials is better. As the HSPs of common organic solvents are already known, the HSP values (as well as molar volume, surface tension, and boiling point) of the probe liquids employed in this investigation were obtained from the official HSP database (listed in Table S1).10 The SPHERE method9 was applied to determine the HSP of GeH. In the SPHERE method, experimental compatibility data (such as dispersibility, solubility, and swelling) of the HSP-unknown material, with respect to the probe liquids, were acquired. From the compatibility data, the probe liquids were divided into good and poor solvents by the appropriate threshold (the dispersibility scores in this case). The HSPs of the good and poor solvents were plotted in the HSP space (Cartesian coordinates with axes of δD, δP, and δH). From the HSP plots, the HSP sphere was drawn by fitting the good solvents within the sphere and poor solvents outside the sphere. The center of the HSP sphere (as well as its radius) was determined through the minimization of the errors [good solvents outside the sphere (wrong-out) and poor solvents inside the sphere (wrong-in)]. The minimization process was performed using the commercially available HSPiP v4.1.3 program. The center position of the obtained HSP sphere was the HSP value of the objective material. In addition, the radius of the HSP sphere (R0) was referred to as the interaction radius, which represents the tolerance for the objective material to interact with other materials. Thus, the HSP sphere obtained from the GeH dispersibility data represents the HSPs of materials compatible with GeH. In other words, solvents with HSPs inside the HSP sphere are potentially favorable for GeH sheet dispersion and the exfoliation (permeability) of layered GeH to GeH sheets.
EXPERIMENTAL METHODS
Synthesis. In a typical GeH synthetic procedure, CaGe2 crystals are used as precursors (Figure 1a). The precursors are prepared by RF heating of Ca and Ge pieces; thereafter, the CaGe2 is reacted with concentrated HCl cooled to −30 °C.7 Figure S1a presents the X-ray diffraction (XRD) pattern of the obtained sample. The broad reflection peaks can be indexed to a hexagonal unit cell with a = 3.8 and c = 11.0 Å, consistent with the previously reported peaks for GeH. GeH has been reported to be a 2H (bilayered hexagonal) unit cell; therefore, the crystal thickness of the GeH unit cell was estimated to be 5.5 Å.13 The in-plane hexagonal lattice constant coincides with that of CaGe2 (a = 3.987 Å), indicating that the reaction is topotactic and the 2D germanium network of CaGe2 is preserved, as shown in Figures 1a and b. Fourier-transform infrared spectroscopy (FT-IR) in the attenuated total reflection mode (Figure S1b) indicates Ge−H stretching and multiple wagging modes at 2000 and 570 cm−1, respectively. Additionally, weak vibrational modes (Ge−H2) at 770 and 825 cm−1 are observed. Considering the XRD and FT-IR data together, the host CaGe2 matrix undergoes a structural change to a crystalline covalently bonded GeH solid: a layered hydrogenated germanium hydride (Figure 1b). Liquid Exfoliation. In the present work, we used an ultrasonic cleaning bath for the agitation of GeH powder (5 mg each) in combination with 35 probe liquids (2 mL). The probe liquids were rationally chosen to cover a wide variety of molecular interactions spanning a large HSP range (Table S1). The GeH and probe liquidfilled vials were sonicated for 10 min in an ultrasonic bath to prepare the exfoliated GeH-nanosheet dispersions (liquid exfoliation, Figure 1c). The dispersions were imaged after 1 h and 1 day of standing (sedimentation). Characterization of the GeH Dispersions. The intensity of the transmitted light was divided into the three primary colors [red (R), green (G), and blue (B)]. Light through the dispersions was labeled IRD, IGD, and IBD, respectively. The background light intensity was labeled IRB, IGB, and IBB, respectively. The light transmittance (T) of the dispersions was calculated according to the following equation: T=
IRD + IGD + IBD IRB + IGB + IBB
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RESULTS AND DISCUSSION Figures 2 and S2 show the images of GeH dispersions in the probe liquids after sonication for 10 min and the subsequent
(1)
T can be transformed to absorbance per unit length (A/l) and directly correlates to the concentration of GeH exfoliated and dispersed in the probe liquids using the following equation:
A /l = − log T
(2)
In this investigation, the light path length (l) corresponds to the inner diameter of the vials (identical for all samples); therefore, T is inversely correlated to the concentration of the exfoliated GeH in the dispersions. Thus, we categorized the dispersibility of the probe liquids for the exfoliation and dispersion of GeH into four-grade scores using the T value: Scores 1, 2, 3, and 4 indicate very good (T < 0.4), good (0.4 ≤ T < 0.6), poor (0.6 ≤ T < 0.8), and very poor (T ≥ 0.8) solvents, respectively. The solvents with Scores 1 and 2 were defined as good solvents and those with Scores 3 and 4 as poor solvents. Calculation of HSP. HSPs are indicators for molecular interactions and are effective for predicting and/or examining the compatibility (dispersibility, solubility, wettability, etc.) of two different materials.9 HSPs consist of three terms that originate from the corresponding molecular interactions: δD (London dispersion term), δP (polar term), and δH (hydrogen bonding term). The compatibility of two different materials (with the respective HSPs of [δD1, δP1, δH1] and [δD2, δP2, δH2]) can be estimated by the HSP distance (Ra) according to eq 3:9
Ra =
Figure 2. Photographs of GeH dispersions in probe liquids #1−35.
sedimentation procedures (for 1 h and 1 day), respectively. The light intensities and background intensities from the raw spectroscopy obtained data are summarized in Tables S2 and S3, respectively. The transmittance (T), calculated in eq 1, and the resultant dispersibility score are also listed in Tables S2 and S3. Probe liquid #24 solidified during the sedimentation
4 × (δD1 − δD2)2 + (δP1 − δP2)2 + (δH1 − δH2)2 (3) 5334
DOI: 10.1021/acs.chemmater.8b02153 Chem. Mater. 2018, 30, 5333−5338
Article
Chemistry of Materials procedure and was excluded from the results. The original color of liquid #35 was yellowish (i.e., absorption of blue light), as shown in Figure 2; therefore, the transmittance (T) was calculated by precluding the effect of blue light absorption (IBD = 0, IBB = 0 for eq 1). By applying the criterion described above to divide the probe liquids into good/poor solvents, the HSPs of the good (blue circle) and poor (red square) solvents were plotted, as shown in Figure 3. The good/poor solvents outside/inside the HSP sphere were deemed as anomalies, probably due to uncertainty of the official HSPs, limitations to HSP theory (e.g., insufficient treatment on Lewis acid−base interactions), and/or unexpected experimental errors. Although there are a considerable number of anomalies (7 and 8 for 1 h and 1 day sedimentation, respectively) in this investigation, the obtained
HSP spheres are still considered to be useful, as will be discussed later. Table 1 summarizes the HSP values obtained by the SPHERE method; the HSP values for GeH #1 and GeH #2 were calculated from the data sets of 1 h- and 1 daysedimentation results, respectively. Although the HSP of GeH #2 was slightly shifted from that of GeH #1, they were still quite similar. Because both the FIT values9 were ∼0.5 (FIT = 1 represents a perfect fit without any anomalies), due to the considerable number of anomalies, the accuracy of the HSP values should not be considered very high. However, the trends for all the HSP terms were quite high (δD term was extremely high); therefore, GeH must have high cohesive energy (strong molecular interactions). It is noteworthy that the HSP values of GeH were quite consistent with that of the ionic liquid 1,3-butyl methyl imidazolium tetrafluoroborate ([δD, δP, δH] = [23, 19, 10] [J/cm3]1/2),10 which can effectively diffuse into CaSi2 and CaGe2 source materials to form bilayer silicene14 and bilayer germanene,15 respectively. Comparing the three HSP terms of GeH with tentatively calculated those of graphene,10,16 the δD and δH values of GeH were much larger tha those of graphene. This was especially true for the δD values of GeH (∼24 [J/cm3]1/2), which were extraordinarily large in comparison with those of the common chemical compounds (15−20 [J/cm3]1/2) that are primarily composed of hydrocarbons, oxygen, and nitrogen. It is known that the δD value is dependent on the size of the constituent atoms (i.e., larger atoms than carbon, such as chlorine, sulfur, and bromine, lead to a larger δD value)9 as well as the molecular size and atomic bonding state. In a more precise expression, the δD correlates with the polarizability of the constituent functional groups.17 Although the polarizability values of graphene and GeH are unknown, the polarizability values for the ground state atoms of carbon and germanium are calculated to be 1.67 × 10−24 and 5.84 × 10−24 cm3,18 respectively; thus, the extraordinarily large δD value of GeH might be attributed to the large polarizability of the germanium atom. On the other hand, the δH value of GeH (13−18 [J/ cm3]1/2) was larger than that of graphene (∼7 [J/cm3]1/2); however, the δH value was not extraordinarily large and comparable to the moderately large δH values of hydroxyl compounds, such as alcohols and polyols. In this study, the δH value of GeH #2 (after 1 day sedimentation) was larger than that of GeH #1 (after 1 h sedimentation). Therefore, this moderately large δH value of GeH might imply that the Ge− Ge bonds are partially oxidized or a portion of the Ge−H bonds gets substituted with Ge−OH. Atmospheric moisture and water impurities in organic solvents work as an efficient GeH oxidizer. In fact, the presence of Ge−O bond was confirmed by XPS analysis for GeH samples exposed to ambient atmospheres over 1 month.7 Figure 4a shows the dependences of T through GeH dispersions on the relative energy difference [RED = Ra (HSP distance between GeH and solvent calculated by eq 3)/R0 (interaction radius of GeH)]. RED = 1 indicates the threshold for differentiating between good and poor solvents. In both the 1 h and 1 day sedimentation results, the RED threshold did not perfectly divide the good/poor solvents; however, a correlation between dispersibility and RED seems apparent [i.e., smaller RED (better compatibility in terms of HSP) resulted in lower T (better dispersibility of GeH in the solvent)]. Therefore, we believe that the HSP framework still works well in this system.
Figure 3. Plots of probe-liquid HSPs in Cartesian coordinates of δD, δP, and δH (HSP space) at the sedimentation time of (a) 1 h and (b) 1 day. Probe liquids judged as good and poor solvents were drawn by blue (●: inside a GeH HSP sphere, ○: outside the sphere) and red (■: outside the sphere, □: inside the sphere) plots, respectively. The HSP center value and HSP sphere of GeH were calculated using the SPHERE method and drawn as a green plot (●) and green wireframe, respectively. 5335
DOI: 10.1021/acs.chemmater.8b02153 Chem. Mater. 2018, 30, 5333−5338
Article
Chemistry of Materials Table 1. HSPs Obtained for GeH with Different Sedimentation Times and That for Graphene for Comparison HSP [(J/cm3)1/2] GeH #1 (10 min exfoliation +1 h sedimentation) GeH #2 (10 min exfoliation +1 day sedimentation) graphenea (literature)
δD
δP
δH
R0
FIT
remarks
23.24 24.69 20
14.61 11.21 11.2
13.89 17.73 7.3
17.9 17.9 6.7
0.526 0.510 1.000
G/Tb = 19/34 G/T = 10/34 G/T = 5/12
a
HSP of graphene was tentatively calculated by the SPHERE method with a small set of dispersibility data from ref 13. bG/T = numbers of good solvents/number of test solvents.
Figure 5 depicts the room-temperature absorption spectra for the GeH dispersions. The data can be characterized by
Figure 4. (a) Dependence of T through GeH dispersions on the relative energy difference to HSP center values after 1 h and 1 day of sedimentation. (b) Dependences of relative energy difference and T through GeH dispersions on probe-liquid boiling point.
The large data dispersion might be attributed to the experimental errors, i.e., the degradation via oxidation and/or hydrolysis from the presence of water impurities in the probe liquids (residual concentrations of water in probe liquids was not accounted for in this study). Figure 4b shows the relation of RED and T with the probeliquid boiling points. The probe liquids located in the lower left region were preferable for GeH exfoliation, both in terms of the GeH dispersibility and procedural easiness to remove the solvents. From Figure 4b, 1,3-dioxolane and N,Ndimethylformamide (DMF) were deemed as the preferable solvents from the RED values (
