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Liquid-phase exfoliation of germanane (GeH) based on Hansen solubility parameters Daisuke Nakamura, and Hideyuki Nakano Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02153 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 22, 2018
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Chemistry of Materials
Liquid-phase exfoliation of germanane (GeH) based on Hansen solubility parameters Daisuke Nakamura* and Hideyuki Nakano* Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192 (Japan) ABSTRACT: Two-dimensional (2D) materials combine the collective advantages of individual building blocks and synergistic properties, and they 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 (HSP). 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 it 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.
INTRODUCTION After the mechanical exfoliation of graphite was reported, many studies based on the exfoliation of other twodimensional (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 non-trivial 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 analog 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 (18200 cm2/Vs); however, the only available experimental data for GeH indicate a much lower mobility value (70 cm2/Vs).5 The ideal mobility of GeH is five times higher than that of bulk germanium (3900 cm2/Vs) 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. Till date, GeH has been successfully exfoliated into individual sheets by mechanical means (utilizing ScotchTM tape), but this procedure would require transferring the
sample to specific substrates.7 Using another approach, CaGe2 has directly been converted into methylterminated 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.
Figure 1. Schematic illustration of (a) CaGe2, (b) layered germanane (GeH), and (c) germanane dispersion. Orange; Ge, green: Ca, white; H.
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 Å 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 thick-
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ness 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 1b. 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 liquid-filled 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:
I RD + I GD + I BD I RB + I GB + I BB . (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) T=
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
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[δD1, δP1, δH1] and [δD2, δP2, δH2]) can be estimated by the HSP distance (Ra) according to Eq. (3):9 2
2
2
Ra = 4 × (δD1 − δD2 ) + (δP1 − δP2 ) + (δH1 − δH 2 )
.
(3)
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.
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 sedimentation procedures (for 1 hour 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 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)).
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Chemistry of Materials
Figure 2. Photographs of GeH dispersions in probe liquids #1–#35.
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 hour 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 datasets of 1 hour- and 1 daysedimentation results, respectively. Although the HSP of GeH #2 was slightly shifted from that of GeH #1, they were still quite similar. Since 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 1,3butyl 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 germanene15, respectively. Comparing the three HSP terms of GeH with tentatively-calculated those of graphene,10, 16 the δD and δH values was especially true for the δD values of GeH (~24
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.
[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
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Table 1. HSPs obtained for GeH with the different sedimentation times, and that for graphene for comparison. HSP [(J/cm3)1/2]
FIT
δP
δH
R0
23.24
14.61
13.89
17.9
0.526
G/T[b] = 19/34
24.69
11.21
17.73
17.9
0.510
G/T = 10/34
1.000
G/T 5/12
GeH #1 (10 min exfoliation
Remarks
δD
+ 1 h sedimentation)
GeH #2 (10 min exfoliation + 1 day sedimentation)
Graphene[a] (literature)
20
11.2
7.3
6.7
=
[a]
HSP of graphene was tentatively calculated by the SPHERE method with a small set of dispersibility data from [b] Ref. 13. G/T = numbers of good solvents/ number of test solvents.
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 1h 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
deemed as the preferable solvents from the RED values (< 1) and boiling points (< 160 °C) while 1,3-dioxolane, 2butanol, cyclohexylchloride, and DMF were deemed as preferable liquids from T values (< 60%) and boiling points (< 160 °C). Among these preferable solvents, the boiling point of 1,3-dioxolane was the lowest, 78 °C, and 1,3-dioxolane can be removed quickly, even at room temperature, to avoid the thermal degradation of GeH (amorphization begins to occur over 75 °C)7. Thus, 1,3dioxolane is considered as the best solvent for the liquid exfoliation of GeH. Figure 5 depicts the room-temperature absorption spectra for the GeH dispersions. The data can be characterized by Beer–Lambert behavior (as shown in Figure 5b, the peak absorbance at 550 nm exhibits a linear relationship to GeH concentration), indicating that the GeH sheets are almost monodisperse (negligible intermolecular interactions). The particle size distribution of the exfoliated GeH measured by laser diffractometry (in volumetric basis, Figure S3) ranged from 10 to 100 µm, which well corresponds to the lateral sizes of coarse GeH sheets observed in optical micrographs (see inset image in Figure S4). The particle diameter values are mainly influenced by the lateral sizes of GeH sheets (not sheet thickness) due to its measurement principle. The bandgap energy for the GeH dispersion is 1.37 eV, as shown in the inset of Figure S5, smaller than that of bulk GeH (1.59 eV). This approximately 0.2-eV energy difference may be associated with the nanometer thickness of the GeH sheets, which is about one order of magnitude smaller than the thickness of bulk GeH.
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 hour 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. 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 probe-liquid 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, 1,3dioxolane and N,N-dimethylformamide (DMF) were
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 probeliquid boiling point.
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Chemistry of Materials O (see also Figure S4 for the Raman spectrum of almost non-oxidized GeH sheet). The Ge:Cl:Ca atomic ratio was estimated to be 83:15:2 (Figure 6b). During the synthesis of GeH in HCl solution, HCl becomes intercalated into the GeH sheets and only small amounts of CaCl2 remain in the GeH layers. These findings are in line with the FTIR results of the layered GeH solid (strong Ge–H stretching mode (2000 cm-1) and CaCl2 (1600 cm-1)). Figures 6c and 6d show the atomic force microscopy (AFM) image of the exfoliated GeH sheets using 1,3dioxolane coated onto a SiO2 (300 nm)/Si substrate. The thickness is about 3.2 nm with flat surfaces and sharp edges. The GeH sheet thickness was nearly uniform, distributed between 3 and 4 nm. From the sheet thickness, the exfoliated GeH sheets were composed of 4–5 layers. Although X-ray photoelectron spectroscopy (XPS) analysis of the deposited GeH sheets was also carried out to
Figure 5. Room-temperature optical properties and oxidation state of the GeH dispersions in 1,3-dioxolane as the dispersion medium. (a) UV/Vis spectra of the dispersions at various concentrations (charged concentrations). (b) Absorbance at 550 nm plotted against the GeH sheet concentration (c) Ge K-edge XANES spectra of the GeH dispersions in 1,3-dioxolane [5 h (blue line) and 15 month (dashed brown line) after the exfoliation] as well as those of GeH source powder (black line) and GeO2 powder (dashed black line) for reference. The XANES data were collected at the BL5S1 of Aichi Synchrotron Radiation Center.
To assess the stability (oxidation) of the exfoliated GeH dispersion, X-ray absorption near edge structure (XANES) spectra of GeH dispersions were measured 5 h and 15 month after the exfoliation (Figure 5c). The XANES spectrum obtained from the fresh GeH dispersion (5h after the exfoliation) is perfectly identical to that from the GeH source powder before the exfoliation; however, that from the aged GeH dispersion (15 month after the exfoliation) exhibits the slight shift in peak energy toward the peak of GeO2. This fact indicates the GeH sheets dispersed even in organic solvents gradually oxidized during long-time storage as is the case with the long-time exposure to ambient atmospheres.7 From the transmission electron microscopy (TEM) results, the exfoliated GeH can be seen to exhibit a transparent layered morphology (Figure 6a). The energydispersive X-ray (EDX) spectrum has a strong Ge signal, the presence of trace amounts of Cl and Ca, and very little
Figure 6. (a) Low-magnification TEM micrograph of germanane, (b) EDX spectrum of germanane, (c) non-contact mode AFM image, and (d) line profile taken along the green line in panel (c).
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verify the bonding state of the as-deposited GeH sheets, not enough Ge peak intensity (
