Article pubs.acs.org/cm
Surface Modification of Layered Polysilane with n‑Alkylamines, α,ωDiaminoalkanes, and ω‑Aminocarboxylic Acids Hirotaka Okamoto,*,† Yusuke Sugiyama,†,‡ Koji Nakanishi,§,∥ Toshiaki Ohta,§ Takuya Mitsuoka,† and Hideyuki Nakano†,⊥ †
Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan The SR Center, Ritsumeikan University, 1-21-1, Noji-Higashi, Kusatsu, Shiga 525-8577, Japan ⊥ Japan Science and Technology Agency, PRESTO, Kawaguchi, Saitama 332-0012, Japan §
S Supporting Information *
ABSTRACT: Surface modification of a layered polysilane consisting of two-dimensional crystalline silicon layers was investigated using n-alkylamines, α,ω-diaminoalkanes, and ωaminocarboxylic acids. All products of the reactions with nbutyl- to n-hexadecylamines were dispersed as nanosheets in chloroform and exhibited the same blue light emission and photoluminescence spectra when exposed to UV light. After removal of the solvent, these products formed regularly stacked structures with an interlayer distance that increased in proportion to the alkyl length. The structure consists of bilayered alkyl chains with a tilt angle of ca. 47°. When the layered polysilane was reacted with α,ω-diaminoalkanes, the products were not dispersed in chloroform, because the silicon layers were covalently bonded by the diamines. The interlayer distances for these products also increased in proportion to the alkyl length. The layered polysilane was also successfully reacted with 12-aminododecanoic acid in pyridine or other polar solvents, where the formation of SiN bonds was confirmed by Si Kedge X-ray absorption near-edge structure analysis. This result indicates that a highly reactive carboxyl group can be introduced onto the silicon layer surface. A chloroform solution of the 12-aminododecanoic acid-modified polysilane exhibited the same blue light emission as the polysilane modified with n-alkylamines.
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partial substitution of Ca2+ with Mg2+ or K+.8,9 The stacked structure of Si6H6 can also be destroyed by mechanochemical lithiation.10 However, the exfoliation and expansion of Si6H6 with organic materials could be achieved using a wide variety of mild synthetic processes to obtain products that are expected to exhibit characteristic properties due to the formation of inorganic−organic hybrids. Organic modification of inorganic layered material surfaces is a powerful technique, not only for changing the interlayer structure but also for expanding the range of applications to those such as polymer composites 11−13 and colloidal suspensions that could be used as building blocks for constructing self-assembly multilayers.14−16 Many organically modified layered materials have been prepared by soft chemical procedures or intercalation reactions, most of which are conducted using ion-exchange reactions.17−20 In contrast, the immobilization of organic units through covalent bonds has an advantage in practical applications, because of the thermal and chemical stabilities of the resultant inorganic−organic
INTRODUCTION Layered polysilane, Si6H6, consists of stacked crystalline silicon layers without oxygen, in contrast to silicate materials. The hexagonal silicon layers, which correspond to two-dimensional corrugated Si(111) planes, are terminated above and below by hydrogen atoms and are stacked with a layer spacing of 0.54 nm.1,2 Recently, germanane, GeH, which has the same structure as Si6H6, was synthesized in a similar manner.3 Si6H6 emits strong visible photoluminescence (PL)4 and is also expected to exhibit characteristic electronic and optical properties, due to the crystalline silicon structure and nonoxide composition. The layered structure is suitable for use as an electrode material in rechargeable batteries. It was shown that Si6H6 exhibits a higher capacity and lower volume change than bulk Si in charge− discharge cycles.5 The results of first-principles calculations indicated that the insertion and diffusion of both Na and Li occur more favorably in Si6H6 than in bulk Si.6 Moreover, we reported a unique application of Si6H6 as an anode material in a new type of anion secondary battery.7 The electronic and optical properties of Si6H6 are expected to vary upon exfoliation of the layers or expansion of the interlayer spacing of the stacking structure. A few exfoliated silicon layers have been synthesized from CaSi2, which is a precursor of Si6H6, by the © XXXX American Chemical Society
Received: November 21, 2014 Revised: February 2, 2015
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DOI: 10.1021/cm5042869 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials hybrids.16 For example, for GeH, it was shown that covalent methyl termination improves the thermal stability and increases the band-gap energy. 21 Density functional calculations suggested that the methyl-terminated GeH monolayer exhibits a novel physical phenomenon called the quantum spin Hall effect, which is induced by strain.22 We have successfully synthesized covalently bonded organically modified layered polysilanes in three ways: reaction with phenyl groups through a Grignard reagent,23 addition of nhexene by hydrosilylation,24 and reaction with n-decylamine.25 Each product resulted in the exfoliation of layered polysilanes to provide silicon nanosheets with thicknesses of around 1 nm. In particular, n-decylamine-modified silicon nanosheets were obtained in relatively high yield and were easily formed into regularly stacked structures in a concentrated state. Here, we report the synthesis of several types of nalkylamine-modified layered polysilanes (Cm-Sin, where m indicates the number of carbon atoms included in the amines) and the effect of the alkyl chain length on the stacking structure of the Cm-Sin materials. In addition, the reaction of Si6H6 with α,ω-diaminoalkanes, which have amino groups at both ends of alkyl chains, was also examined. It was expected that the interlayer distances of the products would increase in proportion to the alkyl chain length and form large clusters without dispersion in the solvent because of the tight connection of each layer. Moreover, the modification of Si6H6 with ω-aminocarboxylic acids was also examined for the introduction of carboxyl groups, which is expected to provide versatile reactive sites on the layer surface.
Figure 1. FTIR spectra of Cm-Sin (m = 4, 6, 10, 12, and 16) and Si6H6. The inset shows enlarged spectra of C10- and C12-Sin in the range from 800 to 1200 cm−1.
lowest angles in each pattern correspond to the (001) planes, and the peaks at almost equal intervals are considered to be higher-order peaks, namely, (00l) peaks, where l is a positive integer, for the (001) planes in each sample. The interlayer distances for the m = 4, 6, 10, 12, and 16 samples were calculated from the (001) reflection as d = 1.30, 1.72, 2.48, 2.81, and 3.35 nm, respectively. The very sharp (001) peaks and the appearance of many (00l) peaks indicate a large number of silicon layers that are regularly stacked parallel to the glass plate surface by self-assembly. The thicknesses of the stacked structures were estimated to be around 100 nm based on the full widths at half-maximum of the (001) peaks and the Scherrer equation.27 Agglomerated structures of C10-Sin with thicknesses of a few hundred nanometers were directly observed by atomic force microscopy (AFM) for a sample prepared from a slightly concentrated solution (Figure 3a). Magnification of the image revealed that the agglomerates are constructed by the stacking of nanosheets (Figure 3b). These results indicate that tens of layers or more were included in the stacked structures. The number of stacked layers was much larger than for Si6H6, in which approximately 10 layers are included,2,19 The increment of layers suggests that the stacking structure of Cm-Sin is formed not by mere intercalation of nalkylamines into Si6H6 layers but by self-assembly of the exfoliated modified layers. Figure 2b shows the proportional relationship between the d spacing of the stacked Cm-Sin structures and m, where the slope and the intercept of the line are 0.172 and 0.674 nm, respectively. This relationship suggests that alkyl chains included in the stacked Cm-Sin materials are regularly arrayed and take the same conformation, probably an all-trans conformation. Although this is not directly confirmed, it has been widely observed or proposed in regularly stacked structures such as n-alkylamine-modified layered metal oxides and n-alkylamine intercalated organic polymer crystals.28−35 If this assumption is correct, then the slope of 0.172 nm indicates a bilayered alkyl-chain structure at a tilt angle of ca. 47° with respect to the stacking layers (Figure 2c). The intercept of
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RESULTS AND DISCUSSION Alkylamine-Modified Layered Polysilane (Cm-Sin). Organic modification of Si6H6 was conducted by stirring Si6H6 with primary n-alkylamines such as n-butyl- (C4), nhexyl- (C6), n-decyl- (C10), n-dodecyl- (C12), and nhexadecyl- (C16) amine in chloroform for 12 h at 60 °C in a nitrogen atmosphere. The yellow-colored Si6H6 precipitate powder rose to the solvent surface after the reaction and became reddish-colored (Figure S1, Supporting Information). Trace amounts of white and black precipitates were evident, which were silica and reduced crystalline silicon, respectively. It was concluded that Cm-Sin samples were successfully prepared by the reaction and that the Cm-Sin samples were included in the floating powders and in the solution. Fourier transfer infrared (FTIR) spectra of the refined CmSin samples showed weak peaks at 930 and 1150 cm−1 (Figure 1). These peaks are attributed to the stretching vibration of SiNSi bonds and the bending vibration of the NH bond contained in the SiNHC unit,26 that is, n-alkylamines reacted with SiH bonds and covalently attached to silicon layers in each sample. We previously identified the formation of SiN covalent bonds based on X-ray absorption near edge structure (XANES) analysis.25 The FTIR spectra also show characteristic peaks in the wavenumber range from 2800 to 3000 cm−1 that can be assigned to CH stretching vibrations and also indicate the presence of organic groups. Figure 2a presents X-ray diffraction (XRD) patterns for the various Cm-Sin samples, in which the intensity is shown on a logarithmic scale. Samples were prepared by dropping a chloroform solution of Cm-Sin onto a glass plate and then evaporating the solvent. Strong and narrow XRD peaks are evident in the lower-angle region, and many peaks are aligned at almost equal intervals in all of the patterns. The peaks at the B
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formation of C10-Sin. However, a small amount of chloroform remained and reacted with amine or amine moieties included in the C10-Sin product to form the salt during the reheating treatment. As a result, the XRD pattern of C10-Sin indicated the presence of the C10-HCl salt as an impurity. The Cm-HCl salts also exhibited periodic peaks (Figure S2, Supporting Information) that resemble, but are different from, those of the Cm-Sin samples. Small (001) peaks of the Cm-HCl salts were observed in the XRD patterns of C4-, C6-, and C12-Sin, as shown in Figure 2a. These results indicate that the Cm-HCl salts could be produced as byproducts during the reaction of Si6H6 and amines in chloroform. Therefore, to prevent salt formation, the reaction should be performed without a solvent or in another solvent. It should also be noted that the thickness of the C10-Sin nanosheet formed on a plate from a dilute solution was previously confirmed to be 7.5 nm by AFM.19 We have now confirmed that this value is consistent with a stack of three layers of C10-Sin. In addition, FTIR patterns of the amine hydrochloride salts were distinctly different from those of the amines or Cm-Sin materials, as shown in Figure S3 (Supporting Information). Blue light emission can be observed when a Cm-Sin chloroform solution is exposed to UV light with a wavelength of 365 nm, as reported previously.19 Figure 4a shows PL spectra
Figure 2. (a) XRD patterns of Cm-Sin (m = 4, 6, 10, 12, and 16). (b) Relationship between the d spacing and m for Cm-Sin stacked structures. Peaks marked with circles are derived from the (001) plane of Cm-HCl salts. (c) Structural model of regularly stacked C6-Sin.
Figure 4. PL spectra of Cm-Sin (m = 4, 6, 10, 12, and 16 from bottom to top) in chloroform. Excitation and fluorescence wavelengths are (a) 350 and (b) 450 nm. The intensity was normalized with respect to the maxima, and the curves are vertically shifted by 0.2. Figure 3. (a) AFM image of C10-Sin agglomerated structures. (b) Magnified image of an agglomerate surface.
of the Cm-Sin samples in chloroform that were obtained using an excitation wavelength of 350 nm. All samples displayed a broad main peak at 438 nm (2.83 eV) with two subpeaks at 414 nm (3.00 eV) and 466 nm (2.66 eV). These peak positions were found to be independent of m, but the relative intensities of the peaks at 414 and 466 nm tended to increase and decrease, respectively, with increasing m. Three peaks at 360, 378, and 398 nm were also observed in the PL excitation (PLE) spectra of Cm-Sin (Figure 4b). The peak positions remained unchanged and the relative intensities at 360 and 398 nm tended to increase and decrease, respectively, with increasing m, as in the case of the PL spectra. The presence of three peaks has been observed for other silicon nanosheets even at room temperature,23,36 and subpeaks should not be due to chemical
0.674 nm is slightly larger than the d spacing of Si6H6, which is 0.59 nm.25 This is plausible because Cm-Sin contains larger nitrogen atoms in place of smaller hydrogen atoms. We previously reported that the d spacing of stacked C10-Sin is 2.98 nm;25 however, in the present work, this value was determined to be not for C10-Sin but for a crystal of the dodecylamine hydrochloride salt (C10-HCl salt). This error was due to a mistaken identification of the XRD pattern for the salt as that for C10-Sin. In our previous study, after the reaction of Si6H6 with dodecylamine in chloroform, the evaporated sample was reheated at 120 °C for 24 h to complete the C
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SiH bonds were present in DiC12-Sin or that some unreacted Si6H6 remained in the product sample. The peak at around 1000−1100 cm−1 in the DiCm-Sin spectra suggests the presence of SiOSi bonds, which would indicate that a small amount of silica remained or that unreacted SiH bonds became Si OSi bonds after the washing treatment. The use of shorter diamines is likely to leave a larger amount of unreacted SiH bonds in DiCm-Sin, as suggested by the increase in the peak intensity with decreasing m. Moreover, the two peaks around 2850−2940 cm−1, which correspond to the stretching vibration of CH bonds, are indistinct for the DiC2-Sin and DiC3-Sin samples. Two reasons are offered to explain this: a low CH density due to the short alkyl chain length and a low reactivity of Si6H6 with DiC2 or DiC3. Incidentally, the CH stretching signals in Figure 5 are relatively weaker than those in Figure 1, because the C/Si molar ratio of DiCm-Sin must be about half that of Cm-Sin for the same m value. As discussed later, the dialkylamine moieties form unimolecular layers between silicon layers, whereas n-alkylamine moieties form bimolecular layers. The FTIR results indicate that DiC12-Sin and DiC6-Sin can be obtained by the reaction of alkyldiamines and Si6H6, whereas the successful production of DiC2-Sin and DiC3-Sin remains unconfirmed. The (001) reflection peak appeared at 2θ = 6.49° in the XRD pattern for DiC12-Sin (Figure 6a). Higher-order (00l) peaks
impurities. The main peak in the PL spectra at 438 nm is almost the same as that for a Mg-doped silicon nanosheet8 or a silicon network polymer with hexyl side chains.37 The consistency of the PL peak with these reports supports the formation of a two-dimensional silicon backbone, because the band-gap energy of a two-dimensional silicon material was calculated as 2.5 eV and the dimensionality is considered to play an important role in the band-gap energy.37 The band-gap energy of Cm-Sin was found to shift to a slightly higher energy than 2.3 eV for Si6H6.4 However, the increase in the interlayer distance, which expands by at least 1.49 nm for C4-Sin, is not likely to lead to a further shift in the band-gap energy. α,ω-Diaminoalkane-Modified Layered Polysilane (DiCm-Sin). Products of the reaction of Si6H6 with the α,ωdiaminoalkanes 1,2-diaminoethane (DiC2), 1,3-diaminopropane (DiC3), 1,6-diaminohexane (DiC6), and 1,12-diaminododecane (DiC12) kept their yellow color and precipitated in chloroform after heat treatment. However, the precipitates after reaction with DiC6 and DiC12 appeared swollen, which implies that the precipitates were mainly composed of alkyldiaminemodified layered polysilane (DiCm-Sin). It is reasonable that the DiCm-Sin products did not dissolve in chloroform, because the silicon layers should be covalently bonded through the alkyldiamines. As was the case for Cm-Sin, the FTIR spectrum of DiC12-Sin had weak peaks at 930 and 1150 cm−1 (Figure 5), which
Figure 5. FTIR spectra of precipitates obtained by heat treatment of Si6H6 with DiCm (m = 2, 3, 6, and 12). Figure 6. (a) XRD patterns of DiCm-Sin (m = 2, 3, 6, and 12). Peaks marked with triangles are coincident with those for the DiCm-HCl salts. (b) Relationship between the d spacing and m for DiCm-Sin. (c) Structural model of DiC12-Sin.
indicates that a SiN linkage was formed in DiC12-Sin. The bond formation was also confirmed by Si K-edge XANES analysis (Figure S4, Supporting Information). The two FTIR peaks observed at 3265 and 3320 cm−1 are related to the stretching vibration of the NH bond. The two NH peaks indicate the existence of primary amines. This fact suggests that all diamine ends did not always react with Si6H6. Similar peaks were observed in the spectra of DiC12 and the DiC12-HCl salt; however, the peak positions and intensity ratios were different (Figure S5, Supporting Information). Thus, it is considered that both SiNHC bonds and SiNCSi bonds as well as free NH2 ends were included in DiC12-Sin. Another peak was observed at 2150 cm−1 that corresponds to the stretching vibration of the SiH bond. This suggests that some unreacted
were also observed, which suggests that DiC12-Sin consists of a regularly stacked layer structure. However, there were many other peaks that were unrelated to the stacked structure, in contrast to the XRD patterns for Cm-Sin, which is considered to be due to the different sample preparation method for XRD measurements. Specifically, the DiC12-Sin powders were randomly filled into a sample holder. It should be noted that the XRD pattern of DiC12-Sin is distinctly different from those of DiC12 and the DiC12-HCl salt (Figure S6, Supporting D
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by the reaction in polar solvents was confirmed based on the characteristic similarity with Cm-Sin. Figure 7a also suggests
Information), as the (001) peaks for both DiC12 and the SiC12-HCl salt appeared at 2θ = 4.55°. The (001) peaks for DiC6-Sin and DiC2-Sin appeared at 2θ = 8.95° and 10.45°, respectively, whereas no corresponding peak was observed for DiC3-Sin (Figure 6a). The preparation of DiC3-Sin is thus also considered to be suspect from the results of the XRD measurements. Note that several peaks are coincident with those for DiCm-HCl salts in Figure 6a. However, they should not be attributed to the salts concerning at least DiC12-, DiC6-, and DiC2-Sin, because stronger (00l) peaks of the DiCm-HCl salts cannot be observed in the same figure. The relationship between the d spacing for DiCm-Sin and m are shown in Figure 6b. Although there are only three data points, they are on a straight line, and the slope and intercept of the line are 0.052 and 0.717 nm, respectively. This proportional relationship suggests that the alkyl chains included in the DiCm-Sin materials take the same conformation, probably an all-trans conformation, as in the case of Cm-Sin, although the alkyl chains form a single layer between the silicon layers. The slope of 0.052 nm indicates a highly tilted alkyl-chain structure at an angle of ca. 66° with respect to the silicon layers (Figure 6c). The intercept of 0.717 nm is slightly larger than that for Cm-Sin. The smaller intercept of Cm-Sin might therefore be due to the dense packing at the interface of the two organic layers between the silicon layers. It is considered that, in many cases, both ends of the diamines bind to the silicon layers in DiCmSin. In the case that the alkyl chains take an all-trans conformation, the CN bonds at the two ends of DiCm are in the same direction when m is even, whereas they are not when m is odd. This difference is considered to result in the low reactivity of DiC3 with Si6H6 because the SiNC bonds that can be formed at the two ends of a DiC3 moiety cannot take the same bond angle in DiC3-Sin. ω-Aminocarboxylic Acid-Modified Layered Polysilane (CmCOOH-Sin). The reaction solvent used to prepare Cm-Sin and DiCm-Sin was chloroform. However, chloroform cannot be used as a solvent for the reaction of Si6H6 with ωaminocarboxylic acids, because it does not dissolve these acids. There are no good solvents for ω-aminocarboxylic acids except for water and alcohols; however, polar solvents such as pyridine, dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP) do partially dissolve these acids. Therefore, the reaction of Si6H6 with the acids glycine (C2COOH), β-alanine (C3COOH), 6-aminocaploic acid (C6COOH), and 12-aminolauric acid (C12COOH) were examined in these various solvents. After the heat treatment, it was difficult to distinguish by appearance whether the reaction had successfully progressed or not. When the refined samples were soaked in chloroform, a large part of the sample precipitated, and the solution was colorless and transparent. Samples for XRD measurements were prepared by dropping a solution of the sample onto a glass plate and then evaporating the solvent. Peak patterns different from those for C12COOH or C12COOH-HCl salt were observed for this sample. A strong peak appeared at 2θ = 5.66°, and higher-order peaks were aligned at almost equal intervals, as observed for Cm-Sin, which suggests that a regularly stacked structure formed on the glass plate. The interlayer distance was found to be 1.56 nm, which must be due to the intercalation of C12COOH moieties between the silicon layers. It is considered that the silicon layers were dispersed as a single layer in the chloroform solution and became stacked during solvent evaporation. The successful preparation of C12COOH-modified Si layers (C12COOH-Sin)
Figure 7. (a) XRD patterns of C12COOH-Sin synthesized in pyridine, NMP, and DMSO; the C12COOH HCl salt; and C12COOH. The XRD samples were prepared from chloroform solutions. The two lower patterns were obtained from powder samples. (b) Approximate structural model of C12COOH-Sin.
that C12COOH and the C12COOH-HCl salt were successfully removed by washing with pyridine and acetone, although a certain amount of C12COOH-Sin eluted at the same time. In contrast to C12COOH-Sin, no periodic patterns were observed for the samples that were expected to be CmCOOHSin (m = 2, 3, and 6) if the reactions of lower carboxylic acids with Si6H6 had proceeded successfully. XRD measurements were also performed for these samples by directly filling the precipitates into a glass plate holder. Among these samples, only that obtained from the reaction with C6COOH showed a peak at 2θ = 9.92° that could possibly correspond to the interlayer distance of the expanded silicon layers (Figure S7, Supporting Information). If this peak corresponds to the (001) plane of the silicon layers, then the calculated d spacing would be 0.89 nm, which is considerably small if C6COOH moieties are intercalated between the silicon layers. Therefore, the XRD results suggest that only C12COOH successfully reacts with Si6H6 to form C12COOH-Sin. It is noteworthy that the d spacing of C12COOH-Sin is approximately half that of C12-Sin, which implies that C12COOH moieties attached to adjacent silicon layers mutually overlap each other, as shown in Figure 7b. The refined C12COOH-Sin sample showed an FTIR spectrum different from those of C12COOH and the C12COOH-HCl salt (Figure 8). In comparison with the C12COOH-HCl salt, the peaks at around 1640−1730, 3200− 3300, and 1500−1620 cm−1, which correspond to CO stretching, NH stretching, and NH bending, respectively, E
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Figure 10. PL (solid lines) and PLE (dashed lines) spectra of C12COOH-Sin (thick lines) and C12-Sin (thin lines) in chloroform. Excitation and fluorescence wavelengths were 350 and 450 nm, respectively.
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CONCLUSIONS In this work, we have shown that Si6H6 can be reacted with various types of primary amines such as n-alkylamine, α,ωdiaminoalkanes, and ω-aminocarboxylic acids to form organically modified silicon-layered materials. Modification with all five of the n-alkylamines examined in this work was successfully achieved. The Cm-Sin samples form regular stacking structures by self-assembly when dried from a chloroform solution. The structure is composed of a bilayered alkyl chain with a tilt angle of ca. 47° between the silicon layers. Solutions of Cm-Sin emit strong visible PL that is not affected by the length of the alkyl chains. The reaction of Si6H6 with α,ω-diaminoalkanes, except for diaminopropane, progressed well to produce an expanded silicon layered structure. The differences in reactivity can be attributed to an even−odd effect because both ends of the alkyl chains react with the silicon layers. The formation of DiCm-Sin structures has potential for the construction of large organic silicon clusters. In the case of the ω-aminocarboxylic acids, only 12-aminododecanoic acid was confirmed to form a linkage to the silicon layers. Although the reaction and refinement procedures should be improved to increase the yield and reduce product loss, the introduction of carboxylic groups onto silicon nanosheets creates new possibilities for the development of applications such as combinations with other types of layered materials.
Figure 8. FTIR spectra of C12COOH-Sin, C12COOH-HCl salt, and C12COOH.
appeared at different positions. The latter two peaks were also different from those observed for C12COOH. If this spectrum is correct for C12COOH-Sin, then the C12COOH moieties and the Si layers should be bound in the form of SiNH CH2 because the peak at around 3300 cm−1 for NH stretching appeared as a single peak. The formation of SiN covalent bonds in C12COOH-Sin was indicated by the Si K-edge XANES spectra (Figure 9). The
Figure 9. Comparison of XANES spectra of C12COOH-Sin with those of quartz and β-Si3N4 obtained using TEY mode in a vacuum.
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peak at 1844 eV, detected using total-electron-yield (TEY) mode, suggests that a SiN linkage exists, as in the case of C10-Sin.25 A shoulder at 1846 eV indicates that a SiO linkage formed to a small extent in C12COOH-Sin. It could form not only by oxidation in storage and in the measurement but also by reaction with the carboxylic groups of the C12COOH moieties. As a result, C12COOH-Sin was confirmed as having been successfully obtained; however, it is more easily oxidized than Cm-Sin. The PL and PLE spectra of a chloroform solution of C12COOH-Sin were almost the same as those obtained for C12-Sin (Figure 10), which suggests that C12COOH-Sin was dispersed as nanosheets in the solution. These results also indicate that the PL and the PLE spectra are not affected by organic moieties or the presence of polar functional groups, but rather are dependent on the two-dimensional crystalline silicon structure. Choi and colleagues reported that the PL wavelength increases with the thickness of the silicon nanosheets synthesized by chemical vapor deposition.38 The same group also showed that similar PL and PLE spectra are obtained for ethanol solutions of two-dimensional silicon crystals with a thickness of 2 nm.36
ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures, photographs of Cm-Sin just after preparation; FTIR spectra and XRD patterns of Cm-HCl salts; XANES analysis of DiC2-Sin; FTIR spectra and XRD patterns of DiC12-Sin, DiC12-HCl, and DiC12; XRD patterns of the reactants of CmCOOH with Si6H6. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Addresses ‡
Y.S.: Toyota Industrial Corporation, 8, Chaya, Kyowa, Obu, Aichi 474-8601, Japan. ∥ K.N.: Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Weiss, A.; Beil, G.; Meyer, H. Z. Naturforsch. B 1979, 34, 25−30.
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DOI: 10.1021/cm5042869 Chem. Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/cm5042869 Chem. Mater. XXXX, XXX, XXX−XXX
