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Cite this: Analyst, 2014, 139, 3533
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Two-dimensional MoS2 nanosheets as a capillary GC stationary phase for highly effective molecular screening† Jia Jia,a Fujian Xu,b Shanling Wang,a Xue Jiang,b Zhou Long*a and Xiandeng Hou*ab
DOI: 10.1039/c4an00332b www.rsc.org/analyst
Stable layered MoS2 nanosheets were employed as a stationary phase in gas chromatography. A wide range of different analytes were screened with excellent separation efficiency.
Besides well-known graphene, other inorganic graphene analogues1 such as transition metal dichalcogenides2 with a common formula of MeX2 (Me ¼ Mo, W, Ti, etc., X ¼ S, Se, Te, etc.) have become more and more attractive in recent years. They play an important role in various applications including lubrication,3 catalysis,4 photovoltaics,5 supercapacitors6 and rechargeable battery systems7 due to their excellent properties such as semiconductivity, half-metallic magnetism,8 superconductivity,9 and charge density wave phenomena.10 Up to now, graphene oxide has been demonstrated to be a very good adsorbent11 and commonly used as a stationary phase in chromatography12 and electrophoresis.13 A two-dimensional MoS2 layer has a vertical structure similar to that of graphite14 (Fig. 1a), but unlike graphite or h-BN, the MoS2 layer is made up of hexagons, with Mo and S atoms located at alternating corners (Fig. 1b). Bulk MoS2 has attracted interest for use in photovoltaic and photocatalytic15 materials because of its strong absorption in the solar spectral region. The special electronic structure of monolayer and few-layer MoS2 shows a dramatic improvement in optical properties.16 However, applications of MoS2 as an adsorbent for chromatography has rarely been reported. Recently, the adsorption of exotic molecules onto MoS2 monolayers has been investigated by rst-principle calculations,17 and it has also been reported that a laminar MoS2 membrane could behave as a size-selective molecular sieve.18 Therefore, it could be expected that two-dimensional layered MoS2 nanosheets could be employed as a good platform a
Analytical & Testing Centre, Sichuan University, Chengdu, 610064, China. E-mail: [email protected]
b
College of Chemistry, Sichuan University Chengdu, 610064, China. E-mail: houxd@ scu.edu.cn † Electronic supplementary 10.1039/c4an00332b
information
(ESI)
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available:
See
DOI:
for molecular screening based on the selective adsorption of exotic molecules.19 In this context, we rst developed a stationary phase of MoS2 nanosheets for molecular screening using the gas chromatographic (GC) mode. MoS2 nanosheets were prepared by a mixed-solvent exfoliation strategy.20 In brief, 30 mg of MoS2 powder was dispersed in 10 mL of deionized water/ethanol (4.5/5.5, v/v) in a sealed ask, which was then subjected to ultrasound for 8 h. The obtained suspension was centrifuged at 4000 rpm (about 1484g) for 30 min, and the supernatant was collected (Fig. S1†). The thickness of the obtained MoS2 nanosheets was around 1.5–3.4 nm based on the atomic force microscopy (AFM) image (Fig. 2a & b). Considering that the thickness of monolayer MoS2 nanosheets is 0.9–1.2 nm,1 the obtained nanosheets should be composed of 1–3 monolayers. The morphology of the obtained MoS2 nanosheets was studied by transmission electron microscopy (TEM), which reveals that the majority of the nanosheets are present as two-dimensional thin sheets in the dispersion (Fig. 2c), and the electron diffraction pattern shows that the MoS2 nanosheets have a vertical structure similar to graphite21 (inset of Fig. 2c). By using GC with MoS2 nanosheets as the stationary phase, molecular screening was successfully accomplished (see more details in ESI†). Fig. S2† shows the van Deemter plots for
Fig. 1 Structural diagrams of MoS2 nanosheets. (a) Top view of the honeycomb lattice. (b) Atomic structure of layered MoS2. Different sheets of MoS2 are composed of three atomic layers of S–Mo–S, where Mo (black) and S (yellow) are covalently bonded. The figure was made using the Diamond software (version 3.2i, Crystal Impact GbR, Bonn, Germany).
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(a) AFM image of MoS2 nanosheets; (b) height profile of the red line in (a); (c) TEM image of MoS2 nanosheets (darker area), with the inset showing the electron diffraction pattern; (d) SEM image of the cross section of the capillary open tubular column coated with MoS2 nanosheets. Fig. 2
dodecane on the MoS2 column, which can reveal the minimum plate height. The inner wall of the column was fabricated with MoS2 nanosheets (homogeneously coated) with a thickness of approximately 26 nm (Fig. 2d & S3†). It was also observed that aer 2 month use of the column, there was no obvious loss of the stationary phase. For optimization purposes, we rst prepared three MoS2-coated columns with different inner diameters (250, 100 and 75 mm) using the same fabrication procedure and tested each column with the same mixture of volatile organic compounds (VOCs) under the same separation conditions. It turned out that only the column with an inner diameter of 75 mm could well separate all of the analytes (Fig. S4†), and thus, this column was used for all subsequent separations in this work. Moreover, using n-dodecane as a model molecule for testing at 120 C, the calculated retention factor and plates of the chosen column were much higher than the other two. The performance of the column was then tested with Grob's test mixture containing ve compounds with various functional groups, and they were separated within a short period of time (Fig. S5†). A wide range of VOCs were tested as well (the boiling point and theoretical plates per meter of these compounds are summarized in Table S1†). Both a normal alkane mixture (n-C8 to n-C15) (Fig. 3a) and alcohol mixture (n-C4 to n-C12) (Fig. 3b) were well separated. We also chose ve different monosubstituted benzenes, along with benzene and naphthalene, for testing, with all baseline components separated in a short time (Fig. 3c & d). Furthermore, to demonstrate the energy effect of the interaction between analytes and the layered MoS2 nanosheets, the adsorption enthalpies and entropies calculated from van't Hoff plots22 (Fig. 4) are summarized in Table 1. For comparison purposes, we prepared a capillary column with a stationary phase of poly(dimethylsiloxane) (PDMS, used for commercial DB-1 columns) with the same fabrication procedure used for the MoS2-coated column and then tested the
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Fig. 3 Chromatograms obtained from MoS2 column. (a) Temperature program: at 65 C for 0.4 min, 50 C min1 from 65 C to 150 C and at 150 C until the end. (b) Temperature program: at 40 C for 0.2 min, 60 C min1 from 40 C to 180 C and at 180 C until the end. (c) Temperature program: at 40 C for 0.5 min, 60 C min1 from 40 C to 180 C and at 180 C until the end. (d) Temperature program: at 30 C for 0.3 min, 65 C min1 from 30 C to 180 C and 180 C until the end.
Fig. 4 van't Hoff plots obtained from the tested molecules from 40 C to 100 C. (a) n-Alkanes from n-hexcane to n-decane; (b) n-alcohols from n-butanol to n-heptanol; (c) benzene and four monosubstituted benzenes; (d) McReynolds tested chemicals.
same analytes under the same separation conditions (Fig. S6†). It can be seen that there were no obvious differences between the two columns for separating both Grob's test mixture and alcohols. However, octane and nonane, benzene and toluene and phenol and methyl benzoate could not be separated with the PDMS-coated column. Moreover, McReynolds constants (DI) were calculated and compared (see Table S2†). The average value of ve McReynolds constants in Table S2† was oen used as an approximate polarity scale to evaluate various types of interactions between the stationary phase and analytes.23 Each constant value of the MoS2 column is greater than that of the PDMS-coated column, which suggests that the polarity of MoS2 nanosheets should be stronger than that of PDMS. Therefore,
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Table 1 Experimental adsorption enthalpies (DH), adsorption entropies (DS) and Gibbs free energies (DG) at 90 C (see ESI† for more calculations and details)
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DH (kJ mol1) DS (J mol1 K1) DG (kJ mol1) Hexane Heptane Octane Nonane Decane Butanol Pentanol Hexanol Heptanol Benzene Toluene Chlorobenzene Anisole Aniline Pyridine Nitropropane Pentanone
1.95 0.01 5.10 0.01 9.40 0.04 16.72 0.11 24.13 0.48 5.63 0.05 11.54 0.09 21.81 0.32 29.69 0.09 3.35 0.02 7.45 0.07 10.39 0.09 18.47 0.24 24.45 0.51 5.51 0.03 7.66 0.03 4.03 0.03
2.57 0.02 5.78 0.01 16.69 0.17 35.57 0.59 54.70 0.85 7.49 0.21 22.97 0.66 50.14 0.8 69.62 0.88 0.17 0.01 11.96 0.21 19.4 0.32 40.69 0.54 55.58 0.47 6.65 0.03 12.58 0.06 2.85 0.01
2.89 3.00 3.34 3.80 4.27 2.91 3.20 3.61 4.41 2.94 3.11 3.35 3.70 4.27 3.09 3.09 2.99
0.01 0.01 0.03 0.05 0.08 0.03 0.03 0.05 0.01 0.02 0.03 0.03 0.05 0.09 0.02 0.01 0.02
the commercial perspective of the proposed MoS2-coated column might be promising to certainly extend in the future. Unlike graphene oxide12 or a metal–organic framework (MOF),24,25 the layered MoS2 nanosheet has no abundant functional groups, large pore frameworks, or empty metal sites or helical channels. The distance between each of the two adjacent ˚ 26 which is smaller than the size layers of S–Mo–S is only 2.98 A, of each tested molecule (e.g. kinetic diameter of CH4 is about ˚ (ref. 27)). Therefore, the tested molecules could not diffuse 3.4 A into the space between the adjacent layers, and the layered MoS2 nanosheets could not play a role similar to those of zeolite28 or MOF29 as molecular sieves. Considering the correlated calculations reported before,17 most interactions between analytes and MoS2 nanosheets may occur at the edge of the S layer or the surface of the S layer where there are abundant lone pairs of electrons. Linear alkane or alcohol with a longer molecular chain can interact with MoS2 nanosheets based on a stronger dispersion force with the S layer, and higher adsorption enthalpy was obtained. For benzene and hexane, as the electron donor, the former can form a stronger dispersion force with the S layer than the latter. Higher adsorption enthalpies were obtained from benzene than hexane, while for GC, using single-walled carbon nanotubes as the stationary phase, the reverse result was reported.22 For the tested molecules with an –N (pyridine, aniline, 2,6-dimethylaniline, nitropropane) or –OH (n-alcohols, phenol, 2-naphthol) group, peak tailing was observed. This is probably due to the graphite-like structure of MoS2 nanosheets, which provides strong interactions between MoS2 and the tested molecules including hydrophobicity interaction, hydrogen bonding, and p–p stacking.12 Moreover, different from MOF-based GC in which nitropropane eluted right aer benzene,25 with MoS2 nanosheets used as the GC stationary phase, the elution sequence is benzene, pentanone, pyridine, butanol and nitropropane, and with the increase of the according adsorption enthalpies obtained from each
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Fig. 5 (a) Resolution between adjacent linear alkanes from 50 C to 90 C; (b) resolution between benzene and monosubstituted benzenes from 40 C to 80 C.
molecule. It should also be mentioned that with the increase of temperature, decrease of resolution was observed (Fig. 5).
Conclusions We fabricated an open tube capillary column with layered MoS2 nanosheets as the stationary phase for the rst time. Good separation behaviour was observed by testing a wide range of different analytes, which is better than that obtained from the PDMS stationary phase used for a commercial nonpolar DB-1 column. The adsorption enthalpy of each analyte was obtained, and the energy effect between analyte and layered MoS2 nanosheets was thoroughly discussed. Furthermore, the layered MoS2 nanosheet used as GC stationary phase has high chemical stability with a melting point up to 1100 C; thus, it should have good application perspective for high-temperature GC in the future.
Acknowledgements We acknowledge the nancial support from the National Natural Science Foundation of China (no. 21205083) and Ministry of Education of China (no. 20120181120071), and assistance from our Analytical & Testing Centre for SEM, TEM and AFM data.
Notes and references 1 J. N. Coleman, M. Lotya, A. O'Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H.-Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen, D. W. McComb, P. D. Nellist and V. Nicolosi, Science, 2011, 331, 568. 2 K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov and A. K. Geim, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 10451. 3 L. Rapoport, N. Fleischer and R. Tenne, J. Mater. Chem., 2005, 15, 1782. 4 B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff and J. K. Nørskov, J. Am. Chem. Soc., 2005, 127, 5308. Analyst, 2014, 139, 3533–3536 | 3535
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5 C. L. Stender, E. C. Greyson, Y. Babayan and T. W. Odom, Adv. Mater., 2005, 17, 2837. 6 L. Cao, S. Yang, W. Gao, Z. Liu, Y. Gong, L. Ma, G. Shi, S. Lei, Y. Zhang and S. Zhang, Small, 2013, 17, 2905. 7 J. Xiao, D. Choi, L. Cosimbescu, P. Koech, J. Liu and J. P. Lemmon, Chem. Mater., 2010, 22, 4522. 8 Y. Li, Z. Zhou, S. Zhang and Z. Chen, J. Am. Chem. Soc., 2008, 130, 16739. 9 Y. Zhang, J. Ye, Y. Matsuhashi and Y. Iwasa, Nano Lett., 2012, 12, 1136. 10 J. Heising and M. G. Kanatzidis, J. Am. Chem. Soc., 1999, 121, 638. 11 Q. Liu, J. Shi, M. Cheng, G. Li, D. Cao and G. Jiang, Chem. Commun., 2012, 48, 1874. 12 Q. Qu, Y. Shen, C. Gu, Z. Gu, Q. Gu, C. Wang and X. Hu, Anal. Chim. Acta, 2012, 757, 83. 13 M.-M. Wang and X.-P. Yan, Anal. Chem., 2011, 84, 39. 14 B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nat. Nanotechnol., 2011, 6, 147. 15 E. Gourmelon, O. Lignier, H. Hadouda, G. Couturier, J. C. Bern` ede, J. Tedd, J. Pouzet and J. Salardenne, Sol. Energy Mater. Sol. Cells, 1997, 46, 115. 16 K. Wang, J. Wang, J. Fan, M. Lotya, A. O'Neill, D. Fox, Y. Feng, X. Zhang, B. Jiang, Q. Zhao, H. Zhang, J. N. Coleman, L. Zhang and W. Josef Blau, ACS Nano, 2013, 10, 9260.
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17 J. He, K. Wu, R. Sa, Q. Li and Y. Wei, Appl. Phys. Lett., 2010, 96, 082504. 18 L. Sun, H. Huang and X. Peng, Chem. Commun., 2013, 49, 10718. 19 M. Xu, T. Liang, M. Shi and H. Chen, Chem. Rev., 2013, 113, 3766. 20 K.-G. Zhou, N.-N. Mao, H.-X. Wang, Y. Peng and H.-L. Zhang, Angew. Chem., Int. Ed., 2011, 50, 10839. 21 J. H. Warner, M. H. Rummeli, L. Ge, T. Gemming, B. Montanari, N. M. Harrison, B. Buchner, G. Andrew and D. Briggs, Nat. Nanotechnol., 2009, 4, 500. 22 M. Karwa and S. Mitra, Anal. Chem., 2006, 78, 2064. 23 W. O. McReynolds, J. Chromatogr. Sci., 1970, 8, 685. 24 Z.-Y. Gu and X.-P. Yan, Angew. Chem., Int. Ed., 2010, 49, 1477. 25 S.-M. Xie, Z.-J. Zhang, Z.-Y. Wang and L.-M. Yuan, J. Am. Chem. Soc., 2011, 133, 11892. 26 V. Alexiev, R. Prins and T. Weber, Phys. Chem. Chem. Phys., 2000, 2, 1815. 27 H. Bux, F. Liang, Y. Li, J. Cravillon, M. Wiebcke and J. Caro, J. Am. Chem. Soc., 2009, 131, 16000. 28 J. F. Denayer, G. V. Baron, J. A. Martens and P. A. Jacobs, J. Phys. Chem. B, 1998, 102, 3077. 29 N. Chang, Z.-Y. Gu and X.-P. Yan, J. Am. Chem. Soc., 2010, 132, 13645.
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Cite this: Analyst, 2014, 139, 3533
Received 16th February 2014 Accepted 6th May 2014
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Two-dimensional MoS2 nanosheets as a capillary GC stationary phase for highly effective molecular screening† Jia Jia,a Fujian Xu,b Shanling Wang,a Xue Jiang,b Zhou Long*a and Xiandeng Hou*ab
DOI: 10.1039/c4an00332b www.rsc.org/analyst
Stable layered MoS2 nanosheets were employed as a stationary phase in gas chromatography. A wide range of different analytes were screened with excellent separation efficiency.
Besides well-known graphene, other inorganic graphene analogues1 such as transition metal dichalcogenides2 with a common formula of MeX2 (Me ¼ Mo, W, Ti, etc., X ¼ S, Se, Te, etc.) have become more and more attractive in recent years. They play an important role in various applications including lubrication,3 catalysis,4 photovoltaics,5 supercapacitors6 and rechargeable battery systems7 due to their excellent properties such as semiconductivity, half-metallic magnetism,8 superconductivity,9 and charge density wave phenomena.10 Up to now, graphene oxide has been demonstrated to be a very good adsorbent11 and commonly used as a stationary phase in chromatography12 and electrophoresis.13 A two-dimensional MoS2 layer has a vertical structure similar to that of graphite14 (Fig. 1a), but unlike graphite or h-BN, the MoS2 layer is made up of hexagons, with Mo and S atoms located at alternating corners (Fig. 1b). Bulk MoS2 has attracted interest for use in photovoltaic and photocatalytic15 materials because of its strong absorption in the solar spectral region. The special electronic structure of monolayer and few-layer MoS2 shows a dramatic improvement in optical properties.16 However, applications of MoS2 as an adsorbent for chromatography has rarely been reported. Recently, the adsorption of exotic molecules onto MoS2 monolayers has been investigated by rst-principle calculations,17 and it has also been reported that a laminar MoS2 membrane could behave as a size-selective molecular sieve.18 Therefore, it could be expected that two-dimensional layered MoS2 nanosheets could be employed as a good platform a
Analytical & Testing Centre, Sichuan University, Chengdu, 610064, China. E-mail: [email protected]
b
College of Chemistry, Sichuan University Chengdu, 610064, China. E-mail: houxd@ scu.edu.cn † Electronic supplementary 10.1039/c4an00332b
information
(ESI)
This journal is © The Royal Society of Chemistry 2014
available:
See
DOI:
for molecular screening based on the selective adsorption of exotic molecules.19 In this context, we rst developed a stationary phase of MoS2 nanosheets for molecular screening using the gas chromatographic (GC) mode. MoS2 nanosheets were prepared by a mixed-solvent exfoliation strategy.20 In brief, 30 mg of MoS2 powder was dispersed in 10 mL of deionized water/ethanol (4.5/5.5, v/v) in a sealed ask, which was then subjected to ultrasound for 8 h. The obtained suspension was centrifuged at 4000 rpm (about 1484g) for 30 min, and the supernatant was collected (Fig. S1†). The thickness of the obtained MoS2 nanosheets was around 1.5–3.4 nm based on the atomic force microscopy (AFM) image (Fig. 2a & b). Considering that the thickness of monolayer MoS2 nanosheets is 0.9–1.2 nm,1 the obtained nanosheets should be composed of 1–3 monolayers. The morphology of the obtained MoS2 nanosheets was studied by transmission electron microscopy (TEM), which reveals that the majority of the nanosheets are present as two-dimensional thin sheets in the dispersion (Fig. 2c), and the electron diffraction pattern shows that the MoS2 nanosheets have a vertical structure similar to graphite21 (inset of Fig. 2c). By using GC with MoS2 nanosheets as the stationary phase, molecular screening was successfully accomplished (see more details in ESI†). Fig. S2† shows the van Deemter plots for
Fig. 1 Structural diagrams of MoS2 nanosheets. (a) Top view of the honeycomb lattice. (b) Atomic structure of layered MoS2. Different sheets of MoS2 are composed of three atomic layers of S–Mo–S, where Mo (black) and S (yellow) are covalently bonded. The figure was made using the Diamond software (version 3.2i, Crystal Impact GbR, Bonn, Germany).
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(a) AFM image of MoS2 nanosheets; (b) height profile of the red line in (a); (c) TEM image of MoS2 nanosheets (darker area), with the inset showing the electron diffraction pattern; (d) SEM image of the cross section of the capillary open tubular column coated with MoS2 nanosheets. Fig. 2
dodecane on the MoS2 column, which can reveal the minimum plate height. The inner wall of the column was fabricated with MoS2 nanosheets (homogeneously coated) with a thickness of approximately 26 nm (Fig. 2d & S3†). It was also observed that aer 2 month use of the column, there was no obvious loss of the stationary phase. For optimization purposes, we rst prepared three MoS2-coated columns with different inner diameters (250, 100 and 75 mm) using the same fabrication procedure and tested each column with the same mixture of volatile organic compounds (VOCs) under the same separation conditions. It turned out that only the column with an inner diameter of 75 mm could well separate all of the analytes (Fig. S4†), and thus, this column was used for all subsequent separations in this work. Moreover, using n-dodecane as a model molecule for testing at 120 C, the calculated retention factor and plates of the chosen column were much higher than the other two. The performance of the column was then tested with Grob's test mixture containing ve compounds with various functional groups, and they were separated within a short period of time (Fig. S5†). A wide range of VOCs were tested as well (the boiling point and theoretical plates per meter of these compounds are summarized in Table S1†). Both a normal alkane mixture (n-C8 to n-C15) (Fig. 3a) and alcohol mixture (n-C4 to n-C12) (Fig. 3b) were well separated. We also chose ve different monosubstituted benzenes, along with benzene and naphthalene, for testing, with all baseline components separated in a short time (Fig. 3c & d). Furthermore, to demonstrate the energy effect of the interaction between analytes and the layered MoS2 nanosheets, the adsorption enthalpies and entropies calculated from van't Hoff plots22 (Fig. 4) are summarized in Table 1. For comparison purposes, we prepared a capillary column with a stationary phase of poly(dimethylsiloxane) (PDMS, used for commercial DB-1 columns) with the same fabrication procedure used for the MoS2-coated column and then tested the
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Fig. 3 Chromatograms obtained from MoS2 column. (a) Temperature program: at 65 C for 0.4 min, 50 C min1 from 65 C to 150 C and at 150 C until the end. (b) Temperature program: at 40 C for 0.2 min, 60 C min1 from 40 C to 180 C and at 180 C until the end. (c) Temperature program: at 40 C for 0.5 min, 60 C min1 from 40 C to 180 C and at 180 C until the end. (d) Temperature program: at 30 C for 0.3 min, 65 C min1 from 30 C to 180 C and 180 C until the end.
Fig. 4 van't Hoff plots obtained from the tested molecules from 40 C to 100 C. (a) n-Alkanes from n-hexcane to n-decane; (b) n-alcohols from n-butanol to n-heptanol; (c) benzene and four monosubstituted benzenes; (d) McReynolds tested chemicals.
same analytes under the same separation conditions (Fig. S6†). It can be seen that there were no obvious differences between the two columns for separating both Grob's test mixture and alcohols. However, octane and nonane, benzene and toluene and phenol and methyl benzoate could not be separated with the PDMS-coated column. Moreover, McReynolds constants (DI) were calculated and compared (see Table S2†). The average value of ve McReynolds constants in Table S2† was oen used as an approximate polarity scale to evaluate various types of interactions between the stationary phase and analytes.23 Each constant value of the MoS2 column is greater than that of the PDMS-coated column, which suggests that the polarity of MoS2 nanosheets should be stronger than that of PDMS. Therefore,
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Table 1 Experimental adsorption enthalpies (DH), adsorption entropies (DS) and Gibbs free energies (DG) at 90 C (see ESI† for more calculations and details)
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DH (kJ mol1) DS (J mol1 K1) DG (kJ mol1) Hexane Heptane Octane Nonane Decane Butanol Pentanol Hexanol Heptanol Benzene Toluene Chlorobenzene Anisole Aniline Pyridine Nitropropane Pentanone
1.95 0.01 5.10 0.01 9.40 0.04 16.72 0.11 24.13 0.48 5.63 0.05 11.54 0.09 21.81 0.32 29.69 0.09 3.35 0.02 7.45 0.07 10.39 0.09 18.47 0.24 24.45 0.51 5.51 0.03 7.66 0.03 4.03 0.03
2.57 0.02 5.78 0.01 16.69 0.17 35.57 0.59 54.70 0.85 7.49 0.21 22.97 0.66 50.14 0.8 69.62 0.88 0.17 0.01 11.96 0.21 19.4 0.32 40.69 0.54 55.58 0.47 6.65 0.03 12.58 0.06 2.85 0.01
2.89 3.00 3.34 3.80 4.27 2.91 3.20 3.61 4.41 2.94 3.11 3.35 3.70 4.27 3.09 3.09 2.99
0.01 0.01 0.03 0.05 0.08 0.03 0.03 0.05 0.01 0.02 0.03 0.03 0.05 0.09 0.02 0.01 0.02
the commercial perspective of the proposed MoS2-coated column might be promising to certainly extend in the future. Unlike graphene oxide12 or a metal–organic framework (MOF),24,25 the layered MoS2 nanosheet has no abundant functional groups, large pore frameworks, or empty metal sites or helical channels. The distance between each of the two adjacent ˚ 26 which is smaller than the size layers of S–Mo–S is only 2.98 A, of each tested molecule (e.g. kinetic diameter of CH4 is about ˚ (ref. 27)). Therefore, the tested molecules could not diffuse 3.4 A into the space between the adjacent layers, and the layered MoS2 nanosheets could not play a role similar to those of zeolite28 or MOF29 as molecular sieves. Considering the correlated calculations reported before,17 most interactions between analytes and MoS2 nanosheets may occur at the edge of the S layer or the surface of the S layer where there are abundant lone pairs of electrons. Linear alkane or alcohol with a longer molecular chain can interact with MoS2 nanosheets based on a stronger dispersion force with the S layer, and higher adsorption enthalpy was obtained. For benzene and hexane, as the electron donor, the former can form a stronger dispersion force with the S layer than the latter. Higher adsorption enthalpies were obtained from benzene than hexane, while for GC, using single-walled carbon nanotubes as the stationary phase, the reverse result was reported.22 For the tested molecules with an –N (pyridine, aniline, 2,6-dimethylaniline, nitropropane) or –OH (n-alcohols, phenol, 2-naphthol) group, peak tailing was observed. This is probably due to the graphite-like structure of MoS2 nanosheets, which provides strong interactions between MoS2 and the tested molecules including hydrophobicity interaction, hydrogen bonding, and p–p stacking.12 Moreover, different from MOF-based GC in which nitropropane eluted right aer benzene,25 with MoS2 nanosheets used as the GC stationary phase, the elution sequence is benzene, pentanone, pyridine, butanol and nitropropane, and with the increase of the according adsorption enthalpies obtained from each
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Fig. 5 (a) Resolution between adjacent linear alkanes from 50 C to 90 C; (b) resolution between benzene and monosubstituted benzenes from 40 C to 80 C.
molecule. It should also be mentioned that with the increase of temperature, decrease of resolution was observed (Fig. 5).
Conclusions We fabricated an open tube capillary column with layered MoS2 nanosheets as the stationary phase for the rst time. Good separation behaviour was observed by testing a wide range of different analytes, which is better than that obtained from the PDMS stationary phase used for a commercial nonpolar DB-1 column. The adsorption enthalpy of each analyte was obtained, and the energy effect between analyte and layered MoS2 nanosheets was thoroughly discussed. Furthermore, the layered MoS2 nanosheet used as GC stationary phase has high chemical stability with a melting point up to 1100 C; thus, it should have good application perspective for high-temperature GC in the future.
Acknowledgements We acknowledge the nancial support from the National Natural Science Foundation of China (no. 21205083) and Ministry of Education of China (no. 20120181120071), and assistance from our Analytical & Testing Centre for SEM, TEM and AFM data.
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