- Email: [email protected]
Chemical Syntheses of Two-Dimensional Boron Materials
(2011). Freestanding palladium nanosheets with plasmonic and catalytic properties. Nat. Nanotechnol. 6, 28–32. 7. Duan, X., Huang, Y., Cui, Y., Wang, J., and Lieber, C.M. (2001). Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices. Nature 409, 66–69.
8. Quddus, E.B., Wilson, A., Webb, R.A., and Koley, G. (2014). Oxygen mediated synthesis of high quality InN nanowires above their decomposition temperature. Nanoscale 6, 1166–1172. 9. Luo, L., Li, Y., Sun, X., Li, J., Hu, E., Liu, Y.L., Tian, Y., Yang, X.-Q., Li, Y.Q., Lin, W.-F., et al. (2020). Synthesis and properties of stable
sub-2-nm-thick aluminum nanosheets: oxygen passivation and two-photon luminescence. Chem 6, this issue, 448–459. 10. Toby, B.H., and Egami, T. (1992). Accuracy of pair distribution function analysis applied to crystalline and non-crystalline materials. Acta Crystallogr. A 48, 336–346.
Preview
Chemical Syntheses of TwoDimensional Boron Materials Xi-Meng Chen1 and Xuenian Chen1,* In this issue of Chem, Kondo and co-workers report a facile soft-chemical method for the preparation of layered, electrically conductive hydrogen boride, which is short-range order but amorphous as a result of the geometrical frustration of the terminal B–H bonds. This work offers an alternative route to two-dimensional (2D) boron materials. Two-dimensional (2D) carbon materials, especially graphene and MXenes, have attained great interest recently because of their unique physicochemical properties. As a neighboring element of carbon, boron-based 2D materials are less known. 2D boron hydride materials are expected to show unique electronic properties compared with those of their carbon allotropes because of the electron-deficient nature of boron. However, the existing synthetic methods for 2D boron hydrides cannot provide highly pure homogeneous materials,1 which might limit the investigation on the application of the 2D boron materials. The well-known characteristic of the boron atom is electron deficiency because of the unoccupied 2p orbital in its valence shell.2 As a result, boron tends to form multiple-center bonds in its different allotropes or compounds. In the different bulk boron allotropes, the basic structural unit is the threedimensional B12 icosahedra. Hydrogen boride (noted as HB here and usually
324
named borane or boron hydride in terms of chemistry) is one of the most important type of boron compounds, in which only the B and H elements are involved. Boranes, which usually occur as discrete molecules or anions, such as B2H6, B4H10, BH4 , B2H7 , B3H8 , B10H102 , and B12H122 , have been extensively investigated.3,4 In this issue of Chem, Kondo and coworkers report a soft-chemical method for the synthesis of 2D HB materials.5 In this report, the authors improved the synthetic method for the pure HB by using the Schlenk technique (Figure 1A) on the basis of their recent work.6 By the improved method, the unoxidized layered HB material was synthesized in its powder form by the chemical ion exchange of magnesium cations in MgB2 for protons of the ion-exchange resin in acetonitrile or methanol. The prepared HB sample sheets, some of which were folded and wrinkled (Figure 1B), and their thickness were evaluated. The thinner and thicker sheets were located at the rotational center and surrounding re-
Chem 6, 322–336, February 13, 2020 ª 2020 Elsevier Inc.
gions of the spin-coated surface, respectively (Figure 1C). The authors investigated experimentally and computationally the 2D HB materials from chemical bonds to overall structure. According to the X-ray diffraction results, the HB sheets were amorphous but had short-range orders. The three-center, two-electron (3c-2e) B–H–B bridging bonds were confirmed by X-ray and Fourier transform infrared spectroscopy. The HB structure was composed of corrugated layers formed by hexagonal boron networks of chair-type B6 rings, and the B–H–B bridging bonds were located at the ridge lines of the corrugated layers (Figure 1D). In this structure, each B2 unit only had a single symmetric 3c-2e B–H–B bridging bond, which is different from B2H6-type double B–H–B bridging bonds. Each B2 unit formed a terminal B–H group at one of the equivalent terminal positions, resulting in asymmetric B–HB– B–HT or HT–B–HB–B environments (Figure 1D). The different B–H arrangements (B–HB–B–HT and HT–B–HB–B) were equivalent but resulted in different local distortions of the B layer. As a result, the non-crystalline nature of the material originated from such arrangements of H atoms, i.e., the
1School
of Chemistry and Chemical Engineering, Henan Key Laboratory of Boron Chemistry and Advanced Energy Materials, Henan Normal University, Xinxiang, Henan 453007, China *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2020.01.001
Figure 1. The Synthesis of HB Sheets and Their Structure (A) Depiction of the synthesis of HB sheets by the Schlenk method. (B) Scanning electron microscopy image. (C) Atomic force microscopy images with line profiles for HB on a mica surface. (D) Structural model of HB.
Chem 6, 322–336, February 13, 2020
325
geometrical frustration of terminal B–H groups. The HB materials possessed a good electrical conductivity of 0.13 S cm 1 at 0 C–10 C, and the activation energy for conduction was 0.10 eV, indicating that the conductivity could be on par with that of metal materials and is excellent among 2D materials. However, at temperatures of 30 C or higher, the resistance of HB increased dramatically, and the metal-to-insulator transition was reversible. The authors found that the desorption and adsorption of impurities (residual solvents) had an effect on the reversible transition. When the HB materials adsorbed such impurities, the B–H–B bonds could dissociate, forming terminal B–H bonds and leading to the metallic conductivity of HB. Therefore, local B–H bonds in HB and their response to chemical adsorption have a dominant effect on electronic behaviors. This discovery was believed to open the unique functionalities of the HB materials. Although 2D boron materials have not been chemically synthesized or characterized, a series of size-selected boron clusters, most of which have planar or quasiplanar structures, were predicted and confirmed experimentally and computationally via photoelectron spectroscopy.7 As a typical representative of these boron clusters, the quasiplanar structure of B36 has been extensively investigated, and the concept of borophene was first proposed by Profs. Wang and Li in 2014.7 Borophenes are analogs to graphenes, and the B36 cluster can be considered as the analogous boron unit of the hexagonal C6 unit in graphene to form extended graphenelike boron nanostructures (borophenes) because it is a highly stable quasiplanar boron cluster with a central hexagonal hole. Borophenes are believed to be
326
Chem 6, 322–336, February 13, 2020
a new generation of synthesized 2D materials. Profs. Guisinger’s and Wu’s groups almost simultaneously reported similar methods by molecular beam epitaxy (MBE) for the syntheses of borophenes.8,9 The prepared borophene sheets were planar and inert to oxidization and showed metal-like, highly anisotropic electronic properties in outstanding agreement with theoretical predictions. Their work paved the way to the application of boron-based microelectronic devices. The authors provided soft-chemical methods for the syntheses of 2D boron materials and also pointed out that the bridge hydrogen (B–H–B) and the terminal hydrogen are responses for the electronic conductivity of the 2D materials, as well as that the geometrical frustration of the terminal B–H bond leads to an amorphous state but short-range orders. A few challenges remain for this new method and new materials. According to first-principle computations,10 borophenes can be prepared on noble-metal substrates, such as Cu(111), Ag(111), and Au(111). Thin borophene sheets were successfully synthesized on a Ag(111) surface under ultrahigh vacuum conditions with a pure boron (99.9999%) source and MBE.8,9 On the other hand, in the synthesized 2D HB materials, B:H is a 1:1 ratio; thus, the possible roles of the bridge and terminal hydrogens for stabilizing 2D materials deserve to be explored in depth considering that, like the Cu, Ag, and Au atoms, the H atom possesses ns1 electronic configuration, which might stabilize the 2D HB materials as the noble-metal passivation did. Further exploration following this idea might facilitate finding other ligands with such stabilization roles and realize chemical synthesize the 2D or other types of boron-based materials. Overall, the study provides an alternative route to 2D boron mate-
rials, which will facilitate further exploration of the applications of 2D boron materials in different fields.
1. Jiao, Y., Ma, F., Bell, J., Bilic, A., and Du, A. (2016). Two-dimensional boron hydride sheets: high stability, massless dirac fermions, and excellent mechanical properties. Angew. Chem. Int. Ed. Engl. 55, 10292–10295. 2. Zhao, Q., Dewhurst, R.D., Braunschweig, H., and Chen, X. (2019). A new perspective on borane chemistry: the nucleophilicity of the B-H bonding pair electrons. Angew. Chem. Int. Ed. Engl. 58, 3268–3278. 3. Chen, X.M., Ma, N., Zhang, Q.F., Wang, J., Feng, X., Wei, C., Wang, L.S., Zhang, J., and Chen, X. (2018). Elucidation of the formation mechanisms of the octahydrotriborate anion (B3H8 ) through the nucleophilicity of the B-H bond. J. Am. Chem. Soc. 140, 6718–6726. 4. Hansen, B.R.S., Paskevicius, M., Li, H.W., Akiba, E., and Jensen, T.R. (2016). Metal boranes: progress and applications. Coord. Chem. Rev. 323, 60–70. 5. Tominaka, S., Ishibiki, R., Fujino, A., Kawakami, K., Ohara, K., Masuda, T., Matsuda, I., Hosono, H., and Kondo, T. (2020). Geometrical frustration of B-H bonds in layered hydrogen borides accessible by soft chemistry. Chem 6, this issue, 406–418. 6. Nishino, H., Fujita, T., Cuong, N.T., Tominaka, S., Miyauchi, M., Iimura, S., Hirata, A., Umezawa, N., Okada, S., Nishibori, E., et al. (2017). Formation and characterization of hydrogen boride sheets derived from MgB2 by cation exchange. J. Am. Chem. Soc. 139, 13761–13769. 7. Wang, L.S. (2016). Photoelectron spectroscopy of size-selected boron clusters: from planar structures to borophenes and borospherenes. Int. Rev. Phys. Chem. 35, 69–142. 8. Mannix, A.J., Zhou, X.F., Kiraly, B., Wood, J.D., Alducin, D., Myers, B.D., Liu, X., Fisher, B.L., Santiago, U., Guest, J.R., et al. (2015). Synthesis of borophenes: Anisotropic, twodimensional boron polymorphs. Science 350, 1513–1516. 9. Feng, B., Zhang, J., Zhong, Q., Li, W., Li, S., Li, H., Cheng, P., Meng, S., Chen, L., and Wu, K. (2016). Experimental realization of twodimensional boron sheets. Nat. Chem. 8, 563–568. 10. Liu, Y., Penev, E.S., and Yakobson, B.I. (2013). Probing the synthesis of twodimensional boron by first-principles computations. Angew. Chem. Int. Ed. Engl. 52, 3156–3159.
8. Quddus, E.B., Wilson, A., Webb, R.A., and Koley, G. (2014). Oxygen mediated synthesis of high quality InN nanowires above their decomposition temperature. Nanoscale 6, 1166–1172. 9. Luo, L., Li, Y., Sun, X., Li, J., Hu, E., Liu, Y.L., Tian, Y., Yang, X.-Q., Li, Y.Q., Lin, W.-F., et al. (2020). Synthesis and properties of stable
sub-2-nm-thick aluminum nanosheets: oxygen passivation and two-photon luminescence. Chem 6, this issue, 448–459. 10. Toby, B.H., and Egami, T. (1992). Accuracy of pair distribution function analysis applied to crystalline and non-crystalline materials. Acta Crystallogr. A 48, 336–346.
Preview
Chemical Syntheses of TwoDimensional Boron Materials Xi-Meng Chen1 and Xuenian Chen1,* In this issue of Chem, Kondo and co-workers report a facile soft-chemical method for the preparation of layered, electrically conductive hydrogen boride, which is short-range order but amorphous as a result of the geometrical frustration of the terminal B–H bonds. This work offers an alternative route to two-dimensional (2D) boron materials. Two-dimensional (2D) carbon materials, especially graphene and MXenes, have attained great interest recently because of their unique physicochemical properties. As a neighboring element of carbon, boron-based 2D materials are less known. 2D boron hydride materials are expected to show unique electronic properties compared with those of their carbon allotropes because of the electron-deficient nature of boron. However, the existing synthetic methods for 2D boron hydrides cannot provide highly pure homogeneous materials,1 which might limit the investigation on the application of the 2D boron materials. The well-known characteristic of the boron atom is electron deficiency because of the unoccupied 2p orbital in its valence shell.2 As a result, boron tends to form multiple-center bonds in its different allotropes or compounds. In the different bulk boron allotropes, the basic structural unit is the threedimensional B12 icosahedra. Hydrogen boride (noted as HB here and usually
324
named borane or boron hydride in terms of chemistry) is one of the most important type of boron compounds, in which only the B and H elements are involved. Boranes, which usually occur as discrete molecules or anions, such as B2H6, B4H10, BH4 , B2H7 , B3H8 , B10H102 , and B12H122 , have been extensively investigated.3,4 In this issue of Chem, Kondo and coworkers report a soft-chemical method for the synthesis of 2D HB materials.5 In this report, the authors improved the synthetic method for the pure HB by using the Schlenk technique (Figure 1A) on the basis of their recent work.6 By the improved method, the unoxidized layered HB material was synthesized in its powder form by the chemical ion exchange of magnesium cations in MgB2 for protons of the ion-exchange resin in acetonitrile or methanol. The prepared HB sample sheets, some of which were folded and wrinkled (Figure 1B), and their thickness were evaluated. The thinner and thicker sheets were located at the rotational center and surrounding re-
Chem 6, 322–336, February 13, 2020 ª 2020 Elsevier Inc.
gions of the spin-coated surface, respectively (Figure 1C). The authors investigated experimentally and computationally the 2D HB materials from chemical bonds to overall structure. According to the X-ray diffraction results, the HB sheets were amorphous but had short-range orders. The three-center, two-electron (3c-2e) B–H–B bridging bonds were confirmed by X-ray and Fourier transform infrared spectroscopy. The HB structure was composed of corrugated layers formed by hexagonal boron networks of chair-type B6 rings, and the B–H–B bridging bonds were located at the ridge lines of the corrugated layers (Figure 1D). In this structure, each B2 unit only had a single symmetric 3c-2e B–H–B bridging bond, which is different from B2H6-type double B–H–B bridging bonds. Each B2 unit formed a terminal B–H group at one of the equivalent terminal positions, resulting in asymmetric B–HB– B–HT or HT–B–HB–B environments (Figure 1D). The different B–H arrangements (B–HB–B–HT and HT–B–HB–B) were equivalent but resulted in different local distortions of the B layer. As a result, the non-crystalline nature of the material originated from such arrangements of H atoms, i.e., the
1School
of Chemistry and Chemical Engineering, Henan Key Laboratory of Boron Chemistry and Advanced Energy Materials, Henan Normal University, Xinxiang, Henan 453007, China *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2020.01.001
Figure 1. The Synthesis of HB Sheets and Their Structure (A) Depiction of the synthesis of HB sheets by the Schlenk method. (B) Scanning electron microscopy image. (C) Atomic force microscopy images with line profiles for HB on a mica surface. (D) Structural model of HB.
Chem 6, 322–336, February 13, 2020
325
geometrical frustration of terminal B–H groups. The HB materials possessed a good electrical conductivity of 0.13 S cm 1 at 0 C–10 C, and the activation energy for conduction was 0.10 eV, indicating that the conductivity could be on par with that of metal materials and is excellent among 2D materials. However, at temperatures of 30 C or higher, the resistance of HB increased dramatically, and the metal-to-insulator transition was reversible. The authors found that the desorption and adsorption of impurities (residual solvents) had an effect on the reversible transition. When the HB materials adsorbed such impurities, the B–H–B bonds could dissociate, forming terminal B–H bonds and leading to the metallic conductivity of HB. Therefore, local B–H bonds in HB and their response to chemical adsorption have a dominant effect on electronic behaviors. This discovery was believed to open the unique functionalities of the HB materials. Although 2D boron materials have not been chemically synthesized or characterized, a series of size-selected boron clusters, most of which have planar or quasiplanar structures, were predicted and confirmed experimentally and computationally via photoelectron spectroscopy.7 As a typical representative of these boron clusters, the quasiplanar structure of B36 has been extensively investigated, and the concept of borophene was first proposed by Profs. Wang and Li in 2014.7 Borophenes are analogs to graphenes, and the B36 cluster can be considered as the analogous boron unit of the hexagonal C6 unit in graphene to form extended graphenelike boron nanostructures (borophenes) because it is a highly stable quasiplanar boron cluster with a central hexagonal hole. Borophenes are believed to be
326
Chem 6, 322–336, February 13, 2020
a new generation of synthesized 2D materials. Profs. Guisinger’s and Wu’s groups almost simultaneously reported similar methods by molecular beam epitaxy (MBE) for the syntheses of borophenes.8,9 The prepared borophene sheets were planar and inert to oxidization and showed metal-like, highly anisotropic electronic properties in outstanding agreement with theoretical predictions. Their work paved the way to the application of boron-based microelectronic devices. The authors provided soft-chemical methods for the syntheses of 2D boron materials and also pointed out that the bridge hydrogen (B–H–B) and the terminal hydrogen are responses for the electronic conductivity of the 2D materials, as well as that the geometrical frustration of the terminal B–H bond leads to an amorphous state but short-range orders. A few challenges remain for this new method and new materials. According to first-principle computations,10 borophenes can be prepared on noble-metal substrates, such as Cu(111), Ag(111), and Au(111). Thin borophene sheets were successfully synthesized on a Ag(111) surface under ultrahigh vacuum conditions with a pure boron (99.9999%) source and MBE.8,9 On the other hand, in the synthesized 2D HB materials, B:H is a 1:1 ratio; thus, the possible roles of the bridge and terminal hydrogens for stabilizing 2D materials deserve to be explored in depth considering that, like the Cu, Ag, and Au atoms, the H atom possesses ns1 electronic configuration, which might stabilize the 2D HB materials as the noble-metal passivation did. Further exploration following this idea might facilitate finding other ligands with such stabilization roles and realize chemical synthesize the 2D or other types of boron-based materials. Overall, the study provides an alternative route to 2D boron mate-
rials, which will facilitate further exploration of the applications of 2D boron materials in different fields.
1. Jiao, Y., Ma, F., Bell, J., Bilic, A., and Du, A. (2016). Two-dimensional boron hydride sheets: high stability, massless dirac fermions, and excellent mechanical properties. Angew. Chem. Int. Ed. Engl. 55, 10292–10295. 2. Zhao, Q., Dewhurst, R.D., Braunschweig, H., and Chen, X. (2019). A new perspective on borane chemistry: the nucleophilicity of the B-H bonding pair electrons. Angew. Chem. Int. Ed. Engl. 58, 3268–3278. 3. Chen, X.M., Ma, N., Zhang, Q.F., Wang, J., Feng, X., Wei, C., Wang, L.S., Zhang, J., and Chen, X. (2018). Elucidation of the formation mechanisms of the octahydrotriborate anion (B3H8 ) through the nucleophilicity of the B-H bond. J. Am. Chem. Soc. 140, 6718–6726. 4. Hansen, B.R.S., Paskevicius, M., Li, H.W., Akiba, E., and Jensen, T.R. (2016). Metal boranes: progress and applications. Coord. Chem. Rev. 323, 60–70. 5. Tominaka, S., Ishibiki, R., Fujino, A., Kawakami, K., Ohara, K., Masuda, T., Matsuda, I., Hosono, H., and Kondo, T. (2020). Geometrical frustration of B-H bonds in layered hydrogen borides accessible by soft chemistry. Chem 6, this issue, 406–418. 6. Nishino, H., Fujita, T., Cuong, N.T., Tominaka, S., Miyauchi, M., Iimura, S., Hirata, A., Umezawa, N., Okada, S., Nishibori, E., et al. (2017). Formation and characterization of hydrogen boride sheets derived from MgB2 by cation exchange. J. Am. Chem. Soc. 139, 13761–13769. 7. Wang, L.S. (2016). Photoelectron spectroscopy of size-selected boron clusters: from planar structures to borophenes and borospherenes. Int. Rev. Phys. Chem. 35, 69–142. 8. Mannix, A.J., Zhou, X.F., Kiraly, B., Wood, J.D., Alducin, D., Myers, B.D., Liu, X., Fisher, B.L., Santiago, U., Guest, J.R., et al. (2015). Synthesis of borophenes: Anisotropic, twodimensional boron polymorphs. Science 350, 1513–1516. 9. Feng, B., Zhang, J., Zhong, Q., Li, W., Li, S., Li, H., Cheng, P., Meng, S., Chen, L., and Wu, K. (2016). Experimental realization of twodimensional boron sheets. Nat. Chem. 8, 563–568. 10. Liu, Y., Penev, E.S., and Yakobson, B.I. (2013). Probing the synthesis of twodimensional boron by first-principles computations. Angew. Chem. Int. Ed. Engl. 52, 3156–3159.