Functionalization of the Semiconductor Surfaces of Diamond (100), Si

Jun 5, 2009 - The viability of functionalization of the semiconductor surfaces of diamond (100), Si (100), and Ge (100) by traditional [3 + 2] cycload...
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Functionalization of the Semiconductor Surfaces of Diamond (100), Si (100), and Ge (100) by Cycloaddition of Transition Metal Oxides: A Theoretical Prediction Yi-Jun Xu* and Xianzhi Fu Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, P.R. China Received March 18, 2009. Revised Manuscript Received May 19, 2009 The viability of functionalization of the semiconductor surfaces of diamond (100), Si (100), and Ge (100) by traditional [3+2] cycloaddition of transition metal oxides has been predicted using effective cluster models in the framework of density functional theory. The cycloaddition of transition metal oxides (OsO4, RuO4, and MnO4 -) onto the X (100) (X=C, Si, and Ge) surface is much more facile than that of other molecular analogues including ethylene, fullerene, and single-walled carbon nanotubes because of the high reactivity of surface dimers of X (100). Our computational results demonstrate the plausibility that the well-known [3+2] cycloaddition of transition metal oxides to alkenes in organic chemistry can be employed as a new type of surface reaction to functionalize the semiconductor X (100) surface, which offers the new possibility for self-assembly or chemical functionalization of X (100) at low temperature. More importantly, the chemical functionalization of X (100) by cycloaddition of transition metal oxides provides the molecular basis for preparation of semiconductor-supported catalysts but also strongly advances the concept of using organic reactions to modify the solid surface, particularly to modify the semiconductor C (100), Si (100), and Ge (100) surfaces for target applications in numerous fields such as microelectronics and heterogeneous photocatalysis.

1. Introduction Recent years have witnessed immense interest in organic functionalization of the surfaces of group IV semiconductors (diamond, silicon, and germanium) because of its potential applications in a variety of technological fields, such as molecular microelectronics, nonlinear optics, biological sensors, and photocatalysis.1,2 Incorporation of the vast range of functions of organic molecules into the surface of semiconductors could potentially introduce various novel physical and chemical properties into the matrix of semiconductors. As a result, the new semiconductor-based organic-inorganic hybrid multifunctional material would be achieved, which possesses both the physical functions of semiconductors and the chemical functions of organic functional groups.1,2 In particular, much abundant and fascinating chemistry has been revealed on the (100) surfaces of diamond, silicon, and germanium, which adopt the 21 reconstruction with the first-layer atoms dimerizing to form rows of XdX dimers (X=C, Si, and Ge). The chemical bonding in the *To whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Meyer zu Heringdorf, F. J.; Reuter, M. C.; Tromp, R. M. Nature (London) 2001, 412, 517. (b) Hamers, R. J. Nature (London) 2001, 412, 489. (c) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature (London) 2000, 406, 48. (d) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (e) Yates, J. T.Jr. Science 1998, 279, 335. (f) Piva, P. G.; Dilabio, G. A.; Pitters, J. L.; Zikovsky, J.; Rezeq, M.; Dogel, S.; Hofer, W. A.; Wolkow, R. A. Nature (London) 2005, 435, 658. (2) For reviews, see: (a) Bent, S. F. J. Phys. Chem. B 2002, 106, 2830. (b) Filler, M. A. Bent, S. F. Prog. Surf. Sci. 2003, 73, 1. (c) Bent, S. F. Surf. Sci. 2002, 500, 879. (d) Waltenburg, H. N.; Yates, J. T.Jr. Chem. Rev. 1995, 95, 1589. (e) Buriak, J. M. Chem. Rev. 2002, 102, 1271. (f) Duke, C. B. Chem. Rev. 1996, 96, 1237. (g) Hamers, R. J.; Wang, Y. Chem. Rev. 1996, 96, 1261. (h) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. S.; Russell, J. N.Jr Acc. Chem. Res. 2000, 33, 617. (i) Wolkow, R. A. Annu. Rev. Phys. Chem. 1999, 50, 413. (j) Lu, X.; Lin, M. C. Int. Rev. Phys. Chem. 2002, 21, 137. (k) Yoshinobu, J. Prog. Surf. Sci. 2004, 77, 37. (l) Tao, F.; Xu, G. Q. Acc. Chem. Res. 2004, 37, 882. (m) Buriak, J. M. Chem. Commun. 1999, 1051.

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surface XdX dimers features a σ bond and a much weaker π bond than that in ethylene. Thus, the semiconductors X (100) surfaces can be vividly depicted as a “a highly reactive and big organic molecular alkene”, implying that the chemistry of X (100) might show some similarity to the chemistry of alkenes.2 Accordingly, the typical chemistry reactions related to alkenes could be applied to organically functionalize the X (100) surface (X=C, Si, and Ge). This is indeed the case since the X (100) surface has been found to be subject to [2+2] cycloaddition with simple alkenes,3 1,3-dipolar cycloaddition with 1,3-dipolar molecules,4 and DielsAlder cycloaddition reaction with conjugated dienes.5 All of these strongly demonstrate that the semiconductor X (100) surfaces can be flexibly functionalized by means of traditional synthetic organic chemistry. (3) For example, see: (a) Hovis, J. S.; Coulter, S. K.; Hamers, R. J.; D’Evelyn, M. P.; Russell, J. N.; Bulter, J. E. J. Am. Chem. Soc. 2000, 122, 732. (b) Cho, J. H.; Kleinman, L. Phys. Rev. B 2003, 67, 115314. (c) Liu, H.; Hamers, R. J. J. Am. Chem. Soc. 1997, 119, 7593. (d) Lu, X.; Zhu, M.; Wang, X. J. Phys. Chem. B 2004, 108, 7359. (e) Miotto, R.; Ferraz, A. C.; Srivastava, G. P. Surf. Sci. 2002, 507-510, 12. (f) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D. M.; Wolkow, R. A. J. Am. Chem. Soc. 2000, 122, 3548. (g) Cho, J. H.; Kleinman, L. Phys. Rev. B 2001, 64, 235420. (h) Fink, A.; Huber, R.; Widdra, W. J. J. Chem. Phys. 2001, 115, 2768. (4) For example, see: (a) Barriocanal, J. A.; Doren, D. J. J. Phys. Chem. B 2000, 104, 12269. (b) Barriocanal, J. A.; Doren, D. J. J. Vac. Sci. Technol. A 2000, 18, 1959. (c) Lu, X.; Xu, X.; Wang, N.; Zhang, Q. J. Phys. Chem. B 2002, 106, 5972. (d) Lu, X.; Xu, X.; Wang, N.; Zhang, Q. J. Org. Chem. 2002, 67, 515. (e) Lu, X.; Fu, G.; Wang, N.; Zhang, Q.; Lin, M. C. Chem. Phys. Lett. 2003, 371, 172. (5) For example, see: (a) Fitzgerald, D. R.; Doren, D. J. J. Am. Chem. Soc. 2000, 122, 12334. (b) Okamoto, Y. J. Phys. Chem. B 2001, 105, 1813. (c) Wang, G. T.; Bent, S. F.; Russell, J. N.Jr; Butler, J. E.; D’Evelyn, M. P. J. Am. Chem. Soc. 2000, 122, 744. (d) Konecny, R.; Doren, D. J. J. Am. Chem. Soc. 1997, 119, 11098. (e) Teplyakov, A. V.; Kong, M. J.; Bent, S. F. J. Am. Chem. Soc. 1997, 119, 11100. (f) Hovis, J. S.; Liu, H. B.; Hamers, R. J. J. Phys. Chem. B 1998, 102, 6873. (g) Choi, C. H.; Gorden, M. S. J. Am. Chem. Soc. 1999, 121, 11311. (h) Lu, X.; Zhu, M.; Wang, X.; Zhang, Q. J. Phys. Chem. B 2004, 108, 4478. (i) Lu, X.; Wang, X.; Yuan, Q.; Zhang, Q. J. Am. Chem. Soc. 2003, 125, 7923. (j) Fink, A.; Menzel, D.; Widdra, W. J. Phys. Chem. B 2001, 105, 3828. (k) Mui, C.; Bent, S. F.; Musgrave, C. B. J. Phys. Chem. A 2000, 104, 2457. (l) Lee, S. W.; Nelen, L. N.; Ihm, H.; Scoggins, T.; Greenlief, C. M. Surf. Sci. 1998, 410, L773.

Published on Web 06/05/2009

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Scheme 1. Sketch of Functionalization of X (100) (X = C, Si, and Ge), Fullerene, and Carbon Nanotube Using Traditional [3 + 2] Cycloaddition of Transition Metal Oxides to Alkenes

We have recently predicted, by effective cluster models, that the cycloaddition reactions of carbenes and nitrenes to alkenes and the epoxidation of alkenes by dioxiranes in organic chemistry can be used as new types of surface reaction to functionalize the X (100) surface (X=C, Si, and Ge).6,7 In particular, the prediction on organic functionalization of the diamond (100) surface by cycloaddition of carbenes has been confirmed experimentally by the elegant research work reported by Foord’s research group.8 Clearly, theoretical and experimental chemistry is able to aid each other, making it more effective to design novel semiconductorbased materials with a clear target that represents a very active research field at the border between organic chemistry and material science. In organic chemistry, the concerted [3 + 2] cycloaddition of transition metal oxides (such as OsO4, MnO4 -, and RuO4) to alkenes represents a prominent paradigm for transition-metalmediated oxygen transfer reaction, and its catalytic asymmetric form has proven to be a powerful method for enantioselective synthesis.9 Analogous chemical reaction was successfully (6) (a) Xu, Y. J.; Zhang, Y. F.; Li, J. Q. J. Org. Chem. 2005, 70, 6089. (b) Xu, Y. J.; Zhang, Y. F.; Li, J. Q. J. Org. Chem. 2005, 70, 7773. (7) (a) Xu, Y. J.; Zhang, Y. F.; Li, J. Q. J. Phys. Chem. B 2006, 110, 3197. (b) Xu, Y. J.; Zhang, Y. F.; Li, J. Q. J. Phys. Chem. B 2006, 110, 6148. (c) Xu, Y. J.; Zhang, Y. F.; Li, J. Q. J. Phys. Chem. B 2006, 110, 13931. (d) Xu, Y. J.; Zhang, Y. F.; Li, J. Q. J. Phys. Chem. C 2007, 111, 3729. (8) Wang, H.; Griffiths, J.; Egdell, R. G.; Moloney, M. G.; Foord, J. S. Langmuir 2008, 24, 862. (9) (a) Deubel, D. V.; Frenking, G. Acc. Chem. Res. 2003, 36, 645. (b) Pidun, U.; Boehme, C.; Frenking, G. Angew. Chem., Int. Ed. 1996, 35, 2817. (c) Dapprich, S.; Ujaque, G.; Maseras, F.; Lledos, A.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc. 1996, 118, 11660. (d) Torrent, M.; Deng, L.; Duran, M.; Sola, M.; Ziegler, T. Organometallics 1997, 16, 13. (e) Del Monte, A. J.; Haller, J.; Houk, K. N.; Sharpless, K. B.; Singleton, D. A.; Strassner, T.; Thomas, A. A. J. Am. Chem. Soc. 1997, 119, 9907. (f) Schroeder, M. Chem. Rev. 1980, 80, 187. (g) Kolb, H. C.; Van Nieuwenhze, M. S.; Sharpless, K. B. Chem. Rev. 1994, 94, 2483. (h) Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2024. (10) (a) Hawkins, J. M.; Meyer, A.; Lewis, T. A.; Loren, S.; Hollander, F. J. Science 1991, 252, 312. (b) Hawkins, J. M.; Lewis, T. A.; Loren, S. D.; Meyer, A.; Heath, J. R.; Shibato, Y.; Saykally, R. J. J. Org. Chem. 1990, 55, 6250. (c) Hawkins, J. M. Acc. Chem. Res. 1992, 25, 150. (d) Hawkins, J. M.; Meyer, A. Science 1993, 260, 1918. (e) Balch, A. L.; Olmstead, M. M. Chem. Rev. 1998, 98, 2123.

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extended to functionalize fullerenes and singled-walled carbon nanotubes (SWCNTs) which can be regarded as featuring a similar but less reactive CdC bonding motif on the sidewall than that of alkenes.10,11 For example, osmylation of fullerene with osmium tetraoxide (OsO4) gave birth to the first exohedral fullerene-metal complex single crystal.10 Sidewall osmylation of individual metallic SWCNTs was achieved by exposing the nanotubes to OsO4 vapor under UV photoirradiation; the covalent attachment of OsO4 leads to an increase in the electrical resistance by up to several orders of magnitude.11a Inspired by these chemical precedents, we infer that similar process may proceed on the X (100) surface (X=C, Si, and Ge) since they all feature a much more reactive surface XdX dimer than that of alkenes, fullerenes, and SWCNTs, as depicted in Scheme 1. Such an inference has been confirmed by the present theoretical calculations. Our results demonstrate the [3+2] cycloaddition of transition metal oxides, such as OsO4, MnO4 -, and RuO4, to alkenes can be applied as another new type of surface reaction to modify the semiconductor surfaces of X (100) (X=C, Si, and Ge), which may impart other new functionalities to the semiconductor surface and offer the new possibility of selfassembly on semiconductor surfaces. The as-formed adduct can also act as a good starting point for further manipulations for various potential applications. More significantly, the functionalization of X (100) by cycloaddition of transition metal oxides provides the molecular basis for preparation of semiconductorsupported catalysts but also strongly advances the concept of using organic reactions to modify the solid surface, particularly to modify the semiconductor C (100), Si (100), and Ge (100) surfaces because they behave very much like molecular alkenes.2-7 Bearing in mind that analogous chemistry can lie in materials featuring an analogous bonding motif, reasonable implications for other future work are also discussed and prospected. (11) (a) Cui, J.; Burghard, M.; Kern, K. Nano Lett. 2003, 3, 615. (b) Lu, X.; Tian, F.; Feng, Y.; Xu, X.; Wang, N.; Zhang, Q. Nano Lett. 2002, 2, 1235. (c) Banerjee, S.; Wong, S. S. J. Am. Chem. Soc. 2004, 126, 2073.

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Figure 1. Highest occupied molecular orbital (HOMO) of final products for the addition of MnO4 -, OsO4, and RuO4 onto the X (100) surface (X=C, Si, and Ge).

2. Computational Details and Models As previously done, a X9H12 (X=C, Si, and Ge) cluster model (Figure S1) was employed to model a dimer site of the X (100) surface.4-7 Such a modeling scheme has been successfully and widely used to predict the surface reaction on X (100), for example, the additions of carbenes and nitrenes onto the X (100) 9842 DOI: 10.1021/la900942e

surface and the epoxidation of C (100) by dioxiranes.6,7 It has been demonstrated that size of clusters plays a negligible effect on the predicted trend or reaction energies.4-7 This one-dimer cluster consists of two surface carbon, silicon, or germanium atoms representing the surface dimer and seven atoms representing three layers of subsurface bulk atoms. The dangling bonds of the Langmuir 2009, 25(17), 9840–9846

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subsurface atoms are terminated by a total of 12 hydrogen atoms. Theoretically, this cluster model fulfills the requirements of stoichiometry principle, neutrality principle, and coordination principle so that it can be concluded as a good model of choice.12 Three molecules, namely MnO4 -, OsO4, and RuO4, were chosen as representative model molecules of transition metal oxides, as shown in Figure S1. To simulate the addition of transition metal oxides onto the semiconductor surfaces of X (100) (X=C, Si, and Ge) theoretically, the B3LYP method,13 which is a combination of HF with a DFT based on the Becke three-parameter exchange coupled with the Lee-Yang-Parr (LYP) correlation potential, has been used in our calculations. Previous theoretical studies proved that this method in conjunction with an effective X9H12 cluster model can be effective and reliable in describing the surface reaction energetics and mechanisms on the X (100) surfaces, and in particular, it is very useful in qualitatively predicting the possibility of a given reaction on the X (100) surface.4-7 In principle, cluster model calculations ought to be the most straightforward method to study surface processes theoretically in which the electronic properties are described in terms of local orbitals, allowing one to treat problems occurring on a solid surface with typical language of chemistry, the language of orbitals.6,7,14 Cluster models tend to simulate single isolated adsorbed species at the low surface coverage regime, while periodic slab models are better suited to model adsorbate overlayers and the substrate band structure at higher coverage. Objectively speaking, both methods have their respective advantages and can offer complementary information for each other.6,7,14 Moreover, cluster models present advantages in terms of a compromise between accuracy and computational cost if clusters are appropriately selected. For C, Si, Ge, O, and H atoms, the standard all-electron splitvalence 6-311G(2d, 2p) basis set was used, while the effective core potential (ECP) of the LANL2DZ basis set was used for Mn, Os, and Ru transition metal atoms. Geometry optimizations without constrained degrees of freedom were carried out using analytical gradients and the Berny algorithm. Using this theoretical approach, geometry optimizations reveal that the CdC dimer in the C9H12 cluster is symmetric while the SidSi and GedGe dimers in the Si9H12 and Ge9H12 cluster are asymmetric as can be clearly seen from the optimized geometry and the highest occupied molecular orbitals (HOMOs), as depicted in Figure S1. These results are in good agreement with the experimental observation that, at low temperatures, the surface dimers on Si (100) and Ge(100) are bulked, whereas those on the C (100) surface are symmetric.2 All of the calculations were implemented with the Gaussian-98 suite of programs.15 (12) Xu, X.; Nakatsuji, H.; Lu, X.; Ehara, M.; Cai, Y.; Wang, N. Q.; Zhang, Q. E. Theor. Chem. Acc. 1999, 172, 170. (13) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. A 1988, 37, 785. (14) (a) Pacchioni, G. Surf. Rev. Lett. 2000, 7, 277. (b) Xu, Y. J.; Li, J. Q.; Zhang, Y. F.; Chen, W. K. J. Chem. Phys. 2004, 120, 8753. (c) Xu, Y. J.; Li, J. Q.; Zhang, Y. F. Surf. Rev. Lett. 2003, 10, 691. (d) Xu, Y. J.; Li, J. Q. Chem. Phys. Lett. 2005, 406, 249. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian, Inc., Pittsburgh, PA, 1998.

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3. Results and Discussion Figure S2 lists the optimized geometries and calculated reaction energies for the cycloaddition and complexation of transition metal oxides, i.e., MnO4 -, OsO4, and RuO4, with the semiconductor surface of C (100), Si (100), and Ge (100), respectively. As can be clearly seen, all of the reactions are highly exothermic. In addition, note that the oxidation power of MnO4 - and RuO4 is stronger than OsO4, as reflected by the higher exothermicity for the cycloadditions of MnO4 - and RuO4 onto the surface dimer of C (100), Si (100), and Ge (100). For example, the addition of MnO4 - and RuO4 onto the C (100) surface is highly exothermic by -108.6 and -118.4 kcal/mol, respectively, which is much higher than -84.4 kcal/mol for that of OsO4. In addition, similar to the osmylation of fullerene and SWCNT with OsO4,10,11 it is reasonable to believe that the functionality resulting from the addition of transition metal oxides (MnO4 -, OsO4, and RuO4) onto the X (100) (X=C, Si, and Ge) surface could be finely tuned by the ligand on the transition metal center. For comparison, at the same level of theory, the cycloaddition between ethylene and transition metal oxide (MnO4-, OsO4, or RuO4) is also calculated. Figure 2 displays the reaction profile for the cycloaddition reaction between ethylene and OsO4, for which the predicted reaction energy is -26.6 kcal/mol along with a low activation energy barrier around +7.1 kcal/mol. Obviously, the exothermicity of cycloaddition of OsO4 onto the C (100) surface is significantly enhanced, which is about 58 kcal/mol higher in energy than that between OsO4 and ethylene, indicating the high surface reactivity of C (100). This can be ascribed to the higher reactivity of the surface dimer of C (100) than ethylene due to the reduced overlap of p orbitals.3-7 In analogy, the exothermicity of cycloaddition of OsO4 onto Si (100) and Ge (100) is about 59 and 36 kcal/mol higher in energy than that between OsO4 and ethylene, respectively. In particular, it should be mentioned that no transition state is located for the cycloadditions of OsO4, MnO4-, and RuO4 onto the C (100), Si (100), and Ge (100) surfaces as compared to their ethylene molecular analogues, as seen from Figure S2 and Figure 2. Analogous barrierless chemical processes were also observed for the additions of carbenes and nitrenes onto X (100) (X=C, Si, and Ge),6 the hydroboration of X (100),16 and the epoxidation of C (100) with dioxiranes.7b This indicates that all of these cycloaddition reactions on the semiconductor surface of X (100) are unactivated, hence strongly suggesting the reactivity of the surface dimer of X (100) is much higher than ethylene. These results are quite reasonable since the surface dimer π-bond strength is 5-10 kcal/mol for Si (100) and Ge (100)17 and ∼28 kcal/mol for C (100),18 which are much smaller than the π-bond strength of 56 kcal/mol in ethylene.7a Therefore, the easiness of the cycloaddition of transition metal oxides onto the semiconductor surfaces of X (100) (X=C, Si, and Ge), both thermodynamically and kinetically, should be reasonably expected. Such a remarkable enhancement of the reactivity was also found in previous studies, such as the cycloadditions of carbenes and nitrenes onto the X (100) surface (X=C, Si, and Ge),6 Diels-Alder cycloadditions of X (100) with conjugated dienes,5 the 1,3-dipolar cycloadditions of X (100) with 1,3-dipolar molecules,4 the [2+2] cycloaddition with simple alkenes,3 and epoxidation of C (100) with dioxiranes.7b All of these indicate that the XdX dimer of X (100) can be regarded as a highly reactive and big molecular alkene. (16) (a) Xu, Y. J.; Li, J. Q. Appl. Surf. Sci. 2006, 252, 5855. (b) Long, L.; Lu, X.; Tian, F.; Zhang, Q. J. Org. Chem. 2003, 68, 4495. (17) (a) D’Evelyn, M. P.; Cohen, S. M.; Rouchouze, E.; Yang, Y. L. J. Chem. Phys. 1993, 98, 3560. (b) Doren, D. J. Adv. Chem. Phys. 1996, 95, 1. (18) Hukka, T. I.; Pakkanen, T. A.; D’Evelyn, M. P. J. Phys. Chem. 1994, 98, 12420.

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Figure 2. Reaction profiles of cycloadditions of MnO4 -, OsO4, and RuO4 to ethylene (units in kcal/mol for reaction energies) calculated at the B3LYP/6-311G(2d,2p) level of theory.

Notably, regarding the addition of RuO4 to ethylene, no transition state is located, which can be understood by the more powerful oxidation capability of RuO4 than OsO4 and MnO4 -, as 9844 DOI: 10.1021/la900942e

reflected by the much higher exothermicity of -59.1 kcal/mol than -26.6 kcal/mol and -47.1 kcal/mol for the addition of OsO4 and MnO4 - to ethylene, respectively, as shown in Figure 2. Langmuir 2009, 25(17), 9840–9846

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Figure 3. Prospective sketch of functionalization of various materials featuring an analogous bonding motif to alkenes using traditional [3+2] cycloaddition of transition metal oxides to alkenes.

In addition, it should be mentioned that all of the [3+2] cycloadditions of OsO4, MnO4 -, and RuO4 to ethylene follow a concerted manner. However, the case is different regarding that of transition metal oxides onto the surface dimer of X (100) (X= C, Si, and Ge) due to the specific structure feature of the surface dimer of X (100). The cycloaddition process of transition metal oxides onto the surface dimer of C (100) is quite similar to that of ethylene. Namely, the bond formation of transition metal oxides with the surface symmetric CdC dimer follows a concerted pathway. This can be easily understood since the geometry of the surface CdC dimer of C (100) is quite similar to ethylene; the primary difference between them is that the surface CdC dimer is much more reactive. This also leads to the fact that the cycloadditions of transition metal oxides onto X (100) is barrierless as mentioned above. Nevertheless, the cycloaddition of transition metal oxides onto Si (100) and Ge (100) is different, which follows a stepwise manner. This is quite reasonable because of the surface SidSi and GedGe dimers feature an asymmetric/distorted structure, which results in the manner of bond formation processs of transition metal oxides with the surface of Si (100) and Ge (100) different from that of C (100). Importantly, if such a chemical process could be experimentally realized, it may well be the first step in the atomic layer deposition (ALD) growth of transition metal oxides films on the X (100) surface;19 consequently, it would lead to new hybrid multifunctional materials, which could find specific applications in some potential fields, such as microelectronics and photocatalysis. Figure 1 displays the highest occupied molecular orbitals (HOMOs) of the product for the cycloadditions of MnO4-, OsO4, and RuO4 onto C (100), Si (100), and Ge (100). From them, we can briefly conclude the formation of chemical bonding between transition metal oxides and the X (100) surface (X=C, Si, and Ge). As a result of the additions of MnO4-, OsO4, and (19) Shen, J.; Gai, Z.; Kirschner, J. Surf. Sci. Rep. 2004, 52, 163.

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RuO4 onto Si (100) and Ge (100), the asymmetric surface dimer develops into the symmetric one, as reinforced by the optimized geometries (Figure S2) and their corresponding HOMOs depicted in Figure 1. The mantra of chemistry;“similar structure dictates analogous function”;stimulates us to propose implications for other relative and future work. As the heavier molecular analogues of alkenes, disilenes, digermenes, silenes, and germenes all have bonding motifs similar to that of alkenes, fullerenes, SWCNTs, and the X (100) surface (X=C, Si, and Ge). 19,20 Namely, the typical bonding motifs of all of these substances feature a strong σ bond and a weak π bond, hence implying that analogous chemistry might exist among them. 2-7,20,21 However, analogous cycloaddition reactions of transition metal oxides to alkenes have not been reported regarding disilenes, digermenes, silenes, and germenes. Theoretical studies in this regard would add meaningful richness to the chemistry of disilenes, digermenes, silenes, and germenes as well as offer instructive information for experiments, thus contributing to a joint experiment and theory study on the heavier molecular analogues of alkenes. Of course, it should be noted that, due to the difference in detailed structure among them, the reaction mechanisms such as reaction pathways and activation energy barrier should be different. Disilenes, digermenes, silenes, and germenes have the trans-bending structure, different from the planar structure of ethylene.20,21 (20) (a) Xu, Y. J.; Zhang, Y. F.; Li, J. Q. Chem. Phys. Lett. 2006, 421, 36. (b) Su, M. D. J. Phys. Chem. A 2004, 108, 823. (c) Su, M. D. Inorg. Chem. 2004, 43, 4846. (d) Kira, M.; Ishima, T.; Iwamoto, T.; Ichinohe, M. J. Am. Chem. Soc. 2001, 123, 1676. (e) Mosey, N. J.; Baines, K. M.; Woo, T. K. J. Am. Chem. Soc. 2002, 124, 13306. (21) For reviews on the heavier molecular analogues of alkenes, see: (a) Kira, M. J. Organomet. Chem. 2004, 689, 4475. (b) Raabe, G.; Michl, J. Chem. Rev. 1985, 85, 419. (c) West, R. Pure Appl. Chem. 1984, 56, 163. (d) Cowley, A. H.; Norman, N. C. Prog. Inorg. Chem. 1986, 34, 1. (e) Grev, R. S. Adv. Organomet. Chem. 1991, 33, 125. (f) Weidenburch, M. Coord. Chem. Rev. 1994, 130, 275. (g) Kira, M. Pure Appl. Chem. 2000, 72, 2333. (h) Power, P. P. Chem. Rev. 1999, 99, 3463. (i) Kira, M.; Iwamoto, T. J. Organomet. Chem. 2000, 610, 236.

DOI: 10.1021/la900942e

9845

Article

Xu and Fu

Therefore, it could be anticipated that the detailed reaction pathways for cycloadditions of transition metal oxides should be different between the case of ethylene and their heavier molecular analogues (disilenes, digermenes, silenes, and germenes). However, the qualitative feasibility of functionalization of these structure-like substances by using traditional cycloadditions of transition metal oxides can be expected (as displayed in Figure 3), which has been clearly witnessed by previous chemical precedents in this regard.2-7,20,21 Finally, the present cluster model results can be used as a starting geometry input for periodic calculations of monolayer transition metal oxides on the semiconductor X (100) surface (X=C, Si, and Ge) with different coverages. Periodic results will further provide us with the detailed information on the change of energy band structure resulting from the addition of transition metal oxides onto the X (100) surface. Knowledge in this aspect would offer fundamental information at a molecular level for understanding and steering toward the applications of X (100)-supported transition metal oxides in heterogeneous photocatalysis.22

4. Concluding Remarks In summary, we have predicted the viability of functionalization of the group IV semiconductor surfaces of diamond (100), Si (100), and Ge (100) by traditional [3 + 2] cycloaddition of transition metal oxides by means of effective cluster models in conjunction with density functional theory. Our results reveal that the cycloaddition of transition metal oxides (OsO4, RuO4, and MnO4 -) onto the X (100) (X=C, Si, and Ge) surface is much more facile than that of other molecular analogues including ethylene, fullerene, and single-walled carbon nanotube because of the high reactivity of surface dimers of X (100). That is, the cycloaddition of transition metal oxides onto X (100) is not only highly exothermic but also is a barrierless process. Our computational results clearly demonstrate the plausibility that the wellknown [3+2] cycloaddition of transition metal oxides to alkenes in organic chemistry can be applied as a new type of surface reaction to functionalize the semiconductor X (100) surfaces, which would offer the new possibility for self-assembly or chemical functionalization of X (100) at low temperature. More significantly, the chemical functionalization of X (100) by cycloaddition of transition metal oxides provides the molecular basis for (22) Apatiga, L. M.; Casta~no, V. M. Appl. Phys. Lett. 2003, 83, 4542.

9846 DOI: 10.1021/la900942e

preparation of semiconductor-supported catalysts but also strongly advances the concept of using organic reactions to modify the solid surface, particularly to modify the semiconductor C (100), Si (100), and Ge (100) surfaces for target applications in many fields.2-7 In view of the structural similarity of disilenes, digermenes, silenes, and germenes to that of alkenes, X (100) (X=C, Si, and Ge), fullerene, and single-walled carbon nanotube,19,20 it can be prospected that analogous cycloaddition reactions of transition metal oxides might also be extended to functionalize these substances, which would add much richness to the chemistry of ethylene heavier molecular analogues (disilenes, digermenes, silenes, and germenes) and the chemistry of the X (100) surface, fullerene, and carbon nanotubes. Interest in all of these aspects both theoretically and experimentally will significantly reinforce the well-known mantra in chemistry domain; i.e., similar chemistry can lie in materials that feature an analogous bonding motif. Indeed, this has been witnessed by the functionalization of fullerenes and carbon nanotubes using traditional inorganic/organic reactions associated with alkenes during the past two decades. It is hoped that, with the help of combination of advanced computational simulations and experimental methods, such intriguing chemistry can be well enriched among all of these structure-like substances. Finally, we are looking forward to experimental realization of the reaction predicted herein with the increasing powerfulness of today’s experimental approaches and the potential applications of as-functionalized semiconductor surfaces in potential fields, for example, heterogeneous photocatalysis.22 Acknowledgment. The support of Award Program of Minjiang Scholar Professorship, Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT0818), and the Natural Science Foundation of China is gratefully acknowledged. Supporting Information Available: Optimized geometries of MnO4 -, OsO4, and RuO4; cluster models of X9H12 (X= C, Si, and Ge) and their corresponding highest occupied molecular orbitals (HOMOs); optimized geometries and calculated reaction energies for the addition of MnO4 -, OsO4, and RuO4 onto the X (100) surface (X=C, Si, and Ge). This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2009, 25(17), 9840–9846