Langmuir 2004, 20, 6359-6364
6359
Hidden Electrochemistry in the Thermal Grafting of Silicon Surfaces from Grignard Reagents Samira Fellah, Rabah Boukherroub,† Franc¸ ois Ozanam,* and Jean-Noe¨l Chazalviel Laboratoire de Physique de la Matie` re Condense´ e, CNRS-E Ä cole Polytechnique, 91128 Palaiseau Cedex, France Received February 6, 2004. In Final Form: May 10, 2004 Covalent grafting of alkyl chains on silicon can be obtained by thermal treatment in Grignard reagents. Alkyl halide present in the Grignard solution as an impurity appears to play a key role in the grafting process. Grafting efficiency is improved when the alkyl halide concentration is increased. It is also enhanced on n-type substrates as compared to p-type substrates and when alkyl bromides are present in solution rather than alkyl chlorides. The grafting reaction involves a zero-current electrochemical step. A reaction model in which simultaneous Grignard oxidation and alkyl halide reduction take place at the silicon surface accounts for all these observations. Alkyl halide reduction is the rate-determining step. Negative charging of the silicon surface lowers the energetic barrier for this reaction, allowing for efficient grafting on n-Si.
Introduction In the last 10 years, the covalent grafting of organic species onto a silicon surface through robust Si-C bonds progressively became a subject of intensive work.1,2 Such a possibility offers a convenient means for chemically stabilizing the silicon surface or conferring it specific functions or properties, for example, for chemical sensing. Most of the chemical routes to achieve silicon derivatization start from the hydrogenated surface, taking profit from its mild reactivity. The net result from such processing is the substitution of the initial Si-H groups by Si-R surface species, where R is an organic group of interest. Such a substitution is often the result of a hydrosilylation reaction, the organic function being brought from an alkene precursor and the reaction being activated by heating (with or without3,4 a catalyst5,6 or a chemical promoter7,8), UV irradiation,9,10 or even mechanical scratching.11 Other routes include electrochemical activation of * Corresponding author. E-mail: francois.ozanam@ polytechnique.fr. † Present address: Interdisciplinary Research Institute, IEMN, Cite´ Scientifique, Avenue Poincare´, BP 69, 59652 Villeneuve d’Ascq Cedex, France. (1) Buriak, J. M. Chem. Rev. 2002, 102, 1271. (2) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 2 2002, 23. (3) Sung, M. M.; Kluth, J.; Yauw, O. W.; Maboudian, R. Langmuir 1997, 13, 6164. (4) Sieval, A. B.; Demirel, L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759. (5) Buriak, J. M.; Stewart, M. P.; Geders, T. W.; Allen, M. J.; Choi, H. C.; Smith, J.; Raftery, D.; Canham, L. T. J. Am. Chem. Soc. 1999, 121, 11491. (6) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831. (7) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (8) Schmeltzer, J. M.; Porter, L. A., Jr.; Stewart, M. P.; Buriak, J. M. Langmuir 2002, 18, 2971. (9) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (10) Cicero, R. L.; Chidsey, C. E. D.; Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Langmuir 2002, 18, 305. (11) Niederhauser, T. L.; Jiang, G.; Lua, Y.-Y.; Dorff, M. J.; Woolley, A. T.; Asplund, M. C.; Berges, D. A.; Linford, M. R. Langmuir 2001, 17, 5889.
diazonium salts,12,13 alkynes,14 or alkyl halides15 on the hydrogenated surface or chemical reaction of organometallic compounds on the chlorinated surface.16-18 Finally, the reaction of the hydrogenated surface with Grignard reagents represents a special case, as it has been reported to yield alkylation of the surface either through thermal6 or electrochemical19 activation. The reaction mechanism has been studied in some detail in the case of the electrochemical grafting.20 However, it remains somewhat unclear in the case of the thermal reaction. The purpose of this work is precisely to clarify this issue. In the electrochemical modification, the reaction proceeds through reactive radicals generated by the oxidative decomposition of the Grignard reagent.20 The process is similar on flat Si(111) and porous silicon. Thermal reaction of Grignard reagents with the hydrogenated silicon surface does not take place in the same way on porous and flat silicon. On porous silicon, in agreement with what is observed in organosilane chemistry, the reaction results in back-bond breaking of the surface silicon atoms and does not lead to a substitution of the initial surface SiH groups by SiR species.21 Here, we disregard this latter case. On the contrary, on flat Si, atomic force microscopy demonstrates that the surface morphology is not affected by the reaction and that a genuine substitution takes place, in agreement with infrared and X-ray photoelectron spectroscopy characterization of the grafted surface. (12) Henry de Villeneuve, C.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415. (13) Allongue, P.; Henry de Villeneuve, C.; Cherouvrier, G.; Corte`s, R.; Bernard, M.-C. J. Electroanal. Chem. 2003, 550-551, 161. (14) Robins, E. G.; Stewart, M. P.; Buriak, J. M. Chem. Commun. 1999, 2479. (15) Gurtner, C.; Wun, A. W.; Sailor, M. J. Angew. Chem., Int. Ed. 1999, 38, 1966. (16) Bansal, A.; Li, X.; Lauermann, I.; Lewis, N. S.; Yi, S. I.; Weinberg, W. H. J. Am. Chem. Soc. 1996, 118, 7225. (17) He, J.; Patitsas, S. N.; Preston, K. F.; Wolkow, R. A.; Wayner, D. D. M. Chem. Phys. Lett. 1998, 286, 508. (18) Terry, J.; Linford, M. R.; Wigren; C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. J. Appl. Phys. 1999, 85, 213. (19) Fide´lis, A.; Ozanam, F.; Chazalviel, J.-N. Surf. Sci. 2000, 444, L7. (20) Fellah, S.; Teyssot, A.; Ozanam, F.; Chazalviel, J.-N.; Vigneron, J.; Etcheberry, A. Langmuir 2002, 18, 5851. (21) Kim, N. Y.; Laibinis, P. E. J. Am. Chem. Soc. 1998, 120, 4516.
10.1021/la049672j CCC: $27.50 © 2004 American Chemical Society Published on Web 06/22/2004
6360
Langmuir, Vol. 20, No. 15, 2004
However, how does such a substitution take place? Though conceivable, classical nucleophilic substitution schemes are not favored at a surface due to steric hindrance limitations. Alternately, one might assume that homolytic dissociation of the Grignard takes place according to an equilibrium of the kind RMgX a R• + •MgX. However, in general, this scheme is thought not to occur, as homopolar scission of the organometallic bond of the Grignard does not usually involve release of free radicals into the solution, except when initiated by a coreactant.22 Therefore, it appears reasonable to invoke the implication of an impurity in the reaction, either in order to favor the generation of reactive radicals through homolytic dissociation of the Grignard reagents or in order to make their oxidative decomposition possible. In the latter case, the basic idea would be the existence of an electrochemical mechanism at zero current, allowing for the reductive decomposition of an impurity in order to balance the oxidative decomposition of the Grignard. A possible candidate for this impurity would be the alkyl halide compound RX used as the precursor of the Grignard reagent RMgX synthesis. It is indeed probable that the alkyl halide remains at least at trace levels in the Grignard solution, and such compounds are likely to be more easily reducible than Grignards themselves. Therefore, we will first show that alkyl halides are indeed present in the Grignard solutions used for grafting reactions, and we will then examine how their concentration affects the grafting efficiency after thermal reaction with hydrogenated silicon surfaces and try to discriminate between an electrochemical or a chemical rate-limiting step in the mechanism of the overall reaction. Then, a reaction model will be worked out and finally discussed, with special attention to the thermodynamic properties of the Grignard solutions and the semiconducting properties of the Si substrate. Experimental Section Substrate Preparation and Infrared Characterization. Unless otherwise specified, we performed all our experiments on (111)-oriented, double-side-polished, float-zone-purified, p-type Si (p-Si) crystals of medium resistivity (5-50 Ω cm). The crystals were cut in trapezoid shapes of the typical dimensions 15 × 15 × 0.5 mm3, to allow for infrared characterization in attenuated total reflection (ATR) geometry. A part of the prism surfaces not probed by the infrared beam during the characterization was scratched using carborundum paper in order to act as a sacrificial anode in the etching process for preparing the hydrogenated surface.23 Prior to modification, the hydrogenated surface was prepared by cleaning in a hot (1:1 H2SO4/H2O2) mixture and etching in deoxygenated 40% NH4F solution. Grafting Procedure. Grignard solutions and alkyl halides were purchased from Aldrich and used without further purification. Unless otherwise specified, they were 1 mol/L solutions in diethyl ether. Hydrogenated silicon substrates were introduced under argon into a Schlenk tube containing the reaction solution. The Schlenk tube was left overnight for reaction at 90 °C. Silicon was then removed from the reacting solution and rinsed successively in anhydrous diethyl ether, dichloromethane, and water. Electrochemical Characterization. Voltammetric characterization of the Grignard and reacting solutions was performed in a closed electrochemical cell, filled in a glovebox. This cell consisted of a glass tube with a classical three-electrode arrangement, the working and counter electrodes being Pt wires. Our Grignard solutions were conductive enough to be used as electrolytes without addition of a supporting salt. In these (22) Kharasch, M. S.; Reinmuth, O. Grignard reactions of nonmetallic substances; Prentice Hall: New York, 1954; pp 114-117. (23) Allongue, P.; Henry de Villeneuve, C.; Morin, S.; Boukherroub, R.; Wayner, D. D. M. Electrochim. Acta 2000, 45, 4591.
Fellah et al.
Figure 1. Infrared spectra of grafted p-Si surfaces in the stretching CH region. The surfaces were treated overnight at 90 °C in as received 1 M C10H21MgBr in diethyl ether (a), 0.9 M C10H21MgBr + 0.1 M C10H21Br in diethyl ether (b), and 1 M C10H21MgBr in diethyl ether stirred 16 h in the presence of Mg powder (c). For clarity, curve b is shifted by +0.0002 and curve c by -0.0002. electrolytes, the reference electrode was a Pt wire in situ covered with magnesium by electrochemical deposition. When alkyl halide was added to the electrolyte, this magnesium reference was not stable anymore. A silver wire covered with silver chloride by anodization in an aqueous KCl solution was used instead as a pseudoreference.
Results Alkyl Halides in the Grignard Solution. First, one may question the presence of alkyl halides in the grafting solutions, since, in many Grignard reagents, organohalides react with their magnesium organocompound through the coupling reaction RMgX + RX f R2 + MgX2, especially at high temperature. Nevertheless, this reaction takes place for alkyl halides and their Grignard reagents in the presence of an oxidized transition-metal catalyst only.24 Our experimental cleaning procedures strictly prohibit the introduction of such a contamination. Therefore, we regard the presence of such a metallic impurity as highly unlikely and consider that the coupling reaction, even under our experimental conditions, is unimportant. We have indeed verified that, in the Grignard solutions used for the thermal grafting, alkyl halides are present in sizable amounts. Our solutions have been analyzed by vapor phase chromatography after hydrolysis in water, and alkyl halide was found in addition to the alkane produced by hydrolysis. Quantitative analysis shows that the concentration of alkyl halide in our Grignard solutions is on the order of 1% as compared to the alkylmagnesium halide concentration. Grafting in Grignard/Alkyl Halide Mixtures. Figure 1a shows that when a (111)-oriented p-Si surface is thermally treated for 16 h at 90 °C in 1 M decylmagnesium bromide, a partial substitution of the initial surface SiH species by alkyl chains occurs. There is no apparent oxidation of the surface, as shown by the absence of contribution in the 1000-1100 cm-1 wavenumber range in which νSiOSi vibrations are conveniently observed. The amount of grafted alkyl chains can be evaluated from the intensity of the νasCH2 vibration at 2922 cm-1 and calibration of the absorption cross section in a liquid alkane (here, tetradecane). A value of ∼1.2 × 1014 cm-2 is found, that is, a value corresponding to a substitution of ∼15% (24) Allen, R. B.; Lawler, R. G.; Ward, H. R. Tetrahedron Lett. 1973, 35, 3303.
Thermal Grafting of Si Surfaces
Langmuir, Vol. 20, No. 15, 2004 6361
for by the existence of two reactions in parallel during the thermal treatment of the hydrogenated surface in the alkyl halide RX. The first one would lead to grafting of the alkyl part of the reactant according to
tSisH + RX f tSisR + HX The second one would lead to surface halogenation, which would account for surface oxidation after hydrolysis of the silicon-halogen bond upon rinsing the sample: H2O
tSisH + RX f tSisX + RH 98 SiO2 Figure 2. Infrared spectrum of a p-Si surface after thermal treatment in pure C14H29Br. Notice the presence of vibrations characteristic of grafted C14H29 groups and of a significant surface oxidation.
of the initial SiH species, which is significantly lower than the maximum value 50% imposed by steric hindrance limitations. As shown in Figure 1b, the grafting efficiency is significantly improved when an alkyl halide (here, decyl bromide) is added to the Grignard reagent prior to modifying the hydrogenated silicon surface. In the present case (corresponding to treatment in a 0.9 M decylmagnesium bromide + 0.1 M decyl bromide solution), the areal density of grafted alkyl chains is found to be about twice as large and reaches ∼1.9 × 1014 cm-2. On the other hand, we have tried to lower as much as possible the amount of decyl bromide present in the Grignard solution. For such a purpose, finely divided clean metallic magnesium was added to a nominally pure 1 M decylmagnesium bromide solution and left under stirring overnight in order to lower the residual concentration of organohalide by formation of the Grignard compound. When such a solution is subsequently used for thermal treatment of the hydrogenated silicon surface, the efficiency of the grafting is significantly hindered, as shown in Figure 1c. The areal density of grafted alkyl chains is then found to be twice as low as that with nominally pure decylmagnesium bromide, decreasing to ∼0.6 × 1014 cm-2. The grafting efficiency of alkyl chains obtained after thermal treatment of the hydrogenated silicon (111) surface in a Grignard solution is found to be higher when the Grignard reagent is an alkylmagnesium bromide rather than an alkylmagnesium choride. For example, we have found that modification of p-Si under the usual conditions (90 °C, 16 h) in 1 M tetradecylmagnesium chloride leads to an areal density of grafted chains of ∼0.7 × 1014 cm-2 only. Interestingly, the efficiency is improved to a larger extent when an alkyl bromide is added to the Grignard solution rather than an alkyl chloride. For instance, in the case of tetradecylmagnesium chloride, the areal chain density can be increased up to ∼2.4 × 1014 cm-2 when tetradecyl bromide is added to the solution. Grafting Using Pure Alkyl Halides. It then appears that addition of alkyl halides to Grignard reagents significantly increases the grafting efficiency of alkyl chains and that their removal hinders the process. It is therefore tempting to examine what could be the role of alkyl halides in the grafting process, including in the absence of Grignard reagents. Figure 2 shows that thermal treatment of the hydrogenated p-Si surface in pure tetradecyl bromide actually results in tetradecyl grafting, as evidenced by the νSiH disappearance at 2083 cm-1 and the νCH buildup around 2900 cm-1 in the infrared spectrum, accompanied with a significant Si surface oxidation (presence of a broad νSiO vibration between 1000 and 1100 cm-1). Such a result might be accounted
It could be thought that these reactions are also prevailing in the presence of a Grignard reagent, with the only difference being that, in the second scheme, the siliconhalogen bond chemically reacts with the Grignard, avoiding its subsequent hydrolysis and surface oxidation. This would contribute to the grafting: RMgX
tSisH + RX f tSisX + RH 98 tSisR If relevant, the above reactions could only be overall reactions, the detailed steps remaining to be determined. However, without discussing these issues at the present stage, two main possibilities exist regarding the grafting mechanism in the presence of a Grignard reagent, depending on the nature of the rate-determining step. This step could be of chemical nature, for example, if it involves a classical nucleophilic substitution or an alkyl halide induced homopolar dissociation of the Grignard, or of electrochemical nature, for example, if it involves the electrochemical dissociation of the Grignard or of the alkyl halide. Before attempting to enter the details of the reaction mechanism, we are going to first discriminate between these two possibilities. Effect of Semiconductor Doping. A major difference between a chemical and an electrochemical reaction is that, in the latter case, it involves a charge transfer between a reactant and the silicon crystal. In a very reducing medium, such as in the presence of a Grignard reagent, oxidation kinetics are expected to be fast and it is likely that, if the rate-determining step is electrochemical, it consists of the reduction of a solution reactant. The charge transfer in such a reduction reaction could proceed either by hole injection into the silicon valence band or by electron capture from the silicon conduction band. If the reaction does not involve a strong oxidizer reactant, implausible in the presence of a Grignard reagent, its kinetics should sensitively depend on the silicon doping type. The energy level of a weak oxidizer reactant will indeed be at levels corresponding to the silicon conduction band, and the energy barrier for the reaction will be much lower for electron capture from the silicon conduction band than for hole injection into the silicon valence band. Therefore, the kinetics are expected to be much faster on n-type silicon in which conduction-band electrons are available for capture than on p-type silicon for which the conduction band is completely empty. Figure 3 shows that the grafting efficiency is actually much lower on p-Si than on n-Si. This clearly points to an electrochemical nature of the rate-determining step of the reaction. By the way, it also accounts for the difference in grafting efficiency between the above-reported results (see, e.g., Figure 1a) and the literature.6 In a control experiment, we checked that, if alkyl chains are grafted by UV-activated hydrosilylation (a reaction which is known to occur without any charge transfer9,10), the grafting efficiency is the same on n-Si and p-Si. It then becomes
6362
Langmuir, Vol. 20, No. 15, 2004
Fellah et al.
react with Grignard species and yield surface Si-R species according to
tSisX + RMgX f tSiR + MgX2
Figure 3. Infrared spectra of grafted n- and p-type silicon surfaces in the stretching CH region. The surfaces were treated overnight at 90 °C in as received 1 M C10H21MgBr in diethyl ether. For clarity, the curve for p-Si is shifted by -0.0001.
clear that the rate-determining step in the thermal grafting of alkyl chains from Grignard reagents onto a silicon surface is an electrochemical reaction. In view of the strong effect of alkyl halides on the grafting efficiency, the most plausible candidate for this rate-limiting reaction is the reductive decomposition of an alkyl halide present in the Grignard solution as an impurity. Model In view of the experimental results reported above, we propose a reaction scheme involving alkyl halide reduction. As for the electrochemical grafting from Grignard reagents, the proposed reaction scheme involves alkyl radicals as reactive species and consists of a three-step mechanism.20 The first step is an electrochemical step in which alkyl radicals are generated. During this step, one oxidation and one reduction reaction take place simultaneously and at the same rate, so that the net electrochemical current is null:
RMgX f R• + MgX+ + e-
(1)
RX + e- f R• + X-
(2)
In these reactions, e- represents an electron exchanged with the silicon conduction band (injected by the Grignard decomposition, captured upon alkyl halide reduction). Contrary to the case of the electrochemical method,20 this step is expected to be rate-limiting. The reactive alkyl radicals produced in the first step react with the surface during the two subsequent steps, leading to grafting:
tSisH + R• f tSi• + RH
(3)
tSi• + R• f tSisR
(4a)
As a matter of fact, the last step could proceed through at least two alternative pathways:
tSi• + RMgX f tSisR + MgX+ + e-
(4b)
(4d)
Discussing the last step of the scheme (reactions 4a-d) is beyond the scope of this report. The crucial point is that, since radicals are highly reactive species, reactions 3 and 4a-c are expected to be fast and not rate-limiting. In the first step, the two electrochemical reactions necessarily operate at the same rate, at least in the steady state. However, the energetic barrier for the alkyl halide reduction reaction is certainly higher than that of the oxidative decomposition of the Grignard reagent, so that the reduction reaction is expected to be rate-limiting (if reaction 4b is present, reaction 2 must actually proceed at a higher rate than reaction 1; this does not change the core of the discussion to follow and will be ignored further on). Also, a similar scheme could apply in the absence of a Grignard reagent, the Grignard oxidation, reaction 1, being replaced by silicon surface oxidation. The above reaction scheme straightforwardly accounts for the main experimental results reported above. First, it obviously explains the beneficial effect of adding alkyl halides in the reacting solutions. Since on one hand alkyl halides are present only as impurities in the standard Grignard solution but on the other hand, in the above scheme, their reduction is rate-limiting for the whole reaction, increasing the alkyl halide concentration should indeed make the grafting easier. Also, the alkyl halide reduction requires an electron supply from the Si substrate. On n-Si, such electrons are readily available at the bottom of the conduction band, that is, at an energy significantly higher than that of electrons lying in the valence band, the only ones available from a p-Si substrate. As a result, the reduction should be much easier on n-Si than on p-Si, in agreement with observations. Discussion Electrochemical Thermodynamic Considerations. To assess the plausibility of the proposed scheme, it is desirable to consider the energetic diagram of the system. Doing so appears rather difficult, due to the lack of relevant data in the literature. However, in the following, we try to draw a qualitative energetic picture from orders of magnitude estimates. According to the literature, the oxidative decomposition of an alkyl Grignard in THF takes place at a potential of ∼ -1.2 V with respect to Ag/Ag+25 or, equivalently, ∼ +1.5 V with respect to Mg/Mg2+.26 In view of the value of the normal potential of Ag/Ag+ with respect to the ferrocene/ ferricinium ion (Fc/Fc+) reference redox system,27 an alkyl Grignard in tetrahydrofuran (THF) undergoes oxidative decomposition at a potential of ∼ -1 V versus Fc/Fc+. This value is weakly sensitive to the nature of the halide or the length of the alkyl chain.25 The potential Vcb corresponding to the minimum of the silicon conduction band has been measured for a clean silicon electrode in acetonitrile and lies at -0.75 V versus Ag/Ag+,28 that is , -0.7 V versus Fc/Fc+.29 It means that the oxidative decomposition of the Grignard takes place at a potential ∼0.3 V more negative
or •
tSi + RX f tSisX + R
•
(4c)
The charge imbalance created by reaction 4b could be canceled by an increased rate of reaction 2 over reaction 1. Reaction 4c could be part of a chain reaction with reaction 3. In this latter case, the Si-X species would
(25) Liebenow, C.; Yang, Z.; Lobitz, P. Electrochem. Commun. 2000, 2, 641. (26) Aurbach, D.; Cohen, Y.; Moshkovich, M. Electrochem. SolidState Lett. 2001, 4, A113. (27) Demange-Gue´rin, G.; Caillet, A. C. R. Acad. Sci. C Chim. 1971, 273, 235. (28) Chazalviel, J.-N.; Truong, T. B. J. Am. Chem. Soc. 1981, 103, 7447. (29) Diggle, J. W.; Parker, A. J. Electrochim. Acta 1973, 18, 975.
Thermal Grafting of Si Surfaces
Figure 4. Voltammetric curves at a Pt electrode at room temperature and at 90 °C of (a) 1 M C10H21MgBr in diethyl ether and (b) 1 M C10H21MgBr + 0.05 M C10H21Br in diethyl ether. The latter curves have been corrected for an extra ohmic drop in the electrolyte. The surface area of the Pt electrode is on the order of 0.1 cm2. The potential of our Ag/AgCl pseudoreference electrode in part b is thought to be ∼ +0.75 V vs Mg/Mg2+.
than Vcb and therefore is an energetically favored process (which is not a surprise in view of the strong reducing character of Grignard reagents). The Mg/Mg2+ couple in a Grignard electrolyte is well reversible. When temperature is increased, it moderately shifts to less negative potentials (by ∼0.1 V at 75 °C).30 In addition, we checked that, at 90 °C, under our reaction conditions, the oxidative decomposition of decylmagnesium bromide takes place at +1.4 V versus Mg/Mg2+ (see Figure 4a). In other words, the oxidative and reductive decomposition reactions of this electrolyte occur at nearly the same potentials at 90 °C and at room temperature. Therefore, the transition from a totally inefficient grafting at room temperature to an efficient one (at least on n-Si) at 90 °C is inconsistent with these small potential changes, and another reactant, such as an alkyl halide, must be involved in the process. In the literature, the reduction potentials of the alkyl bromide series (from ethyl to hexadecyl) lie between -1.7 and -1.8 V versus Ag/AgBr in N,N-dimethylformamide (DMF).31 As above, for comparison purposes, it is useful to express these values against the Fc/Fc+ reference redox system: potential values between -1.7 and -1.8 V versus Ag/AgBr in DMF correspond to values between -2.6 and -2.7 V versus Ag/Ag+ in DMF32 and between -2.5 and (30) Genders, J. D.; Pletcher, D. J. Electroanal. Chem. 1986, 199, 93. (31) Sease, J. W.; Chang, P.; Groth, J. L. J. Am. Chem. Soc. 1964, 86, 3154. (32) Salomon, M. In Physical chemistry of organic solvent systems; Covington, A. K., Dickinson, T., Eds.; Plenum Press: London and New York, 1973; p 194.
Langmuir, Vol. 20, No. 15, 2004 6363
-2.6 V versus Fc/Fc+.29 It means that the reduction of the alkyl bromides takes place at a potential ∼2 V more negative than Vcb and is therefore hindered by a considerable energy barrier, comparable to that for alkylmagnesium bromide reduction. The reduction is easier for alkyl iodides and even more difficult for alkyl chlorides.33 In usual organic solvents, alkyl halide reduction proceeds through concerted electron transfer and carbon-halogen bond breaking, leading to an alkyl radical and a halide anion, according to reaction 2 in the above model.34 The reduction is thermodynamically favored and kinetically faster when going from alkyl chlorides to alkyl iodides. Moreover, reduction of the alkyl radical is more difficult than that of the alkyl halide, which leads to the disregard of the possible involvement of R- species.34 We tried to verify the potential values from the literature using voltammetric experiments in a 1 M decylmagnesium bromide + 0.05 M decylbromide electrolyte in diethyl ether (see Figure 4b). At room temperature, the potential window of this electrolyte is ∼1.3 V, in typical agreement with the reported reduction potential for alkyl bromides. In contrast to the alkylmagnesium halide reduction, the alkyl halide reduction is a strongly irreversible process and it suffers from a sizable overpotential at room temperature. However, as shown in Figure 4b, the faster kinetics at 90 °C appreciably reduce this overpotential, by ∼0.7 V. Therefore, at 90 °C, the reduction of decyl bromide is much easier than that of decylmagnesium bromide, which provides support for our model. The rate-determining role of alkyl halide reduction in the grafting reaction accounts for some further experimental facts. First, it has been mentioned that the efficiency of the thermal grafting is higher for alkylmagnesium bromides than for alkylmagnesium chlorides. Such a behavior can be readily interpreted in the framework of the proposed mechanism, since an alkyl chloride (the impurity present in an alkylmagnesium choride solution) is less easily reduced than an alkyl bromide (present in an alkylmagnesium bromide solution). Furthermore, when an alkyl bromide rather than an alkyl chloride is added to an alkylmagnesium chloride solution, a better efficiency is obtained. This is also accounted for using the same argument. In conclusion, we retain that, under our experimental conditions, the alkyl halide reduction is actually much easier than the Grignard one (in agreement with the easy formation of the Grignard reagent from magnesium and alkyl halide). The corresponding picture is drawn in Figure 5a. However, the final value of the energetic barrier obtained above, on the order of ∼1.6 eV, still appears much too high to account for an efficient process. This issue remains to be addressed. Semiconductor Surface Effects. The above picture is certainly incomplete, since it assumes an ideal silicon surface, in particular, the absence of any interfacial charge. This does not appear realistic, especially in view of the energy favored oxidative decomposition of the Grignard reagent. This reaction should take place spontaneously, resulting in electron injection into the silicon conduction band and negative charging of the surface. The question whether the corresponding charge is stored in surface states lying in the silicon band gap or in conduction band states (i.e., in an electron accumulation layer) is open to discussion. However, in either case, this interfacial charge (33) Mann, C. K.; Barnes, K. K. Electrochemical reactions in nonaqueous systems; Marcel Dekker: New York, 1970; pp 202-204. (34) Andrieux, C. P.; Gallardo, I.; Save´ant, J.-M.; Su, K.-B. J. Am. Chem. Soc. 1986, 108, 638.
6364
Langmuir, Vol. 20, No. 15, 2004
Fellah et al.
Figure 5. Energetic schemes of the electronic levels involved in the reaction model. (a) The positions derived from thermodynamic data lead to a fairly large energy barrier for the rate-determining alkyl halide reduction. (b) On p-Si, the negative charging of the silicon surface induces a shift of the silicon band edges, thereby reducing this energy barrier. (c) On n-Si, the availability of conduction-band electrons from the Si bulk further reduces the energy barrier. In parts b and c, the electronic density of states is figured as a light curve, the Fermi level is qualitatively drawn at a plausible location, and the occupied states are schematically highlighted in gray tone.
will be stored within a depth of a few atomic distances from the surface and will result in a large potential drop, aiming at making electron injection more difficult. In another way, this means that the silicon band energy levels shift with respect to the electrolyte energy levels: an interfacial energetic barrier builds up for hindering electron injection from Grignard oxidation, and conversely, the energetic barrier for alkyl halide reduction decreases. Charging of the silicon surface then appears as the process responsible for the adjustment of the respective rates of the Grignard reagent oxidation and the alkyl halide reduction. It reduces the energetic barrier for alkyl halide reduction and makes the reaction possible. This balance between anodic and cathodic reactions will determine the exact value of the open circuit potential: the Si Fermi level will adjust its position somewhere between the energies corresponding to the RMgX/R•, MgX+ and R•, X-/ RX redox couples. This position will depend on factors such as the surface electron concentration or the electron transfer probabilities from or to the surface. The above picture is practically achieved in a somewhat different manner on n- and p-type Si. In both cases, negative charging of the surface should take place with a similar efficiency and result in the filling of surface states and/or the buildup of a two-dimensional electron layer at the silicon surface (an accumulation layer on n-Si, an inversion layer on p-Si). These electrons essentially lie at energies up to the Fermi level. On p-Si, these electrons are those available at the highest possible energy; that is, alkyl halide reduction requires transition from the states located close to the Fermi level to the R•, X-/RX redox level, as shown in Figure 5b. On n-Si, an additional contribution is available, since free conduction-band electrons from the bulk can reach the surface. As sketched in Figure 5c, the large amount of states available within a mean free path from the surface provides an additional
reservoir of electrons available for alkyl halide reduction, at an energy higher than the Fermi level. In other words, the energy barrier for alkyl halide reduction will be lower on n-Si than on p-Si. In addition, the delocalized character of these electrons, as compared to the case of surface states, certainly makes the spatial decoupling between Grignard oxidation and alkyl halide reduction easier, thereby favoring the efficiency of the whole process. In conclusion, even though negative charging of the interface takes place on n- and p-type Si, which tends to make the behavior of n-Si and p-Si rather similar, the reaction is still expected to remain more efficient on n-Si than on p-Si, as experimentally found. Conclusion Thermal grafting of the silicon surface in Grignard reagents proceeds through a mechanism involving a zerocurrent electrochemical step. Alkyl halide, present as an impurity resulting from the Grignard synthesis, appears as an essential ingredient for reaching a good grafting efficiency. All the experimental observations are consistently accounted for in the framework of a reaction model, in which the first step consists of simultaneous Grignard oxidation and alkyl halide reduction at the silicon surface. Since oxidation of the Grignard is a thermodynamically favored reaction, the silicon surface becomes negatively charged, which lowers the activation barrier for alkyl halide reduction. Since the latter step is rate-determining, grafting is more efficient on n-Si than on p-Si, which sets a clear choice for the Si substrate when using this grafting method. More efficient grafting can be obtained on p-Si by adding alkyl bromide or iodide to the Grignard, but the efficiency remains lower than on n-Si. LA049672J
