Surface Modification of Silicon Oxide with Trialkoxysilanes toward

May 2, 2013 - Anne M. Slaney , I. Esmé Dijke , Mylvaganam Jeyakanthan , Caishun Li , Lu Zou , Patrice Plaza-Alexander , Peter J. Meloncelli , Jeremy A...
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Surface Modification of Silicon Oxide with Trialkoxysilanes toward Close-Packed Monolayer Formation Mutsuo Tanaka,*,† Takahiro Sawaguchi,† Masashi Kuwahara,‡ and Osamu Niwa† †

Biomedical Research Institute, Advanced Industrial Science and Technology (AIST), Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan ‡ Electronics and Photonics Research Institute, Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan S Supporting Information *

ABSTRACT: In order to scrutinize potential of trialkoxysilanes to form close-packed monolayer, surface modification of silicon oxide was carried out with the trialkoxysilanes bearing a ferrocene moiety for analysis by electrochemical methods. As it was found that hydrogen-terminated silicon reacts with trialkoxysilane through natural oxidation in organic solvents, where the silicon oxide layer is thin enough to afford conductivity for electrochemical analysis, hydrogen-terminated silicon wafer was immersed in trialkoxysilane solution for surface modification without oxidation treatment. Cyclic voltammetry measurements to determine surface concentrations of the immobilized ferrocene−silane on silicon surface were carried out with various temperature, concentration, solvent, and molecular structure, while the blocking effect in the cyclic voltammogram was investigated to obtain insight into density leading to the close-packed layer. The results suggested that a monolayer modification tended to occur under milder conditions when the ferrocene−silane had a longer alkyl chain, and formation of a close-packed layer to show significant blocking effect was observed. However, the surface modification proceeded even when surface concentration of the immobilized ferrocene−silane was greater than that expected for the monolayer. On the basis of these tendencies, the surface of silicon oxide modified with trialkoxysilane is considered to be a partial multilayer rather than monolayer although a close-packed layer is formed. This result is supported by the comparison with carbon surface modified with ferrocene−diazonium, in which a significant blocking effect was observed when surface concentrations of the immobilized ferrocene moiety are lower than that for silicon oxide modified with ferrocene− silane.

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INTRODUCTION Chloro/alkoxysilane is well-known as one of the most versatile surface modification materials. This versatility is derived from the nature of the chloro/alkoxysilane group, which reacts with oxidized surface regardless of materials. It has been reported that alkyltrichlorosilane forms a monolayer on a glass surface1−3 before finding self-assembled monolayer formation on gold with alkanethiol.4 The mechanism of monolayer formation is considered to proceed through hydration to silanol by water5−7 on the surface covered with hydroxyl groups8 and then adsorption or bond formation with the surface. Although several monolayer formations are reported using chlorosilane,9−13 examples of an exact surface analysis to prove monolayer formation seem to be rather rare in cases of surface modification of silicon oxide with chloro/alkoxysilane compared with the surface modification of gold with alkanethiol. As common surface analytical methods, there are X-ray photoelectron spectroscopy, ellipsometry method, and scanning probe microscopy (e.g., AFM and STM), but they might not afford conclusive evidence for monolayer formation because of lack of quantitative information for surface concentration of immobilized molecules. It is known that electrochemical © 2013 American Chemical Society

analysis is effective to measure surface concentration of immobilized molecules when the concentration is lower than that for close-packed monolayer. For electrochemical analysis, the molecules have to bear redox-active moieties such as ferrocene.14 In the case of self-assembled monolayer of alkanethiol, the covalent bond formed between gold and thiol functions as a redox center being cleaved by reduction.15,16 As the covalent bond formed between silicon oxide and chloro/ alkoxysilane does not function as a redox center, incorporation of a redox functional group into chloro/alkoxysilane is necessary for electrochemical analysis. This electrochemical method is adopted for surface analysis of ITO modified with trichlorosilane bearing a ferrocene moiety, and it was reported a conclusive evidence for formation of a close-packed monolayer formation on ITO.17 A similar attempt has been done for surface modification of silicon by hydrosilylation,18,19 but the ferrocene moiety was immobilized on the surface by multistep reaction, hydrosilylation, and amide bond formation.20 In this Received: March 17, 2013 Revised: April 25, 2013 Published: May 2, 2013 6361

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modification with trialkoxysilane. Therefore, we focused to what extent surface modification of silicon oxide with trialkoxysilane can come closer to surface modification with ideal monolayer (Scheme 1). Surface concentration of

approach, surface concentration of the immobilized ferrocene moiety could not be equal to that of the immobilized molecule by hydrosilylation at the first step as reaction with 100% conversion is impossible. While chlorosilane has been recognized to afford a closepacked monolayer for surface modification of silicon oxide, surface modification using trialkoxysilane seems to be complicated. Various methods for surface modification using trialkoxysilane toward monolayer formation have been examined, especially for trialkoxysilane bearing amino groups21−30 as the surface modified with an amino group is useful for various additional surface modifications.31 On the other hand, incorporation of long alkyl chains to trialkoxysilane is recognized as an effective method for monolayer formation32,33 similar to chlorosilane.9,10 These works contain a lot of important suggestions for formation of closed-packed monolayer; however, conclusive evidence for monolayer formation as shown in the literature17 is not demonstrated. Furthermore, recent scrutinizing studies suggest a nucleation− growth model on chemically heterogeneous surface of silicon oxide34 and only a part of trialkoxysilanes forming bonds with silicon oxide surface.35 These reports reflect difficulties in a precise depiction of the interface between the formed layer and silicon oxide; therefore, most surface modification using chloro/alkoxysilane is carried out according to empirical procedures without further exploration. In our previous work,36 we studied on surface modification of carbon materials and adopted a redox-active species, ferrocene, as a probe molecule for electrochemical analysis to determine surface concentration of immobilized molecules. This electrochemical protocol is very effective as it is possible to detect the immobilized ferrocene with monolayer concentration level. In addition, density of formed layer is evaluated by blocking effect in cyclic voltummogram.37−42 Therefore, we considered this method to apply to surface analysis of silicon oxide modified with chloro/alkoxysilane. There are a few attempts to immobilize a ferrocene moiety on the surface using trialkoxysilane, but they are for surface functionalization but not for electrochemical analysis.43,44 For electrochemical analysis, conductivity of substrate is essential, and an ideal surface of substrate is flat as much as possible. We chose lowresistance silicon as a substrate, as the surface of hydrogenterminated silicon is known to be flat in angstroms order45−52 and oxidized easily to react with chloro/alkoxysilane. While hydrosilylation reaction is extensively applied for direct modification of silicon surface,18,19 oxidation treatment of silicon surface is necessary to modify with chloro/alkoxysilane, and piranha oxidation treatment is common. But the oxidized layer, silicon oxide formed with this treatment is too thick, resulting in lack of conductivity. We found that hydrogenterminated silicon is oxidized naturally in organic solvent to react with chloro/alkoxysilane, where the oxidized layer is thin enough to afford conductivity for electrochemical analysis. This finding made it possible to analyze the surface of silicon oxide modified with chloro/alkoxysilane electrochemically. In this work, electrochemical analysis of silicon oxide surface modified with trialkoxysilane bearing ferrocene moiety has been conducted in addition to XPS analysis. As mentioned above, surface modification of silicon oxide with chloro/alkoxysilane seems to be complicated and difficult to control experimental factors for an ideal monolayer formation intuitively because nucleation−growth model,34 and a part of trialkoxysilanes forming bonds with the surface35 are reported for the surface

Scheme 1. Schematic Representation of Surface Modification with Trialkoxysilane

immobilized trialkoxysilane can be an indicator to judge monolayer formation, as there is limitation of surface concentration for monolayer. On the other hand, the blocking effect in cyclic voltammogram will afford insight into density of formed layer. We considered that surface of silicon oxide modified with trialkoxysilane could be evaluated by taking into account surface concentration of immobilized trialkoxysilane and density of formed layer by trialkoxysilane. The obtained general insight into this work should be instructive for surface modification of silicon oxide with chloro/alkoxysilane.

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EXPERIMENTAL SECTION

Materials and Synthesis. All chemicals were used as received without additional purification. Details of synthesis procedures are described in the Supporting Information. Millipore water was used for electrochemical measurements. Silicon wafers (111, ϕ 100 × 0.5 mm, p or n-type, high (>1000 Ω·cm) or low ( ethanol, THF, acetonitrile. As described before, the surface modification in this study proceeds with two steps: natural oxidation of silicon surface and reaction of silane with silicon oxide. Therefore, there are two possibilities in the enhancement of surface modification in toluene, which are acceleration of silicon oxidation and silanization. In order to obtain insight into those possibilities, natural oxidation of silicon in various solvents was carried out, and the results of surface analysis by XPS are summarized in Figure 5.

Figure 3. Surface concentration of immobilized ferrocene−silane 1 at room temperature with 1 mM solution (●), at room temperature with 5 mM solution (▲), and at 50 °C with 1 mM solution (■) using toluene as a solvent. Figure 5. High-resolution Si 2p XPS spectra of silicon surface after storage for 48 h at room temperature in acetonitrile (a), THF (b), ethanol (c), dichloromethane (d), toluene (e), and toluene in the presence of ferrocene−silane 2 (f).

Although the difference in concentration, 1 and 5 mM, did not show a significant influence on the immobilized surface concentration, the effect of temperature was distinguished. As there was not significant difference in the formation of silicon oxide layer at room temperature and 50 °C (Figure S2), the reaction of silane with silicon oxide seemed to be enhanced effectively at higher temperature. Taking into account that the typical surface concentration for self-assembled monolayer (SAM) of alkanethiols on gold is known ∼7.5 × 10−10 mol/ cm2,59 the maximum surface concentration for monolayer of ferrocene−silane might be less than 10 × 10−10 mol/cm2. The immobilized surface concentrations observed in Figure 3 increased after the concentration reached at 10 × 10−10 mol/ cm2. This tendency indicates multilayer formation. Next, we studied the influence of solvents on immobilized surface concentration. The results are summarized in Figure 4. As shown in Figure 4, the surface modification is significantly enhanced in toluene, and the order of surface modification rate

In cases of acetonitrile (Figure 5a) and tetrahydrofuran (THF) (Figure 5b), the silicon oxidation proceeded similarly as observed for toluene (Figure 5e). In Figure 5f, the silicon oxide peak in the presence of ferrocene−silane 2 did not show significant difference from that in Figure 5e, reflecting that the silanization proceeds slowly. Those results indicate that the silanization in acetonitrile and THF proceeds more slowly than in toluene. On the other hand, the silicon oxidation in ethanol (Figure 5c) and dichloromethane (Figure 5d) proceeded more slowly than in toluene. This slow silicon oxidation considered to be a reason for the slow surface modification in ethanol and dichloromethane. In control experiments, surface modification of silicon oxide was carried out with ferrocene−silane 2 using toluene, ethanol, 6364

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Close-Packed Surface Modification with Ferrocene− Silanes. In Figure 6, the surface modification of silicon with ferrocene−silane 2 showed interesting difference in immobilized surface concentration compared with ferrocene−silanes 1 and 3. We considered this tendency, in which the increase of immobilized surface concentration decreased with time, as a phenomenon for monolayer modification reaching at closepacked form. In order to scrutinize this hypothesis, the relationship between the immobilized surface concentration and the blocking effect in cyclic voltammogram37−42 was investigated. In order to evaluate the blocking effect, Ru(NH3)6Cl3 was adopted as other redox probes such as Ru(bipy)3Cl2, FeCl3, and K3[Fe(CN)6] did not afford clear redox peaks in CV. In our previous work,36 a significant blocking effect was observed when monolayer modification proceeded to reach at close-packed form. Surface concentration of the immobilized ferrocene derivatives was about 8.2 × 10−10 mol/cm2, and the molecule structure was similar to ferrocene− silane 2 bearing a phenyldiazonium group instead of a silane group. Furthermore, the typical surface concentration for selfassembled monolayer (SAM) of alkanethiols on gold is reported to be ∼7.5 × 10−10 mol/cm2.59 Therefore, we tentatively considered 10 × 10−10 mol/cm2 as a limit of immobilized surface concentration for monolayer modification. As the surface modification with ferrocene−silane 2 proceeds slowly, various methods to accelerate surface modification were evaluated. First, we carried out the surface modification at 50 °C in toluene, but the immobilized surface concentration increased straightforwardly above the limit concentration 10 × 10−10 mol/cm2 showing a multilayer formation (Figure S4). Next, the surface modification was performed in the presence of acid, HCl. The acceleration effect for surface modification was dependent on the concentration of HCl as depicted in Figure S5, and the influence was drastic when the HCl concentration was more than 5 μM. The immobilized surface concentration in the presence of 10 μM HCl is summarized in Figure 7, which

and dichloromethane to evaluate influence of solvents on silanization. The modified surface was analyzed by XPS, and the peaks of Fe 2p3 and N 1s were evaluated as both peaks will appear depending on progress of surface modification with ferrocene−silane 2. In Figure S3, there was no meaningful difference in peak intensity between toluene and dichloromethane, but ethanol afforded very weak peaks for Fe 2p3 and N 1s. Therefore, the slow surface modification of silicon in dichloromethane caused by slow silicon oxidation and that in ethanol was attributed to both slow silicon oxidation and silanization. It is demonstrated that silanization proceeds smoothly in nonpolar solvent such as toluene and dichloromethane. It is reported that a longer alkyl chain sometimes affords crucial effect for monolayer surface modification.9,10,32,33 The surface modification with ferrocene−silane 1 and 2 was carried out and compared the tendency in immobilized surface concentration as shown in Figure 6. Introduction of alkyl

Figure 6. Surface concentration of immobilized ferrocene-silane 1 (●), 2 (▲), and 3 (■) with 1 mM toluene solution at room temperature.

chain reduced immobilized surface concentration significantly, and an interesting feature of ferrocene−silane 2 was that the increase rate of immobilized surface concentration decreased with time. This tendency is opposite to that of ferrocene−silane 1, and we considered this tendency as a phenomenon for monolayer modification reaching at close-packed form. The close-packed surface modification is discussed in the next section. On the other hand, incorporation of amide group is also considered to be effective for close-packed layer formation by hydrogen-bond formation. 56 As the hydrogen-bond formation might influence on immobilized surface concentration in nonpolar solvent such as toluene, the surface modification with ferrocene−silane 2 (1 mM in toluene) was carried out in the presence of DMF at room temperature for 24 h. The DMF concentrations were 0, 1, and 5 mM, and the values of immobilized surface concentration were (0.94 ± 0.10) × 10−10, (0.99 ± 0.04) × 10−10, and (0.95 ± 0.22) × 10−10 mol/cm2, respectively. The values are almost equivalent regardless of DMF concentrations; therefore, the influence of hydrogen-bond formation seems to be negligible in this system. It is known that trimethoxysilane is more reactive than triethoxysilane, and surface modification with ferrocene−silanes 1 and 3, triethoxy- and trimethoxysilane derivatives, was carried out to compare their reactivity. Figure 6 shows that ferrocene− silane 3 is far reactive than ferrocene−silane 1, reflecting higher reactivity of the trimethoxysilane moiety. The increase of immobilized surface concentration increased with time similar to ferrocene−silane 1, indicating formation of a multilayer.

Figure 7. Surface concentration of immobilized ferrocene−silane 2 with 1 mM toluene solution at room temperature in the absence of HCl (●) and in the presence of HCl (10 μM) (▲).

clearly shows that the surface modification is depressed around 10 × 10−10 mol/cm2 of the immobilized surface concentration. A slight blocking effect was observed in cyclic voltammogram, when the immobilized surface concentration was around 10 × 10−10 mol/cm2. Although the immobilized surface concentration was greater than 10 × 10−10 mol/cm2 to show a multilayer formation, the blocking effect was almost constant. Generally, surface modification under milder condition is considered to be suitable for close-packed monolayer formation. We examined the surface modification with lower 6365

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10 × 10−10 mol/cm2, resulting in a multilayer formation in Figure 8. This result clearly suggests that the surface modified with ferrocene−silane 2 can afford a close-packed layer, but the layer is not monolayer, namely, a partial multilayer. In order to obtain further insight into packing density, the blocking effect was compared with that for the surface modification of glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) with the diazonium compound in our previous work,36 where a monolayer surface modification was observed. For comparison, the normalized CVs are shown in Figure 10 with ferrocene derivatives used for surface

HCl concentration, but a significant blocking effect to show formation of close-packed monolayer was not observed with this system. For the close-packed monolayer formation, we adopted acetic acid, which works milder than HCl. Similar to HCl, addition of acetic acid to toluene solution prompted the surface modification, and the immobilized surface concentration reached at 9.0 × 10−10 mol/cm2 for 24 h immersion in the presence of 5 mM acetic acid. However, clear blocking effect was not observed in cyclic voltammogram. We decreased acetic acid concentration to 0.1 mM and performed the surface modification under milder conditions. Figure 8 shows the

Figure 8. Surface concentration of immobilized ferrocene−silane 2 with 1 mM toluene solution at room temperature in the presence of CH3COOH (0.1 mM). Figure 10. Cyclic voltammograms to show blocking effect for Si (6 day immersion) (a), Si (7 day immersion) (b), GC (c), and HOPG (d) substrates modified with ferrocene derivatives.

immobilized surface concentration dependent on the immersion time. Similar to Figure 7, the surface modification was depressed around 10 × 10−10 mol/cm2 of the immobilized surface concentration. The blocking effect in cyclic voltammogram depending on the immersion time is summarized in Figure 9. The blocking effect became obvious with immersion time. When the immersion times were 6 and 7 days, the blocking effect was significant, where the immobilized surface concentrations were 7.6 and 8.9 × 10−10 mol/cm2, respectively. Although the surface was modified with a close-packed layer to show blocking effect, the surface modification still proceeded when the immobilized surface concentration became more than

modification. Surface concentrations of the immobilized ferrocene derivatives for parts a, b, c, and d of Figure 10 were 7.6, 8.9, 8.2, and 6.6 × 10−10 mol/cm2, respectively. Despite higher surface concentration of the immobilized ferrocene derivatives, the blocking effect for GC is smaller than that of HOPG as GC is known to have rougher surface than HOPG.36 In Figure 10, the blocking effect of (a) and (b) seems at the level intermediate between (c) and (d) with similar surface concentration of the immobilized ferrocene derivatives to (c). Because the surface of Si is as smooth as HOPG, this tendency also suggests that the formed layer is a partial multilayer. Surface Modification of Silicon with Deteriorated Silanes. On the other hand, it is empirically known that a partly decomposed silane solution is effective for surface modification. Therefore, we stored toluene solution of 1 mM ferrocene−silane 2 at room temperature until precipitate was formed. The period for precipitate formation was more than three months at least. We noticed that ferrocene−silane has self-decomposition nature, and the period is dependent on the purification process for ferrocene−silane. When chloroform− ethanol eluent was used for silica gel column chromatography, it took more than six months until the precipitate formation. But the period shortened to three months when chloroform− methanol eluent was used. We considered this difference caused by an alkoxy group exchange of triethoxysilane moiety to methoxy group. Although the metoxy group was not detected by 1H NMR, we confirmed that ethoxy−methoxy group exchange occurred in the presence of abundant methanol during silica gel column chromatography, and ferrocene−silane 2 purified with chloroform−methanol eluent was always more

Figure 9. Cyclic voltammograms to show blocking effect for silicon modified with ferrocene−silane 2 of 1 mM toluene solution at room temperature in the presence of CH3COOH (0.1 mM). 6366

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reactive than that purified with chloroform−ethanol eluent. 1H NMR spectra for the deteriorated solution showed almost intact peaks with small peaks derived from eliminated ethanol. However, reactivity of the deteriorated solution is far higher than that of the original solution, as the ferrocene−silane 2 in the deteriorated solution was absorbed by silica gel completely. As such a reactive species, silanol could be a candidate, but silanol is known as a reactive intermediate compound. The immobilized surface concentration using the deteriorated solution is shown in Figure S6 with that using the original solution for comparison. Figure S6 shows that the deteriorated solution has far higher reactivity than the original solution. Furthermore, a significant blocking effect was observed when the immersion time was more than 96 h. These results suggest that the deteriorated solution of silane has high performance for surface modification.

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CONCLUSIONS The surface modification behavior of the ferrocene−silanes bearing a ferrocene moiety as a molecular probe was studied by electrochemical analysis with XPS measurement. It was found that the surface of hydrogen-terminated silicon was oxidized in organic solvent naturally, resulting in the surface modification with ferrocene−silanes and the oxidized layer; namely, the SiO2 layer is thin enough to keep conductivity for electrochemical analysis. The result of quantitative electrochemical analysis shows that surface concentration of the immobilized ferrocene−silane is dependent on used solvents, in which silanization proceeds smoothly in toluene and dichloromethane, but it is sluggish in acetonitrile, THF, and ethanol. The properties of modified surface are strongly dependent on the molecular structure of ferrocene−silane and modification conditions. A monolayer modification tends to proceed with silanes bearing a longer alkyl chain under milder condition, and a close-packed layer to show blocking effect in cyclic voltammogram can be obtained. However, the surface modification proceeds after the formation of a close-packed layer. Therefore, the obtained close-packed layer with the silanes in this work is considered to be a partial multilayer.

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ASSOCIATED CONTENT

S Supporting Information *

Additional CV, XPS data, and experimental procedure. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +81-29-861-6233; Fax +81-29-861-6177 (M.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Kuraoka (Kobe University) for practical advise and thank Mrs. Takashima (AIST) for experimental assistance.

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REFERENCES

(1) Sagiv, J. Organized monolayers by adsorption. 1. Formation and structure of oleophobic mixed monolayers on solid surfaces. J. Am. Chem. Soc. 1980, 102, 92−98. 6367

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dx.doi.org/10.1021/la4009834 | Langmuir 2013, 29, 6361−6368