Chapter 4 Reductant-activated Surface Functionalization of Silicon Nanocrystals
4.3 Results and Discussion
65 transferred to the XPS. The instrument was operated using Vision Manager software v.
2.2.10 revision 5 and data were analyzed using CasaXPS software (CASA Software Ltd).
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Figure 4.2 Representative FT-IR spectra measured for 5 nm diameter Si NCs treated with neat MA under the presence of (a) no reductant, (b) decamethylcobaltocene, and (c) cobaltocene. Highlighted peaks in orange represent vibrations associated with Si bonds, and highlighted peaks in blue represent vibrations associated with MA.
To corroborate the IR spectra, XP spectra were also collected for each of the samples.
Figure 4.3 demonstrates a representative set of spectra measured for Si NCs with a diameter of 5 nm treated with MA in the presence of decamethylcobaltocene and cobaltocene and in the absence of reductant. In the absence of reductant, the Si 2p spectra demonstrates peaks at 100.5 eV and 101.6 eV corresponding to Si(0) atoms and Si–C bonds resulting from functionalization with MA through the C=C bond (Figure 4.3a). The presence of the small Si–C peak corroborates the presence of C–H, C=O, and Si–CH2 vibrations in the IR spectra (Figure 4.2a), indicating that minimal surface functionalization occurs even in the absence of reductant. The Si 2p spectra of the Si NCs treated in the presence of decamethylcobaltocene illustrate peaks at 98.3 eV, 98.9 eV, and 102.4 eV corresponding to Si(0) atoms, Si–C bonds, and silicon oxide species, respectively (Figure 4.3b). Si NCs treated with MA in the presence of cobaltocene display peaks at 98.7 eV, 99.6 eV, and 102.9 eV corresponding to Si(0) atoms, Si–C bonds, and silicon oxide species (Figure 4.3c). In both cases where the Si NCs were treated with MA in the presence of a reductant, the Si–C peak appears with greater area than the samples treated in the absence of reductant
67 due to the formation of a surficial Si–C bond through the Cβ atom of the C=C bond in MA.
The presence of oxide species in the samples treated with reductant may be attributed to oxidation of the surface due to small amounts of oxygen introduced during the sonication and centrifugation process or during sample preparation prior to measurement.
Figure 4.3 Representative XP spectra measured for Si NCs of 5 nm diameter treated with neat MA in the presence of (a, d) no reductant, (b, e) decamethylcobaltocene, and (c, f) cobaltocene. Each column represents each reductant condition tested. The top row represents Si 2p XP spectra and the bottom row displays C 1s XP spectra.
Figure 4.3d-f displays a representative set of C 1s XP spectra of 5 nm diameter Si NCs treated with MA in the presence of decamethylcobaltocene and cobaltocene and in the absence of reductant. Si NCs treated with MA in the absence of reductant illustrate peaks at 283.7 eV, 285.1 eV, 286.0 eV, 287.8 eV, and 289.5 eV corresponding to C–Si, adventitious C–C, adventitious C=C, C–OMe, and C=O bonds, respectively (Figure 4.3d).
The presence of a small C–Si peak indicates that a small amount of MA reacted with the Si NC surface. The C–OMe and C=O peaks in the XP spectra likely result from a combination of a small amount of surface functionalization as well as adsorbed residual MA. Si NCs treated in the presence of decamethylcobaltocene show peaks at 284.0 eV, 285.0 eV, 285.7 eV, 286.6 eV, and 288.6 eV can be attributed to C–Si, adventitious C–C,
68 adventitious C=C, C–OMe, and C=O bonds, respectively. Samples treated with cobaltocene display peaks at 283.8 eV, 284.9 eV, 286.2 eV, 287.5 eV, and 289.0 eV represent C–Si, adventitious C–C, adventitious C=C, C–OMe, and C=O bonds, respectively. In contrast to the Si NCs treated without reductant, samples treated with decamethylcobaltocene and cobaltocene display stronger C–Si peaks due to greater extent of covalent functionalization. Furthermore, the C=O peak present in the spectra of Si NCs treated with decamethylcobaltocene appear with greater intensity than the C=O peak of Si NCs reacted with cobaltocene, which appears as a weaker shoulder. This further demonstrates that reductant strength increases the reactivity of the surface toward electrophilic addition, validating the reductant strength trends observed in the IR spectra (Figure 4.2b and c).
4.3.2 Si NC size dependency of surface reactivity
To determine the NC size dependency of the reaction, 8 nm and 3 nm diameter hydride- terminated Si NCs were treated similarly with neat MA under the presence of no reductant, decamethylcobaltocene, or cobaltocene. Figure 4.4 displays zoomed-in regions containing the Si–H, C=O, and Si–O vibrations. The spectral trends observed for the 5 nm diameter Si NCs hold for each size tested, where the peak intensity of C=O increases dramatically with respect to the hydride peak when treated with stronger reductants. However, no distinct trend was observed correlating the size of the Si NC to the relative intensity of the C=O peak, indicating that there is no strong reaction dependency on Si NC size. This contrasts oxidant-activated addition,6 where NC size strongly influenced the extent of the reaction, displaying a distinct “on-off” mechanism in which the weakest reductant tested only activated 8 nm Si NCs toward nucleophilic addition and zero reactivity was observed for smaller NCs. In oxidant-activated nucleophilic addition, a major requirement for reactivity is that the redox solution potential of the oxidant must lie near or above the valence band of the Si NC used. Analogously, it was expected that the redox solution potential of the reductant must lie near or above the conduction band edge of the Si NC for reductant-activated electrophilic addition. Here, the lack of Si NC size dependency in reductant-activated addition may be attributed to the significantly smaller energy
69 difference between the conduction bands of the different sized Si NCs (Figure 4.1b) compared to the valence bands.
Figure 4.4 Zoomed-in regions of FT-IR spectra for (a) 8 nm, (b) 5 nm, and (c) 3 nm diameter Si NCs treated with neat MA under the presence of no reductant (top, blue), decamethylcobaltocene (middle, red), or cobaltocene (bottom, green). Highlighted peaks in orange represent vibrations associated with Si bonds, and highlighted peaks in blue represent vibrations associated with MA.
To further investigate the dependency of Si NC size on the reactivity of the surface toward electrophilic addition, XPS was performed and the size dependency of the reaction was compared using the C 1s XP spectra (Figure 4.5). Similar spectral trends were observed correlating reactivity to reductant strength as in Figure 4.3, where Si NCs treated with neat methyl acrylate in the presence of decamethylcobaltocene resulted in C=O peaks with greater area than the samples treated in the presence of cobaltocene. Those treated with cobaltocene result in a C=O peak that appears as a weak shoulder compared to decamethylcobaltocene-treated samples where the C=O peak is more distinct. As in Figure 4.3, samples treated in the absence of reductant displayed a very weak to no C–Si peak indicating poor reactivity of the surface toward electrophilic addition, and peaks attributed to C=O and C–OMe can be associated with residual adsorbed methyl acrylate remaining from incomplete washing. No distinct trend was observed across the 8 nm, 5 nm, and 3 nm sized Si NCs, confirming the lack of size dependency of the reaction observed in the IR spectra (Figure 4.4).
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Figure 4.5 C 1s XP spectra collected for Si NCs with diameters of (a-c) 8 nm, (d-f) 5 nm, and (g-i) 3 nm treated with neat methyl acrylate in the presence of (top row) no reductant, (middle row) decamethylcobaltocene, and (bottom row) cobaltocene.