Chapter 2: Reductant-Activated Functionalization of MoS 2
2.3 Results and Discussion
2.3.2 Coverage as a function of reductant potential
The coverage for methyl-MoS2 was 41% ± 4% (average ± standard deviation, N = 3), slightly higher with previously published 30% coverage for the maximum coverage using methyl iodide and ceMoS2.19 If the coverage obtained in the absence of reductant is limited by the charge density stored in the ceMoS2, we would expect that the coverage would increase with negative charge injection. In addition, removing charge from ceMoS2 prior to the reaction would reduce the surface methyl coverage. On the other hand, if this coverage reflected a sterically limited packing density, then negative charge injection should have minimal effect.
We sought to evaluate whether an appropriately chosen reducing agent could transfer negative charge to ceMoS2 or functionalized ceMoS2, thereby enabling further functionalization after consumption of the initially stored negative charge. The coverage produced by functionalization was thus determined as a function of the reducing strength of the metallocenes present during the functionalization reaction. Five metallocenes, ferrocene (Cp2Fe), nickelocene (Cp2Ni), octamethylnickelocene (Me8Cp2Ni), cobaltocene (Cp2Co), and decamethylcobaltocene (Me10Cp2Co), provided reduction potentials that spanned a range from above to below the Fermi levels (EF) of ce-, methyl-, and propyl-MoS2, estimated using ultraviolet photoelectron spectroscopy (UPS) to determine the work function of 3 samples of each material drop cast onto p+-Si (Figure 2.7). The average work function of 4.78 eV obtained from these measurements is close to literature reports of the 1T MoS2 work function of 4.7 eV that were determined using UPS as well.23 This shift towards lower work function in methyl- and propyl-MoS2 may be due to the contribution of the dipole, which has been a well-documented effect on silicon band edges,67, 94-95 but may also be due to the trapping of negative charges contributed by the reductants in the S–C bonds themselves, increasing the number of electrons in the bands as the coverage increases.
Figure 2.8a describes the change in functional group coverage as a function of the effective solution potential as determined by the reductant, or in the no-reductant case, by the open- circuit voltage of a ceMoS2 electrode (see Appendix A for measurement details). The effective reduction potentials, Eeff, of the metallocene solutions ranged from –0.1 to –1.9 V vs ferrocenium/ferrocene (Fc+/0) and were estimated by assuming a reductant to oxidant concentration ratio of 50:1, as detailed in Table 2.1.96 All reactions were performed in triplicate with error bars indicating the standard deviation. To assess the significance of the area under the fitted S–C curve in XPS spectra, unfunctionalized ceMoS2 spectra, in which the S–C bond is absent, were fit with the additional sulfur peaks at the S–C binding energy (Figure 2.8c), resulting in a “coverage” of 4% ± 3% (N=3). Thus, we interpret that S–C bond formation occurred only for XPS S2p peaks with coverage values above ~10% in the absence of additional data such as NMR. Figure 2.8b shows the standard deviations of the peak areas (~1–3%) obtained from Monte Carlo simulations for each sample, indicating that the trends in Figure 2.8a are well outside the range of error that can be attributed to errors in XPS peak fitting. In addition to quantification by XPS, 13C MAS NMR on ceMoS2 functionalized with
13C-methyl iodide with and without nickelocene and cobaltocene, verified the trend of increasing coverage with reductant strength (Figure 2.8d).
Figure 2.7. Ultraviolet photoelectron spectra (UPS) for chemically exfoliated MoS2, methyl-MoS2, and propyl-MoS2 synthesized using methyl iodide and 1-iodopropane, respectively. (a) The high binding energy cut-off used to obtain the work function for these powders. Displayed work function and error range is the average of three samples and the standard deviation. (b) Valence-band regime for the same samples, showing a ≤ 0.3 eV band gap, consistent with theoretical calculations.14, 16 Samples were not heated to remove adventitious carbon before obtaining spectra.
Figure 2.8. (a) Coverage per MoS2 for MoS2 functionalized with methyl iodide, 1-iodopropane, 1- bromopropane, and 1-chloropropane as a function of the effective potential for the no-reductant case (open-circuit voltage –0.07 V vs E0(Fc+/0), see Appendix A.4) and for the reductants ferrocene, nickelocene, octamethylnickelocene, cobaltocene, and decamethylcobaltocene, corresponding to – 0.1, –0.5, –1.1, –1.3, and –1.9 V vs E0(Fc+/0). Coverages were quantified using the peak areas from high-resolution XPS S2p spectra, as the fraction of the total S2p peaks that corresponds to covalently functionalized sulfur. (b) Standard deviations in percentage (%std) of the functionalized S2p peak areas fitted during Monte Carlo simulations (n = 400) for a portion of the data presented in (a).
Error bars are the standard deviations for the %std. Note that the mean for the %std are below 2%
except for conditions involving 1-chloropropane and minimal functionalization, where a larger % error is expected. (c) High-resolution XPS of S2p for ceMoS2 with two types of peak fitting: (top) three sets of S2p double peaks: one for 1Tʹ-MoS2, one for 2H-MoS2, and one for sulfur defects (S*).
This peak fitting was used to determine the percentage of sulfur in the 2H phase in ceMoS2. The percentage of 2H-MoS2 for all functionalized samples was constrained based on this fitting; (bottom) four sets of S2p peaks, the fourth (simulating S–C) constrained to have the same binding energy relative to the 1Tʹ-MoS2 as the functionalized S–C peak observed in fct-MoS2 samples. This peak fitting was used to determine the error associated with peak fitting when using peak areas for coverage quantification. (d) 13C MAS NMR of 13C-methyl-MoS2 functionalized under three conditions: without reductant, with nickelocene, and with cobaltocene. This technique is semi- quantitative and shows the trend of increasing coverage. Error bars indicate ±10% based on 5%
error of external standard and an estimate of 5% error from peak fitting.
Table 2.1. The standard reduction potential for each redox couple, the effective reduction potential assuming 50:1 ratio of reductant to oxidant, and the potential on an absolute energy scale using E(Fc+/0)
= 0.4 V vs ESHE (standard hydrogen electrode) and ESHE = 4.44 V vs EVac.
As expected, exposure to the ferrocene solution did not result in a substantial change in coverage. Figure 2.8a shows an approximately linear increase in coverage for methyl iodide, 1-iodopropane, and 1-bromopropane as the strength of the reductant increases from ferrocene (Eeff = –0.1 V vs Fc+/0) to cobaltocene (Eeff = –1.3 V vs Fc+/0). At even higher reduction potentials using decamethylcobaltocene (Eeff = –1.9 V vs Fc+/0), the coverage did not increase and even appears to decrease, possibly due to competing side reactions and reduction of methyl iodide. Thus, the highest coverage achieved peaked using cobaltocene at ~64%, which amounts to ~32% of all sulfurs being functionalized. This coverage is lower than we hypothesized, considering that silicon surfaces can achieve ~100% methyl coverage with an Si–Si distance of 3.8 Å on Si(111),97 compared to the S–S distance of ~3.3 Å on ceMoS2. To assess the factors contributing to this coverage and understand the point at which the coverage is limited by sterics, we will use density functional theory (DFT) calculations, discussed in Chapter 3.
In reactions involving 1-chloropropane, S–C bonds were only visible in the XPS data for reactions performed in the presence of the two strongest reductants used: cobaltocene or decamethylcobaltocene. Because of the similarity of some XP spectra to spectra for unfunctionalized ceMoS2, 13C NMR and ATR-FTIR were used to verify that
Reductant (A-) Standard Reduction Potential for (A/A-) vs
E(Fc+/0) (V)
Effective Reduction Potential, 50:1 reductant to oxidant vs
E(Fc+/0) (V)
Potential vs EVac
Ferrocene (Cp2Fe) 0 –0.10 4.74
Nickelocene (Cp2Ni) –0.42 –0.52 4.32
Octamethylnickelocene
(Me8Cp2Ni) –0.95 –1.05 3.79
Cobaltocene (Cp2Co) –1.16 –1.26 3.58
Decamethylcobaltocene
(Me10Cp2Co) –1.77 –1.87 2.97
functionalization was successful using cobaltocene, whereas minimal coverage resulted when octamethylcobaltocene was used (Figure 2.9). Notably, the NMR peak positions and peak splitting for propyl-MoS2 synthesized using chloropropane and cobaltocene were in accord with those for propyl-MoS2 (Figure 2.5). However, an additional peak was present at 87 ppm, and the intensity of the peak at 54 ppm was relatively small, consistent with a downfield shift for the anchoring carbon that can be observed in R–CH2–O species. An additional 4% of both molybdenum and sulfur oxides were observed in the XPS data for the chloropropane/cobaltocene functionalization compared to functionalization using iodopropane, further suggesting a linkage through metal or sulfur oxides.
To evaluate the effects of removing charge density using a metallocene oxidant, ceMoS2 was oxidized by stirring in DMF containing ferrocenium tetrafluoroborate for 3 days, prior to reaction of the ceMoS2 with methyl iodide. The overall composition of the products showed a 15% increase in 2H-MoS2 and a 15% decrease in S–C bonds relative to ceMoS2 handled in a nominally identical way but not exposed to ferrocenium tetrafluoroborate, which showed a coverage of 35%, as expected (Figure 2.10). In another experiment, ceMoS2 was oxidized with ferrocenium tetrafluoroborate for 4 hours instead of 3 days, followed by methyl iodide, which yielded 49% coverage, similar to the case with no oxidant. These results suggest that removal of negative charge affects the coverage by destabilizing the 1Tʹ phase and favoring the reversion to the 2H phase. This reasoning is consistent with the hypothesis that the 2H to 1Tʹ phase transformation occurs due to the stabilization of the 1Tʹ phase by negative charge
Figure 2.9. (a) High-resolution XPS of S2p region, (b) ATR-FTIR, and (c) 13C CPMAS NMR data for the functionalization of ceMoS2 with chloropropane in the presence of either octamethylnickelocene (Me8Cp2Ni) or cobaltocene (Cp2Co).
injection. We would like to deconvolute the effects of the charge density as opposed to the phase on ceMoS2 reactivity, since these two processes can both occur in the presence of an oxidant prior to methyl iodide addition. From these results, it is clear that sections of ceMoS2 that convert back to the 2H phase are excluded from functionalization, but it remains unclear whether oxidized ceMoS2 in the 1Tʹ phase react with methyl iodide. Future experiments in this area will need to elucidate the kinetics of 3 processes to determine a suitable experiment design: ceMoS2
oxidation, phase transition, and the functionalization reaction.