XPS peaks for methyl-MoS2 synthesized using methyl iodide, as a function of the reducing agent used. S7 and S14, minimized with a negative charge of 2, (b) the MoS2 surface before the addition of the 3rd methyl.
Introduction to Molybdenum Disulfide
- Properties and Phases of Molybdenum Disulfide
- Areas of Research
- MoS 2 Functionalization Methods
- Scope of This Thesis
1Tʹ-MoS2 was functionalized with electrophiles that are unreactive in combination with 2H-MoS2 by chemical exfoliation of 2H-MoS2 to increase the nucleophilicity of sulfur. We also investigate the types of electrophiles that are compatible and react with ceMoS2 by changing the leaving group and steric hindrance of the electrophile, and share our preliminary results for electrochemical functionalization and for 2H-MoS2 functionalization.
Reductant-Activated Functionalization of MoS 2
- Introduction
- Experimental Design
- Results and Discussion
- Characteristics of XPS, ATR-FTIR, NMR, and Raman spectra
- Coverage as a function of reductant potential
- Reactivity of ceMoS 2 as a function of the halide leaving group
- S N 2 vs S N 1 mechanism: reactivity of ceMoS 2 with primary, secondary, and
- Electrochemical reduction-activated functionalization of ceMoS 2
- Reductant-activated functionalization of 2H-MoS 2
- Conclusion
Energy diagram showing the relative energy positions of the ceMoS2 Fermi level (eV vs vacuum) relative to the formal potentials of five one-electron metallocene redox pairs (V vs E(Fc+/0)). This reasoning corresponds to the hypothesis that the 2H to 1Tʹ phase transformation takes place as a result of the stabilization of the 1Tʹ phase by negative charge. 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).
Density Functional Theory Modeling of 1Tʹ-MoS 2 Methylation
Introduction
For this purpose, previous DFT studies on the effects of functional groups on the stabilization and bandgap tuning on 1T-MoS298 cannot be applied to our work in Chapter 2. The scope of this chapter will focus on the methylation reaction of 1Tʹ-MoS2 with an alkyl halide.
Computational Design
To obtain the thermodynamics of a reaction (ΔG), the free energy of the final state of a reaction is subtracted from the initial state of a reaction, with the initial and final states similar to the examples shown in Figure 3.1. Reaction scheme for the 1Tʹ-MoS2 methylation reaction to be discussed in this chapter, viewed along the b-axis (top) and c-axis (bottom) of the rectangular cell.
Results and Discussion
- Potential dependence of 1Tʹ-MoS 2 on the number of electrons
- Thermodynamic and kinetic differences during methylation of the two types of
- Thermodynamics of progressive methyl functionalization of 1Tʹ-MoS 2 as a
- Effects of neighboring methyl groups on the rotational barrier
- Models of expected coverage under various constraints
The free energy of the methylation reaction, ΔG, versus the potential for 4th and 5th methylation, comparing the effect of adjacent groups on the ΔG value. The potential of the end products becomes more negative as the number of methyls increases due to the additional electrons in the system.
Conclusion
However, since the coverage distribution of "global 3-in-a-row" is higher than that of "local 1-neighbor max", it would not affect the upper bound of the expected coverage under our current hypothesis. Similarly, we argue that the "0-neighbor max" model reflects the lower bound of coverage distributions in the region, as we saw that methylation is more thermodynamically favorable than the first methylation reaction regardless of the number of methyl groups on the surface, as long as each reaction is surrounded by empty spaces (Figure 3.17). If we assume that methylation in the no-reduction and ferrocene conditions only proceeds for the sites that are easiest to functionalize, then the “0-neighbor max” model represents an upper bound on coverage distribution for these conditions.
In the next chapter, we discuss our research on reducing agent-activated WS2 methylation, compare the results with MoS2 methylation, and explain the differences in coverage using a combination of experimental and theoretical techniques.
Experimental Design
To compare the effects of reduction-activated functionalization on chemically exfoliated WS2 (ceWS2) versus MoS2 (ceMoS2), ceWS2 was reacted with methyl iodide in the presence of one-electron metallocene reductants. To maintain consistency with the metallocene reductants previously used to increase the methyl coverage on ceMoS2, the same five reductants were used: ferrocene (Cp2Fe), nickelocene (Cp2Ni), octamethylnickelocene (Me8Cp2Ni), cobaltocene (Cp2Co), and decamethylcobaltocene (Me10Cp2Co ). We decided to compare only the case of methyl iodide functionalization in order to observe reduction effects without steric constraints.
Experiments showed that these 1-day variations did not affect the coverage to an observable degree (see Results and Discussion), indicating that the achieved coverages were within the thermodynamic limit.
Results and Discussion
- Methyl coverage of WS 2 vs MoS 2 using reductant-activated functionalization
- Zeta potentials and particle sizes of ceWS 2 , ceMoS 2 , fct-WS 2 , and fct-MoS 2
- Comparison of the thermodynamics and kinetics of WS 2 and MoS 2 methyl
Using zeta potential as an indirect measure of reaction progress, ceMoS2 appears to react orders of magnitude faster than ceWS2 with methyl iodide. The interpolation procedure uses a quadratic fit of the free energy versus electron number shown in Figure C.1 and discussed in Appendix C. This difference in rate constant is consistent with our observations that the zeta potential of the MoS2 methylation reaction reaches that of the functionalized product within 30 min, at WS2 and up to 24 hours.
From the linear fit of the experimental coverage data in Figure 4.3, we can calculate the potential at which WS2 achieves the same methyl coverage as MoS2.
Conclusion
This difference in thermodynamics between WS2 and MoS2 may be a result of tungsten having a higher electronegativity and larger size compared to molybdenum, allowing it to stabilize the negative charges on the sulfur, which reduces the nucleophilicity of the sulfur atoms. and makes WS2 less likely to engage. in a nucleophilic addition reaction. The results from this study suggest that reductively activated functionalization using one-electron metallocenes has a promising potential to be widely applied to the functionalization of chalcogenides, and possibly other nucleophilic compounds. Absolute coverage even between the most similar materials will depend on subtle differences in atomic properties, and DFT together with the grand canonical potential kinetics formulation can be useful tools to estimate expected differences in reactivity and coverage prior to experimental testing as method. of screening promising candidates.
To assess the possibility of extending reductant-activated functionalization beyond chalcogenides, reductant-activated functionalization should be tested on non-chalcogenide compounds, and theoretical studies to predict the coverage as a function of reduction potential can help guide research efforts.
Conclusion and Outlook
Materials
All organic solvents and reagents were purchased commercially and used as received without further purification. Molybdenum disulfide powder (99%), tungsten disulfide powder (99.8%), bis(cyclopentadienyl)nickel(II) (nickelocene) and bis(tetramethylcyclopentadienyl)nickel(II) (octamethylnickelocene) were purchased from Alfa Aesar. Anhydrous hexanes, anhydrous N,N-dimethylformamide (DMF), n-butyllithium (1.6 M in hexanes), bis(cyclopentadienyl)iron(II) (ferrocene), bis(pentamethylcyclopentadienyl)cobalt(II) (decamethylcobaltocene) and bis (cyclopentadienyl)cobalt(II) (cobaltocene), were purchased from Sigma-Aldrich.
Synthesis of Chemically Exfoliated MoS 2 (ceMoS 2 ) and WS 2 (ceWS 2 )
Reductant-Activated Functionalization of ceMoS 2 and ceWS 2
Preparation of ceMoS 2 Electrodes and Open-Circuit Voltage Measurement
Tetrabutylammonium perchlorate (0.10 M) was dissolved in acetonitrile to serve as the working electrolyte, with methyl iodide (0.10 M) added immediately prior to the electrochemical tests. A Pt wire served as a counter electrode and an Ag/Ag+ wire was separated from the working electrolyte with a Vycor frit and served as a reference electrode. The voltammetric responses for three samples of ceMoS2 film on polished pyrolytic graphite were obtained in a cell containing 0.10 M methyl iodide.
The mean open circuit voltage immediately after placement in the cell was V vs Fc/Fc+.
Sample Characterization
Raman spectra were collected with a Renishaw inVia Raman microprobe equipped with a Leica DM 2500 M microscope, Leica N Plan 50x objective (numerical aperture lines mm-1 grid, and CCD detector configured in a 180° backscatter geometry. A 532 nm diode-pumped solid-state (DPSS) laser (Renishaw RL532C50) was used as the excitation source. The solid-state carbon-13 (125.4 MHz) nuclear magnetic resonance (NMR) spectra were obtained on a Bruker DSX-500 MHz NMR spectrometer using a Bruker 4.0 mm magic angle spin probe at ambient conditions.
A Cahn C-35 microbalance with a sensitivity of 10 μg was used to determine the concentration of MoS2 and WS2 dispersions.
Density Functional Theory Methods
Electrodes were placed in the sample cuvette and each set of zeta potential measurements consisted of 10 runs of 10 seconds each, fitted to the Smoluchowski model using Zeta-PALS software. To determine the free energy and potential of the optimized structures with greater accuracy, we used jDFTx (v with the CANDLE109 solvation model and 0.1 M KF in water, with the corresponding charge used to obtain the minimized input structure .
Analysis and Quantification of Coverage from XPS Data
- Lineshapes
- Constraints
In addition, the ratios of the areas and positions of the 2H to 1T′ S2p peaks were determined based on the average of the unrestricted fitting of ceMoS2 and ceWS2 samples (2 each) treated completely under inert atmosphere, including air-free transfer to the XPS instrument. These ratios were used to constrain the ratios of 2H to 1T ′ S2p in the analysis of functionalized samples to avoid over- or under-sizing the functionalized sulfur peak. Note that since the above values were determined from two samples, additional samples will improve the accuracy of the estimate for these constraints and change the quantification values for individual samples accordingly.
For example, increasing the distance between the 2H and 1T S2p3/2 peaks from 1.12 to 1.17 eV, holding all other parameters constant, results in a 0.7% decrease in the calculated coverage at WS2, which is negligible given the variability between samples 2 –5% under most conditions.
Quantification of Coverage from 13 C MAS NMR Data
Work Functions of ceMoS 2 and ceWS 2
Effective Reduction Potential for One-Electron Reductants
The effective reduction potential for a solution adding only the reducing agent was estimated from the Nernst equation, assuming a reducing agent to oxidant ratio of 50:1 (i.e. 98% purity), resulting in a correction of – 0.1 V to obtain the effective potentials in Figure 2.8 and Figure 4.3, and Table 2.1.
Vibrational Frequencies of Methanethiol and Propanethiol
DFT Modeling of Grand Canonical Potential Free Energy
Although F(n) appears to be linear with respect to n, as shown in the top row of plots in Figure C.1, this apparent linearity is largely due to the linear contribution of the free energy of electrons. Linear correction for the free energies of electrons by plotting F(n) – nUSHE as shown in the plots in the bottom row of Figure C.1, shows that F(n) is quadratic with respect to n. Free energy as a function of the number of electrons relative to the neutral charge (n0) for the initial state (left column), transition state (middle column) and final state (right column) of the MoS2 + ClCH3 reaction on S7 and S10.
The quadratic nature of the free energy dependence on the number of electrons n is more apparent when corrected for the free energy contribution of the electrons (bottom row).
Calculations Using the Boltzmann Distribution and Eyring Equation
The points are energies calculated using DFT and dotted lines indicate the interpolated fit which is a quadratic fit versus the number of electrons n.
Python Code for Modeling Coverage Distributions
- Coverage distribution model for low-S with no neighbors (“0-neighbor max”) 85
- Coverage distribution model with at most one adjacent functional group to
- Coverage distribution model allowing a maximum of three functional groups in
Then remove that position and the position above and below it from the set of available positions. J.; Sofer, Z.; Luxa, J.; Sedmidubski, D.; Pumera, M., The 3R phase of MoS2 and WS2 outperforms the corresponding 2H phase for hydrogen evolution. Banerjee, K., Functionalization of transition metal dichalcogenides with metal nanoparticles: Implications for doping and gas sensing.
Y.; Zhang, Y.; Quan, X., Covalent functionalization of MoS2 nanosheets synthesized by liquid phase exfoliation for the fabrication of electrochemical sensors for Cd(II) detection. Zboril, R.; Tomanec, O.; Ugolotti, J.; Sofer, Z.; Pumera, M., Functional nanosheet synthons by covalent modification of transition metal dichalcogenides. D.; Smith, R.; Henkelman, G., An improved network-based algorithm for Bader cost allocation. 100) Qiu, L.; Xu, G., Peak overlaps and corresponding solutions in an X-ray photoelectron spectroscopic study of hydrogen desulfurization catalysts. M.; Cho, K., Band alignment of two-dimensional transition metal dichalcogenides: application in field-effect tunnel transistors. 102) Terrones, H.; López-Urías, F.; Terrones, M., New heteroplastic materials with tunable direct band gaps by doping different metal disulfides and diselenides.
