4.3 Results and Discussion
4.3.2 Zeta potentials and particle sizes of ceWS 2 , ceMoS 2 , fct-WS 2 , and fct-MoS 2
To determine whether the surface potential of ceWS2 is as negative as ceMoS2 after intercalation and exfoliation, we performed zeta potential measurements for ceMoS2, ceWS2,
Figure 4.5. Charge density plot of 1Tʹ-MoS2 and 1Tʹ-WS2 using density functional theory (DFT) calculations after optimizing the structures. Charge density was calculated for neutral 1Tʹ-MoS2
and 1Tʹ-WS2, and with a –1 charge for both cases, to simulate a net charge of 0.25 per MS2 (M = Mo, W). The neutral charge density plot was subtracted from the –1 charge density plot for both cases to obtain the charge density difference whose isosurface is shown above. Both structures show similar distribution of the excess negative charge.
and reaction mixtures of ceMoS2/ceWS2 with methyl iodide. The zeta potential of a particle is the electric potential at the boundary of the double layer on the particle surface, between the mobile fluid and the fluid attached to the particle, and can be used to characterize the dispersion stability of particles in solution.104-105 Zeta potential values typically range from – 100 mV to +100 mV, with highly dispersed particles possessing values greater than +30 mV or less than –30 mV. Since the zeta potential is sensitive to variations in pH and ionic strength and dilution is necessary in most cases to achieve a concentration low enough to measure particle mobility by light scattering techniques, the measured zeta potential does not reflect the true value of the zeta potential.104 Even so, we can gain a qualitative and comparative understanding of the zeta potentials between ceMoS2 and ceWS2 without knowing their true zeta potentials under reaction conditions.
The particle sizes and zeta potentials for solutions of ceMoS2, ceWS2, ceMoS2 with ICH3, and ceWS2 with ICH3, were determined using dynamic light scattering. Each set of measurements consisted of 10 individual measurements, and particle size measurements
Figure 4.6. (a) Particle sizes of ceMoS2, ceWS2, ceMoS2 + ICH3, and ceWS2 + ICH3, as a function of time after sample preparation plotted on a log10 y-axis. Linear fits are made using least squares linear regression. The accuracy of particle size measurements decreases for values above 1 µm. The increase in particle size after methyl functionalization indicate increased aggregation. (b) Mobility measurements using dynamic light scattering of suspensions of ceMoS2, ceWS2, ceMoS2 + ICH3, and ceWS2 + ICH3, as a function of time since sample preparation. For both plots, each point is an individual measurement, with each cluster containing 10 data points, and each point is set to 60%
opacity to visualize the distribution of overlapping points. Values plotted here were used to obtain the zeta potentials plotted in Figure 4.7 using the Smoluchowski model.
lasted ~ 10 seconds while zeta potential measurements lasted ~ 2 minutes. The particle size and mobility values were used to compute the zeta potential and are shown in Figure 4.6.
Figure 4.7 graphs the zeta potential measurements for ceMoS2, ceWS2, and the reactions ceMoS2 + ICH3, and ceWS2 + ICH3, as a function of the time elapsed since their preparation.
The time of preparation for ceMoS2 and ceWS2 corresponds to the first addition of water to intercalated MoS2 and WS2, and for ceMoS2 + ICH3 and ceWS2 + ICH3 corresponds to the addition of methyl iodide. Since the exfoliated MS2 (M = Mo, W) needed to be purified, there was a 5-hour period between the addition of water and the zeta potential measurement for ceMS2, whereas the methyl iodide was added just before the particle size and zeta potential measurements for ceMS2 + ICH3. A second round of measurements for all four conditions was obtained after 24 hours. Table 4.1 lists the average and standard deviation of 10 points taken at each time cluster. Both ceMoS2 and ceWS2 had a zeta potential of ~ –60 mV 5 hours after exfoliation and ~ –50 mV 29 hours after exfoliation, suggesting that the surface potential is similar for both compounds. The ceMoS2 zeta potential is similar to the reported value of –41 mV in a previous study.19
Figure 4.7. Zeta potential measurements for solutions of ceMoS2, ceWS2, ceMoS2 + ICH3, and ceWS2 + ICH3. Each point represents an individual measurement, with 60% opacity to show overlapping points. Left graph zooms in on the time frame from 5–25 minutes after sample preparation to show changes in zeta potential immediately after methyl iodide is added to ceMoS2 and ceWS2. Linear fits were made using least squares linear regression.
Table 4.1. Zeta potentials for solutions of ceMoS2, ceWS2, with and without methyl iodide, as a function of time after preparation. Averages and standard deviations are based on 10 measurements, each of which is plotted in Figure 4.7.
Time elapsed since preparation
(minutes)
Zeta Potential (mV) (average ± standard deviation)
ceMoS2 ceWS2 ceMoS2 + ICH3 ceWS2 + ICH3
5 – – –31 ± 7 –56 ± 2
300 (5 hours) –57 ± 2 –62 ± 3 – –
420 (7 hours) –51 ± 5 –65 ± 1 – –
1440 (24 hours) – – –32 ± 3 –32 ± 6
1740 (29 hours) –52 ± 4 –47 ± 4 – –
The zoom-in graph from 5–25 minutes highlights a notable difference in kinetics between the ceMoS2 + ICH3 and ceWS2 + ICH3 reactions. Within the first 30 minutes of reacting ceMoS2 with methyl iodide, the zeta potential shifted from ~ –45 mV to ~ –30 mV. In contrast, the zeta potential values for ceWS2 + ICH3 stayed relatively consistent in the first 30 minutes of reaction at ~ –55 mV. We hypothesize that at t = 0 in the ceMoS2 + ICH3
reaction, the zeta potential was similar to that of ceMoS2 at ~ –60 mV and began to move in the positive direction as the reaction proceeded during the 5 minutes that elapsed for the particle size measurement. 24 hours later, the zeta potentials for both reactions averaged –32 mV. Using the zeta potential as an indirect measurement of the reaction progress, it appears that ceMoS2 reacts orders of magnitude faster than ceWS2 with methyl iodide. Given that the zeta potentials before and after functionalization are similar for both ceMoS2 and ceWS2, this data suggests that the lower methyl coverage on WS2 compared to MoS2 cannot be attributed to differences in surface potential.
4.3.3 Comparison of the thermodynamics and kinetics of WS2 and MoS2 methyl