Sonication is a highly required process for material characterization and electrochemical study. The process agitates particles in a solution by applying acoustic energy waves. The waves are produced by a piece of sonication equipment in which samples are contained in a water bath. In particular, the time-dependent sonication effect on particles is essential for identifying or separating the ingredients of a substance. Thus, the optimum sonication time is important in nanoparticle synthesis and preparation[1-3]. Next, the surface properties and size of nanoparticles in solutions are influenced by the sonication[4]; hence, understanding the sonication effect on particle characteristics is essential, such as particle size and dispersion property, as a function of the sonication time. Sonication could also cause exfoliating effects, which change the size and dispersion of the particles[5].

Molybdenum disulfide (MoS₂) is a two-dimensional transition metal dichalcogenide with a layered structure[6] like graphene. MoS₂ is a semiconductor demonstrating a bandgap of 1.8 eV[7], making it suitable for 2D applications, such as en¬ergy storage devices[8], hydrogen and oxygen evolution reactions[9], field-effect transistors[10], and lithium-ion batteries[11]. Our group investigated the hydrogen and oxygen evolution reaction of MoS₂ by electrochemical measurements[12-15]. For the electrochemical measurements, a homogeneous solution containing MoS₂ is necessary so that the solution is sonicated for 5 min, sometimes up to 1 h. Researchers are concerned about whether the size and distribution of particles (or dispersion of solution) after sonication change[4,5]. Generally, more than 4 h, even 10 h, of sonication is necessary to change the particle size or dispersion[5]. Sonication for 1 h might not change the particle size, but systematic experimental evidence is needed to confirm no change in the particle size by sonication for 1 h.

Herein, we investigated the sonication effect on MoS₂ particles systematically. Five samples were prepared at different sonication times, up to 1 hr, and to investigate the sonication effect on the MoS₂ particle size, they were analyzed using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), impedance spectroscopy (IS), and capacitance and electrochemical measurements.

Molybdenum disulfide (MoS₂ powder, 99%, 2 μm) was purchased from Merck and used without further purification. MoS₂ powder of 200 mg was mixed with 20-ml deionized (DI) water. Figure 1(a) shows the mixed solution before sonication. Also, a clear separation of MoS₂ and DI water is shown. The samples were sonicated for 0, 5, 10, 30, and 60 min, and the sample solution just after the sonication is shown in Fig. 1(b). Well-mixed solutions were obtained, except from the 0-min sample (no sonication sample). Even after 1 h, the solutions showed no significant change (Fig. 1(c)). However, the sample solutions show a clear separation of MoS₂ and DI water (Fig. 1(d)) after 2 h, similar to the beginning. Thus, the distribution of particles (or solution dispersion) after sonication remained unchanged. After sonication, the sample is suitable for electrochemical measurements because it is in the homogeneous solution. However, after 2 h, the solution should be sonicated again for electrochemical measurements. Then, the prepared sample solution was vacuum-filtrated and dried for the measurements. The dried samples are shown in Fig. 1(e).

Figure 1. (Color online) Photos of sample solutions (a) before and (b) just after sonication for 0, 5, 10, 30, and 60 min. Photos taken after (c) 1 and (d) 2 h afterward. (e) Photo of prepared samples after filtration.

An ultrasonication device system (Branson 3510-DTH ultrasonic cleaner and NIST, USA; input power of 320 W) was used for sonication, and OM (OM, Material Science Microscope BX53M, OLYMPUS) was used to investigate the change in the particle size. Furthermore, field-emission SEM (FE-SEM, S4300, Hitachi, Japan) was used to investigate the surface morphologies of the samples. XRD spectrometer (XRD, D/MAX-2500/ PC, Rigaku, Japan) at 40 kV with Cu Kα (λ = 1.54 Å) was used to identify the crystal structure, crystalline grain size, and lattice parameters of the samples.

The electrochemical performance of the samples was investigated using a three-electrode system in 1 M of H₂SO₄ solution using potentiostat/galvanostat with IS (Bio-Logic, SP-150, France). Ag/AgCl (3 M KCl saturated) and platinum wire were used as the reference and counter electrodes. Next, the glassy carbon electrode (GCE, outer diameter = 6 mm and inner diameter = 3 mm) was used as the working electrode. Five milligrams of the samples sonicated for 0, 5, 10, 30, and 60 min were mixed separately in 2-ml isopropyl alcohol, and then, 5 μL homogeneous suspension was drop cast on the GCE to form the working electrode. The weighted active mass of working electrodes was 2.5 gm cm^-2.

To investigate the electrochemical properties of the samples, cyclic voltammetry (CV), chronocoulometry (CC), and potential electrochemical IS (PEIS) measurements were performed. Potential from 0-0.8 V was applied at different scan rates of 10, 25, 50, 100, and 200 mV s^-1 in the CV measurements, and the specific capacitance was calculated[16,17]. CC was used to obtain the active electrochemical surface area of the samples[16,17]. PEIS was performed at an amplitude of 5 mV in the frequency range of 100 mHz to 500 kHz to obtain the internal resistance, charge transfer resistance, and other equivalent impedance behavioral characteristics. All measurements were conducted with reference to the reference electrode.

The SEM images in Fig. 2 show the morphologies of the MoS₂ particles before and after sonication for 5, 10, 30, and 60 min. The flat and two-dimensional structures of the MoS₂ particles are presented. The MoS₂ particles look denser after more sonication. However, no significant change in particle size after the sonication occurs, even for 60 min.

Figure 2. SEM results of samples after sonication for (a) 0, (b) 5, (c) 10, (d) 30, and (e) 60 min.

For the detailed statistical investigation of the particle size after the sonication, OM was used to measure the particle size of the samples (Fig. 3). More than thirty particles were taken from each sample, and the various dimensions of the particle sizes were measured using the diameter scale (2 μm × 2 μm) on the optical microscope. Figure 3(a) shows the optical microscope used herein. Figure 3(b) presents the results obtained from the sample with no sonication. Figure 3(c)-(f) show the particles sonicated for 5, 10, 30, and 60 min, respectively.

Figure 3. (Color online) (a) Photo of optical microscope. Sample results after sonication for (b) 0, (c) 5, (d) 10, (e) 30, and (f) 60 min.

Figure 4 presents the particle sizes of the samples for different sonication times. Average particle sizes of 2.10, 2.08, 2.07, 2.06, and 2.05 μm were calculated accordingly for 0, 5, 10, 30, and 60 min of sonication. An average particle size of 2.07 ± 0.13 μm was obtained. Figure 4(f) shows the average particle size with the standard deviation as a function of the sonication time. After the sonication, no significant change (2 μm) was observed. Therefore, the sonication effect on the particle size is not a concern, even after 60 min of sonication.

Figure 4. (Color online) Particle size analysis results of samples sonicated for (a) 0, (b) 5, (c) 10, (d) 30, and (e) 60 min. (f) Average particle size as a function sonication time.

Further investigation was performed using XRD (Fig. 5), demonstrating similar diffraction patterns for the five samples. Most intense (002) and (103) peaks were centered at 14.4° and 39.6°. The other diffraction peaks of (004), (100), (101), (102), (006), (105), (110), (008), (203), and (204) were observed at 29°, 32.8°, 33.6°, 36°, 44.1°, 49.9°, 58.6°, 60.1°, 73.1°, and 76.3°. respectively[16], showing polycrystalline MoS₂. No additional diffraction peaks were observed before and after sonication. Furthermore, the particle sizes from the (002) peak were calculated using the Scherrer equation[18] and obtained as 2.04, 1.99, 1.97, 1.91, and 1.88 μm for the samples sonicated for 0, 5, 10, 30, and 60 min, respectively. Like the previous OM results (2 μm), the particle size decreases minimally with the sonication, but it is within the precursor particle size of 2 μm.

Figure 5. (Color online) XRD results of samples sonicated for (b) 0, (c) 5, (d) 10, (e) 30, and (f) 60 min.

The electrochemical active surface area was derived from the CC measurement, and Fig. 6(a) shows the CC results, where the slope is proportional to the active surface area[14,17]. The obtained surface areas were 2.58 × 10^-6, 2.60 × 10^-6, 2.61 × 10^-6, 2.63 × 10^-6, and 2.66 × 10^-6 cm² for the samples sonicated for 0, 5, 10, 30, and 60 min sonication, respectively. Electrochemical impedance is also analyzed in Fig. 6(b). The obtained internal resis¬tance and charge-transfer resistance are 6.5 and 44.5 Ω for 0 min, 6.4 and 39 Ω for 5 min, 6.2 and 38 Ω for 10 min, 6.1 and 37.5 Ω for 30 min, and 6 and 37 Ω for 60 min by the Z-fitting program [13]. Researchers concluded that the 1-h sonication did not considerably change the surface area and impedance, even though the surface slightly increased by 0.1 × 10^-6 cm², and a slight reduction in internal resistance by 0.5 Ω was observed after the 60-min sonication.

Figure 6. (Color online) (a) CC and (b) impedance (inset shows the equivalent circuit) results. (c) CV results at 50 mV s^-1 and (d) specific capacitance of the samples.

Figure 6(c) shows the CV results of the samples at a scan rate of 50 mV s^-1 within the potential window of 0–0.8 V. The specific capacitance (C_s) could be determined from the CV results, which is proportional to the surface area[19].

where A is the specific area of the CV curve, $\Delta V$ denotes the potential window, V is the scan rate, and m denotes the loaded mass of the active material. The resultant specific capacitance as a function of scan rates is plotted in Fig. 6(d). The capacitances of the samples at a scan rate of 50 mV s^-1 are 35.6, 36.2, 36.5, and 37 F g^-1 for the samples sonicated at 0, 5, 10, 30, and 60 min, respectively. The capacitance results are consistent with the results of Fig. 6(a) and (b) because it is proportional to the surface area and inverse to the resistance. However, the difference in capacitance between the samples is minimal. Figure 7 shows the other CV results at different scan rates from 10 to 200 mV s^-1. The CV results of five samples are similar at scan rates ranging from 10 to 200 mV s^-1.

Figure 7. (Color online) CV results of samples sonicated for (a) 0, (b) 5, (c) 10, (d) 30, and (e) 60 min.

Sonication is an important process before electrochemical measurements. Many reviewers and researchers are concerned about the sonication effect on the particle size of materials. Herein, we investigated the size changes of MoS₂ particles after sonication for up to 1 hr. The average particle size reported from all five samples was 2.07 ± 0.13 μm, which is similar to the size of the precursor MoS₂ (2 μm). Furthermore, the XRD results also confirmed that the particle size after sonication is the same as that (2 μm) before sonication. Additionally, other electrochemical properties of the samples, such as electrochemical surface area, impedance, CV results, and capacitance, were similar.