Chan Byon

Fabrication and Characterization of the Capillary Performance of Superhydrophilic Cu Micropost Arrays

We report the fabrication of dense arrays of super-hydrophilic Cu microposts at solid fractions as high as 58% and aspect ratios as high as four using electrochemical deposition and chemical oxidation techniques. Oxygen surface plasma treatments of photoresist molds and a precise control of the initial electrodeposition current are found to be critical in creating arrays of nearly defect-free Cu posts. The capillary performance of the micropost arrays is characterized using capillary rate of rise experiments and numerical simulations that account for the finite curvatures of liquid menisci. For the given wick morphology, the capillary performance generally decreases with increasing solid fraction and is enhanced by almost an order of magnitude when thin nanostructured copper oxide layers are formed on the post surface. The present work provides a useful starting point to achieve optimal balance between the capillary performance and the effective thermal conductivity of advanced wicks for micro heat pipes.

The effect of meniscus on the permeability of micro-post arrays

This study aims to investigate the effect of meniscus curvature on the permeability of the micro-post arrays, which are widely used for applications of microfluidics. An analytical model that accounts for the meniscus curvature is developed. The model considers two common array types: quadratic and hexagonal arrays. The permeability of micro-post arrays is estimated using the capillary rate of rise experiment and numerical simulation. The results obtained from the analytical model match the experimental and numerical results within the error of 5% over the range of parameters commonly found in microfluidic applications (0.06 0.2), where d * and H * are the post-diameter and the post-height, respectively, which are normalized by the pitch. Based on the analytic results, the effects of the post-diameter, post-height and the contact angle on the permeability of post-arrays are investigated. It is shown that the previous permeability models based on the flat meniscus assumption overestimate the experimental value by 26% for the quadratic array and 24% for the hexagonal array when cos θ = 1, d * = 0.5 and H *=1. The effect of the meniscus curvature is shown to become more pronounced as the contact angle or the post-height decreases.

Improved photocatalytic activity of MoS2 nanosheets decorated with SnO2 nanoparticles

MoS2 nanosheets decorated with SnO2 mesoporous nanoparticles were successfully prepared by a facile two-step method. The MoS2 nanosheets were pre-synthesized using a solvothermal method and then decorated with the SnO2 mesoporous nanoparticles through a wet chemical method. The nanocomposite was characterized with powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersed spectrometry (EDX), high-resolution transmission electron microscopy (HRTEM), thermal gravimetric and differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). SnO2 mesoporous nanoparticles can be selectively formed and attached to the peripheral surface of the layered MoS2, which was confirmed by FESEM and HRTEM. The photocatalytic activity of the nanocomposite was examined with Rhodamine B (RhB) in aqueous solution under UV light irradiation. The SnO2 nanoparticles remarkably suppressed the electron–hole recombination effect on the MoS2 photocatalyst and improved the photocatalytic activity compared to a pristine MoS2 catalyst. A higher rate of pollutant degradation was accomplished within 50 min that was three times higher than that of the pristine MoS2 catalyst.

Drag reduction in Stokes flows over spheres with nanostructured superhydrophilic surfaces

Nanostructured surfaces offer opportunities to modify flow induced drag on solid objects. Measurements of the terminal velocity reveal that the drag associated with laminar Stokes flows can be reduced for spheres with nanostructured superhydrophilic as well as superhydrophobic surfaces. Numerical simulations suggest that the formation of recirculating or nearly stagnant flow zones leads to significant reduction in the friction drag. Such reduction, however, is offset by an increase in the form drag that arises from nonuniform pressure distributions. Our work motivates further studies to optimally balance the friction and form drag and control resistance to laminar flows over objects with nanostructured surfaces.

Synthesis and characterization of molybdenum disulfide nanoflowers and nanosheets: nanotribology

This paper reports the solvothermal synthesis of MoS2 nanoflowers and nanosheets. The nanoflowers have a mean diameter of about 100 nm and were obtained using thioacetamide (C2H5NS) as a sulfur source. The few layered nanosheets were obtained using thiourea (CH4N2S) as a sulfur source. The obtained powders were characterized using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The lubricating effect of MoS2 nanoflowers and nanosheets were analyzed using four-ball test, the topography of the wear scar was analyzed using SEM, EDS, and 3D surface profilometry. The relationship between the tribological properties and morphology of the materials was determined. It is observed that the engine oil containing the MoS2 nanomaterials penetrated more easily into the interface space, and it formed a continuous film on the interface surface. The tribological performance showed that the synthesized nanosheets had superior antiwear and friction-reducing properties as a lubrication additive compared with nanoflowers. Also, the wear scar of balls lubricated with nanoflowers revealed a larger diameter compared to nanosheets. In conclusion, nanosheets dispensed in oil have better tribological performance compared to nanoflowers oil in terms of capability to reduce friction.

A numerical study on the dynamics of droplet formation in a microfluidic double T-junction

In this study, droplet formations in microfluidic double T-junctions (MFDTD) are investigated based on a two-dimensional numerical model with volume of fluid method. Parametric ranges for generating alternating droplet formation (ADF) are identified. A physical background responsible for the ADF is suggested by analyzing the dynamical stability of flow system. Since the phase discrepancy between dispersed flows is mainly caused by non-symmetrical breaking of merging droplet, merging regime becomes the alternating regime at appropriate conditions. In addition, the effects of channel geometries on droplet formation are studied in terms of relative channel width. The predicted results show that the ADF region is shifted toward lower capillary numbers when channel width ratio is less than unity. The alternating droplet size increases with the increase of channel width ratio. When this ratio reaches unity, alternating droplets can be formed at very high water fraction (wf = 0.8). The droplet formation in MFDTD depends significantly on the viscosity ratio, and the droplet size in ADF decreases with the increase of the viscosity ratio. The understanding of underlying physics of the ADF phenomenon is useful for many applications, including nanoparticle synthesis with different concentrations, hydrogel bead generation, and cell transplantation in biomedical therapy.

ZrO2/MoS2 heterojunction photocatalysts for efficient photocatalytic degradation of methyl orange

We report a simple solution-chemistry approach for the synthesis of ZrO2/MoS2 hybrid photocatalysts, which contain MoS2 as a cocatalyst. The material is usually obtained by a wet chemical method using ZrO(NO3)2 or (NH4)6Mo7O24·4H2O and C8H6S as precursors. The structural features of obtained materials were characterized by X-ray diffraction (XRD), highresolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), thermal analysis (TG-DTA), N2 adsorption-desorption, and photoluminescence (PL). The influence on the photocatalytic activity of the MoS2 cocatalyst concentration with ZrO2 nanoparticles was studied. The MZr-2 hybrid sample had the highest photocatalytic activity for the degradation of methyl orange (MO), which was 8.45 times higher than that of pristine ZrO2 ascribed to high specific surface area and absorbance efficiency. Recycling experiments revealed that the reusability of the MZr-2 hybrid was due to the low photocorrosive effect and good catalytic stability. PL spectra confirmed the electronic interaction between ZrO2 and MoS2. The photoinduced electrons could be easily transferred from CB of ZrO2 to the MoS2 cocatalyst, which facilitate effective charge separation and enhanced the photocatalytic degradation in the UV region. A photocatalytic mechanism is proposed. It is believed that the ZrO2/MoS2 hybrid structure has promise as a photocatalyst with low cost and high efficiency for photoreactions.

Hydrothermally synthesized highly dispersed Na2Ti3O7 nanotubes and their photocatalytic degradation and H2 evolution activity under UV and simulated solar light irradiation

Photocatalytic water splitting technologies are currently being considered for alternative energy sources. However, the strong demand for a high H2 production rate will present conflicting requirements of excellent photoactivity and low-cost photocatalysts. The first alternative may be abundant nanostructured titanate-related materials as a photocatalyst. Here, we report highly dispersed Na2Ti3O7 nanotubes synthesized via a facile hydrothermal route for photocatalytic degradation of Rhodamine B (RhB) and the water splitting under UV-visible light irradiation. Compared with commercial TiO2, the nanostructured Na2Ti3O7 demonstrated excellent photodegradation and water splitting performance, thus addressing the need for low-cost photocatalysts. The as-synthesized Na2Ti3O7 nanotubes exhibited desirable photodegradation, and rate of H2 production was 1,755 μmol·g−1·h−1 and 1,130 μmol·g−1·h−1 under UV and simulated solar light irradiation, respectively; the resulting as-synthesized Na2Ti3O7 nanotubes are active in UV light than that of visible light response.

Sacrificial-template-free synthesis of core-shell C@Bi 2 S 3 heterostructures for efficient supercapacitor and H 2 production applications

Core-shell heterostructures have attracted considerable attention owing to their unique properties and broad range of applications in lithium ion batteries, supercapacitors, and catalysis. Conversely, the effective synthesis of Bi2S3 nanorod core@ amorphous carbon shell heterostructure remains an important challenge. In this study, C@Bi2S3 core-shell heterostructures with enhanced supercapacitor performance were synthesized via sacrificial- template-free one-pot-synthesis method. The highest specific capacities of the C@Bi2S3 core shell was 333.43 F g−1 at a current density of 1 A g−1. Core-shell-structured C@Bi2S3 exhibits 1.86 times higher photocatalytic H2 production than the pristine Bi2S3 under simulated solar light irradiation. This core-shell feature of C@Bi2S3 provides efficient charge separation and transfer owing to the formed heterojunction and a short radial transfer path, thus efficiently diminishing the charge recombination; it also facilitates plenty of active sites for the hydrogen evolution reaction owing to its mesoporous nature. These outcomes will open opportunities for developing low-cost and noble-metal-free efficient electrode materials for water splitting and supercapacitor applications.

Synthesis and structural characterization of Al 2 O 3 -coated MoS 2 spheres for photocatalysis applications

This paper reports the synthesis of novel monodisperse Al2O3-coated molybdenum disulfide nanospheres (i.e., core-shell structures) using a one-step facile hydrothermal method. XPS analysis confirmed the purity and stable structure of the Al2O3- coated MoS2 nanospheres. A possible growth mechanism of the core-shell structure is also reported, along with their influence on the photodegradation process of rhodamine B (RhB). The Al2O3-coated MoS2 nanospheres demonstrate good photocatalytic activity and chemical stability compared to MoS2 spheres. TG-DTA analysis provided insight into the decomposition process of the precursor solution and the stability of the nanoparticles. The enhanced photocatalytic activity makes the Al2O3-coated MoS2 nanospheres a promising candidate as a photocatalyst that could be used in place of traditional Al2O3/MoS2 photocatalyst for the removal of pollutants from waste water.