A facile chemical conversion synthesis of Sb2S3 nanotubes and the visible light-driven photocatalytic activities

We report a simple chemical conversion and cation exchange technique to realize the synthesis of Sb2S3 nanotubes at a low temperature of 90°C. The successful chemical conversion from ZnS nanotubes to Sb2S3 ones benefits from the large difference in solubility between ZnS and Sb2S3. The as-grown Sb2S3 nanotubes have been transformed from a weak crystallization to a polycrystalline structure via successive annealing. In addition to the detailed structural, morphological, and optical investigation of the yielded Sb2S3 nanotubes before and after annealing, we have shown high photocatalytic activities of Sb2S3 nanotubes for methyl orange degradation under visible light irradiation. This approach offers an effective control of the composition and structure of Sb2S3 nanomaterials, facilitates the production at a relatively low reaction temperature without the need of organics, templates, or crystal seeds, and can be extended to the synthesis of hollow structures with various compositions and shapes for unique properties

Linear stability analysis of magnetohydrodynamic duct flows with perfectly conducting walls

<div><p>The stability of magnetohydrodynamic flow in a duct with perfectly conducting walls is investigated in the presence of a homogeneous and constant static magnetic field. The temporal growth and spatial distribution of perturbations are obtained by solving iteratively the direct and adjoint governing equations with respect of perturbations, based on nonmodal stability theory. The effect of the applied magnetic field, as well as the aspect ratio of the duct on the stability of the magnetohydrodynamic duct flow is taken into account. The computational results show that, weak jets appear near the sidewalls at a moderate magnetic field and the velocity of the jet increases with the increase of the intensity of the magnetic field. The duct flow is stable at either weak or strong magnetic field, but becomes unstable at moderate intensity magnetic field, and the stability is invariance with the aspect ratio of the duct. The instability of magnetohydrodynamic duct flow is related with the exponential growth of perturbations evolving on the fully developed jets. Transient growth of perturbations is also observed in the computation and the optimal perturbation is found to be in the form of streamwise vortices and localized within the sidewall layers. By contrast, the Hartmann layer perpendicular to the magnetic field is irrelevant to the stability issue of the magnetohydrodynamic duct flow.</p></div

The spatial structures of optimal perturbations at initial with <i>Re</i> = 7000, <i>Ha</i> = 20 and <i>r</i> = 1 (left column) and <i>r</i> = 9 (right column).

<p>(a) <i>τ</i> = 13, <i>α</i> = 1.2; (b) <i>τ</i> = 13, <i>α</i> = 1.2; (c) <i>τ</i> = 50, <i>α</i> = 3.0; (d) <i>τ</i> = 50, <i>α</i> = 0.2.</p

The optimal perturbation at different final time <i>τ</i> for <i>Re</i> = 7000 and <i>Ha</i> = 20 in the square cross-section duct.

<p>(a) <i>τ</i> = 13, <i>α</i> = 1.2; (b) <i>τ</i> = 50, <i>α</i> = 3.0. The iso-surfaces correspond to the ±0.45 and ±0.90 of the maximum magnitude of the streamwise velocity component.</p

Basic velocity profiles for different Hartmann numbers <i>Ha</i> in the square cross-section duct.

<p>(a) Velocity profiles <i>U</i>(<i>y</i>, <i>z</i> = 0) within the sidewall layers; (b) Velocity profiles <i>U</i>(<i>y</i> = 0, <i>z</i>) in the central cross-section parallel to the sidewall of the duct.</p

Basic velocity profiles in ducts with different aspect ratio <i>r</i> at <i>Ha</i> = 10 (a, b) and <i>Ha</i> = 50 (c, d).

<p>Velocity profiles <i>U</i>(<i>y</i> + <i>r</i>, <i>z</i> = 0) within the sidewall layers (a, c); Velocity profiles <i>U</i>(<i>y</i> = 0, <i>z</i>) in the central cross-section parallel to the sidewall of the duct (b, d).</p

(a) Energy amplification of perturbations <i>G</i> with different modes at <i>Re</i> = 3000 and <i>Ha</i> = 0; (b) Energy amplification of perturbations <i>G</i> in the duct with different aspect ratios <i>r</i> at <i>Re</i> = 3000 and <i>Ha</i> = 0.

<p>(a) Energy amplification of perturbations <i>G</i> with different modes at <i>Re</i> = 3000 and <i>Ha</i> = 0; (b) Energy amplification of perturbations <i>G</i> in the duct with different aspect ratios <i>r</i> at <i>Re</i> = 3000 and <i>Ha</i> = 0.</p

Energy amplification of perturbations <i>G</i> and the corresponding streamwise wavenumber <i>α</i> as a fuction of <i>τ</i> at <i>Re</i> = 7000.

<p>(a, b) <i>Ha</i> = 10; (c, d) <i>Ha</i> = 20; (e, f) <i>Ha</i> = 30; (g, h) <i>Ha</i> = 50.</p