Meng-Hsiung Weng

Structure-dependent mechanical properties of ultrathin zinc oxide nanowires.

Mechanical properties of ultrathin zinc oxide (ZnO) nanowires of about 0.7-1.1 nm width and in the unbuckled wurtzite (WZ) phase have been carried out by molecular dynamics simulation. As the width of the nanowire decreases, Youngs modulus, stress-strain behavior, and yielding stress all increase. In addition, the yielding strength and Youngs modulus of Type III are much lower than the other two types, because Type I and II have prominent edges on the cross-section of the nanowire. Due to the flexibility of the Zn-O bond, the phase transformation from an unbuckled WZ phase to a buckled WZ is observed under the tensile process, and this behavior is reversible. Moreover, one- and two-atom-wide chains can be observed before the ZnO nanowires rupture. These results indicate that the ultrathin nanowire possesses very high malleability.

Adsorption and Dissociation of NH3 on Clean and Hydroxylated TiO2 Rutile (110) Surfaces: A Computational Study

The adsorption and dissociation of NH3 on the clean and hydroxylated TiO2 rutile (110) surfaces have been investigated by the first‐principles calculations. The monodentate adsorbates such as H3NTi(a), H2NTi(a), NTi(a), H2NO(a), HNO(a), NO(a) and HO(a), as well as the bidentate adsorbate, TiNTi(a) can be formed on the clean surface. It is found that the hydroxyl group enhances the adsorption of certain adsorbates on the five‐fold‐coordinated Ti atoms (5c‐Ti), namely H2NTi(a), HNTi(a), NTi(a) and TiNTi(a). In addition, the adsorption energy increases as the number of hydroxyl groups increases. On the contrary, the opposite effect is found for those on the two‐fold‐coordinated O atoms (2c‐O). The enhanced adsorption of NHx (x = 1 − 2) on the 5c‐Ti is due to the large electronegativity of the OH group, increasing the acidity of the Ti center. This also contributes to diminish the adsorption of NHx (x = 1 − 2) on the two‐fold‐coordinated O atoms (2c‐O) decreasing its basicity. According to potential energy profile, the NH3 dissociation on the TiO2 surface is endothermic and the hydroxyl group is found to lower the energetics of H2NTi(a)+HO(a) and HNTi(a)+2{HO(a)}, but slightly raise the energetic of TiNTi(a)+3{HO(a)} compare to those on the clean surface. However, the dissociation of NH3 is found to occur on the hydroxylated surface with an overall endothermic by 31.8 kcal/mol and requires a barrier of 37.5 kcal/mol. A comparison of NH3 on anatase surface has been discussed. The detailed electronic analysis is also carried out to gain insights into the interaction nature between adsorbate and surface.

Dynamical behaviour of 7-1 gold nanowire under different axial tensile strains

The phonon spectrum has in recent years attracted much interest in investigating electrical transfer in nanowires. In this study, molecular dynamics is employed to calculate the phonon property of thinness for 7-1 gold nanowire at room temperature. This study finds that the phonon density distribution of 7-1 nanowire is different from both that of bulk face-centred cubic (fcc) structure and that of nanoparticles. We also observed that the high frequency of the phonon spectrum in the bulk fcc structure has a higher intensity than the low frequency of the phonon spectrum. This phenomenon, however, does not appear in the 7-1 nanowire. The strain effect of 7-1 nanowire on the phonon property has also been studied; moreover, the phonon spectra of core–core and shell–shell structures of nanowire exhibit two different modes. The relationship between the phonon profile of these core and shell structures is analysed in this study.

Adsorption of water molecules inside a Au nanotube: a molecular dynamics study.

A molecular dynamics simulation of water molecules through a Au nanotube with a diameter of 20 A at bulk densities 0.8, 1, and 1.2 gcm(3) has been carried out. The water molecules inside a nanoscale tube, unlike those inside a bulk tube, have a confined effect. The interaction energy of the Au nanotube wall has a direct influence on the distribution of water molecules inside the Au tube in that the adsorption of the water molecules creates shell-like formations of water. Moreover, the high number of adsorbed molecules has already achieved saturation at the wall of the Au nanotube at three bulk densities. This work compares the distribution percentage profiles of hydrogen bonds for different regions inside the tube. The structural characteristics of water molecules inside the tube have also been studied. The results reveal that the numbers of hydrogen bonds per water molecule influence the orientational order parameter q. In addition, the phenomenon of a group of molecules bonded inside the tube can be observed as the number of hydrogen bonds increase.

Potential applications of two ultrathin Pd nanowires to the hydrogen economy

The structures of two ultrathin Pd nanowires were predicted by the simulated annealing basin-hopping method (SABH) with the tight-binding potential. Their material properties for applications to the hydrogen economy were further examined by density functional theory (DFT) calculation and DFT molecular dynamics (DFT-MD) simulation. In terms of thermal stability, these two Pd nanowires are still very stable at temperatures as high as 400 K. For the dissociation of hydrogen molecules, results show the dissociation is almost barrierless, and their catalytic reactivity is very similar to the Pd bulk material. The thermal stability of the H atom within these Pd nanowires was also investigated by DFT-MD, with results showing that the H atom can only stay within Pd nanowires at temperatures much lower than room temperature (298 K). This phenomenon is very different from that of H atoms within Pd bulk material or other reported nanomaterials, leading to hydrogen embrittlement, the main drawback for Pd materials applications. Our results reveal that these two ultrathin Pd nanowires not only possess the same excellent catalytic activity for hydrogen molecules as the bulk Pd material or other Pd nanomaterials, but also avoid the hydrogen embrittlement plaguing Pd bulk materials, which is the main limit to its applications in such things as hydrogen purification, storage, and detection.

The collective motion of carbon atoms in a (10,10) single wall carbon nanotube under axial tensile strain

Molecular dynamics is employed to investigate the collective motion of carbon atoms of a (10,10) single walled carbon nanotube under tensile loading. The “slip vector” parameter is used to study the collective motion of a group of atoms and deformation behavior in three different directions (axial, tangential, and radial directions). According to the variations in the slip vectors in different directions and the stress-strain profile, the axial tensile process can be divided into three stages. In tensile loading process, two local deformation types (symmetrical and nonsymmetrical deformations) are observed before reaching the maximum stress. The variations in the slip vector values of each atom indicate a symmetrical rupture which is oriented 45° to the axis of the nanotube after reaching the maximum stress. By tensile loading, chainlike structures can be formed before the breaking of the nanotube.

Electronic properties of a zinc oxide nanotube under uniaxial tensile strain: a density functional theory study

In this article, density functional theory calculations were employed to investigate the electronic properties of (4,4) armchair zinc oxide single-walled nanotubes (ZNONTs) under uniaxial mechanical deformations. It was found that the highest-occupied molecular orbital and the lowest-unoccupied molecular orbital gap and the value of radial buckling will both decrease linearly with the increase of axial strain. The elongation of the ZNONT mainly originates from the decrease and increase of two characteristic bond angles rather than Zn–O ionic bond elongation. This mechanical behavior is very different from the uniaxial tensional processes of carbon nanotubes and silicon carbide nanotubes formed by covalent bonds. The partial densities of states of the Zn atom and O atom show that the unoccupied states are gradually left-shifted as ZNONT elongates from 0 to 15%. Neither Mulliken charge nor deformation density clearly changes with the different tension strains. Bond order analysis also indicates the bonding strength will decrease as the strain increases from 0 to 15%.

Dynamic behaviour of multi-shell 14-7-1 gold nanowire under different axial tensile strains

The dynamic behaviour of a multi-shell 14-7-1 Au nanowire is investigated and compared with that of a 7-1 Au nanowire under various tensile strains. The molecular dynamics (MD) simulations performed in this study compute the frequency shift and change in intensity of the characteristic frequency peaks of the phonon density of states of the global and substructure atoms in the radial and axial directions. The results show that the frequency shift and intensity change of the characteristic frequency peaks in the strained 14-7-1 nanowire are caused by a change in the radial atomic coupling and a higher degree of structural anisotropy due to the applied strain.

The Diffusion Behaviors of Hydrogen Atoms Inside an Octahedral Pd Nanowire

In our previous study, we have proven two ultrathin Pd nanowires can prevent from Pd embrittlement. In this study, we further demonstrate the material properties of a larger Pd nanowire. Both classical potential function and density functional theory calculation were used. The results show once the larger Pd nanowire has interior Pd atoms, hydrogen atoms can stably stay within the nanowire. Consequently, the hydrogen embrittlement could happen with a higher probability than two ultrathin Pd nanowires reported in our previous study.

Mechanical and dynamical behavior of carbon nanotube with defects:A molecular dynamics simulation

In this paper, molecular dynamics simulation is carried out to investigate the mechanical properties and dynamic behaviors of carbon nanotubes with vacancies and related defects. We have shown the Yangs modulus and tensile strength of nanotubes with vacancy-related defects are dependent on the characteristics of vacancies. Meanwhile, the phonon density of state (DOS) are used to study the vibration properties of carbon nanotubes of various types. and further determine the dynamical behavior of carbon nanotubes under different tensile strains.