Materials and Nanotechnology modeling involves building models of physical and chemical processes, often driven by experimental data, generalizing the solutions of those models to make predictions and explain the experimental result. Starting from both the empirical and first principle simulation, we mainly work on applying these methods to cutting edge problems in materials science and nanotechnology to provide both qualitative and quantitative insights into many phenomena that are too complex to be dealt with by analytical methods or to determine material properties if experiment is not practical.

Hydrogen has been touted as the ultimate clean fuel since its combustion yields only water. However, there are still technological challenges on how to store hydrogen in a way that is safe and has a high energy density.

Carbon nanotube have been proposed as one possible storage medium. However, experiment on hydrogen storage in Nanotube could not be later reproduced by independent group. Theoretically, there is deficiency for using current DFT calculation because it neglects the overlap of electron at long range. It is not possible to give a good description on the interaction of H₂ with nanotube by using DFT calculation only. We incorporated a van der Waals term into DFT to study hydrogen interaction with graphite, nanotube and its bundle. Our preliminary results show VDW correction is very important when using DFT calculation, especially GGA.

I.   Background

Layered double hydroxides (LDH) are anionic clay materials. Their importance in areas such as catalysis, medicine and oil-field technology has greatly increased in the last two decades. The general formula of the most important group of LDH is described as followed Mg_1-xAl_x(OH)₂(NO₃)_x∙nH₂O. A variety of organic and inorganic structures can be intercalated into LDH structures, which makes them extremely promising for drug delivery and gene therapy applications. The interactions between these organic (DNA) or inorganic (NO₃^-) molecules and the nanoparticle layers are still poorly understood.
Computational techniques have been extremely valuable tools in understanding the structures of bio-organic and polymeric systems. The crystal structure of LDH nanoparticles has not been characterized yet, the closest solved structure for this family of LDH being the original crystalline brucite, MgOH₂, which consists of M²⁺ metallic cations coordinated octahedrally by hydroxyl groups. The charge density of the hydroxide layers is directly linked to the M²⁺/M³⁺ ratio of the cationic metallic matrix. Charge neutrality is maintained by intercalation of interlayer anions which are very often labile. DNA is typically negatively charged, which makes it a possible intercalating species. A model structure of the LDH containing NO₃^- counterions has been made, starting with the parent brucite cell parameters. A combination of different forcefields, CLAYFF and Amber, is being used to study the interlayer complex system via real time molecular dynamics (MD), which allows the starting structure to relax into the equilibrium structure.
　

Figure 1. Model of Brucite crystal cell and natural LDH crystals

II.   Proposed Mechanism

It was found that upon hybridization between inorganic layered double hydroxides and biomolecules such as DNA, the transfer efficiency of nanohybrid complexes through cell membranes is enhanced.  The charge interactions after nanohybridization would facilitate the penetration of nanohybrids into cells.  The proposed mechanism is generally an endocytosis phenomenon.  The charge neutralization between the nanoparticle layer and the DNA reduces the repulsion between the negatively charged cell surface and the positively charged DNA structure.

Figure 2. General mechanism illustrating the nanohybridization process and the endocytosis

phenomenon allowing penetration of DNA structures into the cell and then the nucleus

Figure 3. Animated representation illustrating the pinocytosis phenomenon

III.   Methodology and Program

We are currently importing forcefield parameters for this hybrid system.  We will use the Amber force field for the DNA structure and the modified Dreiding forcefield for the nanoparticle system.  The simulations will be run inside the simulation package DL_POLY.

III.   Current Research Status

The first trend, we can observe between the two model structures containing respectively Cl^- and NO₃^- counterions is that there is an increase of cell parameters, a and b. The original brucite structure being the closest known crystal solved structure for this family of LDH, we started with the brucite and we inserted the counterions between the layers. The second important observation concerning NO₃^- is that it seems to prefer a planar conformation than a “tilted” conformation like it was suggested in Xu et al. publication. NO₃^- undergoes successive rotation without really being “tilted”. Finally in both model structures, the interlayer distance is very stable, the atoms are reorganized in the layer plans for the Mg₂⁺ and Al₃⁺ metals. Al₃⁺ is more “mobile” than Mg₂⁺ and is easily displaced out of the layer plan.
　

These results are early results obtained in a short time length with a small number of atoms. The main goal was to generate suitable models for larger systems running at picoseconds or nanosecond time scale. It will be more accurate to compare to experimental results. Nevertheless, it was extremely useful to observe general trends that can be confirmed later on. We expect to be able to reproduce our results for larger system very soon. It will be very important monitor the solvatation level of LDH-bio-organic nanohybrids. Many experimental results were obtained in a solid stated using crystallographic analysis.

Molecular hydrogen storage in Magnesium alloy is another possible way for “on-board” vehicular storage of hydrogen. The hydrogen storage capacity of pure MgH₂ is 7.6 wt%, which is greater than the automobile industry standard (5 wt%). But there are still some problems for its application. One is the rate at which hydrogen absorbs and desorbs is too low and the other is that hydrogen atoms bind too strongly with the Mg atoms, a higher temperature and enough pressure (over 1 atm) are needed to release the hydrogen. We mainly work on the calculation of H₂ dissociation barrier and study the catalysis of other atom such as C, Ti and Fe by first principle method. This project is running in parallel with an experimental program on the synthesis and treatment of carbon nanotubes at the ARC Centre for Functional Nanomaterials.

The noncovalent functionalization of single-walled carbon nanotubes has possible application toward Hydrogen energy and chemical sensing technology. Current Density Functional Theory(DFT) calculation gives a poor description on the physisorption energy and equilibrium distance. We will combine the DFT calculation with a long range van der walls(VDW) interaction to study the physisorption of H₂, NO₂ and NH₃ on SWCNT. The covalent functionalization of SWCNT is believed to open the road towards real nanotechnology applications and bioapplication. We will first perform ab initio calculation to study the structural, electronic properties and stability of the functionalized group on SWCNT. And then we will simulate the reaction path way using a newly developed reactive Car-Parrinello Molecular Dynamics method.