Article pubs.acs.org/JPCC
Molecular Dynamics Study on Metal-Deposited Iron Oxide Nanostructures and Their Gas Adsorption Behavior Jeffrey Yue, Xuchuan Jiang,* and Aibing Yu School of Materials Science and Engineering, The University of New South Wales, Sydney, New South Wales 2052, Australia ABSTRACT: This study presents a comprehensive theoretical and experimental study on the deposition mechanism of precious metal nanoparticles (e.g., Au, Pt, Ag, and Pd) onto iron oxides (e.g., α-FeOOH, α-Fe2O3, and Fe3O4), which can be generated through a hydrothermal synthesis at ambient conditions. By using molecular dynamics (MD) method, the surface interaction energy between metal nanoparticle and iron oxide surface can be quantified. The analysis shows that the total potential energy of the metal nanoparticles decreases significantly when the metal particles are deposited onto porous or defected sites. The interaction is heavily dependent on nanoparticle size, elemental species, and surface structure of the iron oxide (porous or defects). The van der Waals force was found to play a dominant role in the deposition process. The MD simulation shows that the gases (e.g., CO, propene) on such nanocomposites have a lower diffusion coefficient at the Au/Fe2O3 surface and hence enhance catalytic activity, which may help understand the catalytic mechanism of metal oxide nanocomposites.
1. INTRODUCTION Iron oxide nanoparticles, such as goethite (α-FeOOH), hematite (α-Fe2O3), and magnetite (Fe3O4), have exhibited unique electronic and magnetic properties,1 thus making them desirable for diverse applications in many areas such as catalysts, biomedicine, and gas sensors.2,3 Recently, these materials have gained attention for their enhanced performance by doping a small amount (95% to the electrostatic energy. However, for the cases regarding metal−metal oxide interactions, the neutral Au atoms may not supply electrostatic forces and thus remain constant since there is no significant movement of charges during the deposition process, even though 80% of the total energy of the system originates from electrostatic forces. With respect to particle geometry, the contact surface area is minimal between the sphere and the flat surface but significantly increases on a porous surface. Even though a weak vdW force exists in the system, the increased contact surface area creates a better configuration for the deposited nanoparticle. This is consistent with the MD simulations on the Au/TiO2 nanocomposite where the surface defects TiO2 matrix can result in a strong nanocluster−surface interaction.17 It is found that the size and shape of the deposited Au nanocluster can be controlled by introducing a suitable defect, which may prevent Au colloid aggregation and shape transformation. In general, the degree of defects on the surface structure and the types of defects or porous sites can highly influence the morphology of the deposited AuNP and increase the overall interaction energy. More details are under investigation by MD simulation. It is worth noting that the Au deposition also depends on whether a porous or defect surface is created on the surface of
Table 2. Calculated Energies before and after Deposition of AuNP onto the α-Fe2O3 Surface averaged potential energy (kcal/mol) energy component
before deposition (0−50 ps)
after deposition (150−200 ps)
difference
vdW electrostatic total
−10 102.73 −48 950.54 −59 053.27
−10 751.86 −48 950.54 −59 701.40
−649.13 0.0 −649.13
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dx.doi.org/10.1021/jp212139u | J. Phys. Chem. C 2012, 116, 8145−8153
The Journal of Physical Chemistry C
Article
α-Fe2O3, and Fe3O4 are −64.1, −168.3, and −170.0 kcal/mol, respectively; while the interaction energies on a porous surface increase to −357.8, −524.7, and −605.0 kcal/mol, respectively. Similar phenomena can also be found for Pd and Pt nanoparticles. In addition, the difference in interaction energies can be observed on the porous surface, especially for α-FeOOH and αFe2O3. The interaction energies of Au and Pt particles are relatively higher than those of Ag and Pd particles: for example, Au/α-FeOOH (−400 kcal/mol) and Pt/α-FeOOH (−450 kcal/mol) have higher values than Ag/α-FeOOH (−355 kcal/ mol) and Pd/α-FeOOH (−380 kcal/mol), respectively. These differences can probably provide a reasonable explanation why certain metals (e.g., Au, Pt) are more compatible on the iron oxide surfaces than others (e.g., Ag, Pd). For instance, Zheng and Stucky39 demonstrated deposition of AuNPs with dimensions less than 10 nm onto different oxide supports assisted by strong reducing agents, where the nanocomposites show good performance in ethanol oxidation due to the sizedependent properties of AuNP in the material. On the contrary, Ag and Pd nanoparticles are more stable on SiO2 or the surfaces modified by surfactants and/or alkanethiols.40 Although it is possible to produce Ag-deposited SiO2 and polymer structures, the Ag particles still tend to grow into larger particles (15−60 nm) or thin film coatings.7,41 This can be further explained through analysis of the crystalline properties of metal nanoparticles by using the X-ray diffraction scattering function. Due to the strong scattering factor of α-Fe2O3(110) and Fe3O4(311), which coincide with M(111) (M = Au, Pt, Ag, and Pd) peaks, only the α-FeOOH pattern could be shown in this study. The peaks are in agreement with the reported values of the corresponding metals (JCPDS cards 04-0784, gold; 70-2057, platinum; 04-0783, silver; and 99-2335, palladium). Comparison of the curves reveals that Au and Pt form a crystalline phase with high intensity on the Au(111) and Pt(111) planes (Figure 10). The
Figure 8. (A) Graphic plot of the relationship between interaction energies and the number of Au atoms contained in an Au cluster and (B) radial distribution function of the Au atoms against the porous α-Fe2O3 surface interacting with AuNP composed of different gold atoms.
and particle size needs systematic investigations. Much more work will be performed in the near future. 3.4. Deposition of Ag, Pd, and Pt Nanoparticles. To understand the compatibility of metals on iron oxide surface, we also investigated other noble metals (Ag, Pd, and Pt) under similar conditions. Figure 9 shows the interaction energies of
Figure 10. Simulated X-ray diffraction patterns showing the structural crystallization of noble metal particles deposited on the porous αFeOOH(010) surface.
Figure 9. Column graph showing the interaction energies of various metals depositing on smooth and porous iron oxide surfaces.
the noble metals on smooth and porous iron oxide surfaces. They exhibit similar interactions of
