Group members
Lijun Xu
Introduction
Small metal clusters and thin films supported on oxide surfaces have many interesting properties and potential technological applications, such as in heterogeneous catalysis, magnetic nanostructures, and photonic devices. The understanding of the metal-oxide interface can help achieve the goal of tuning the functionality. We select the extensively studied MgO(100) as the oxide support and several different metals as the adsorbates. We hope the studies of those prototypes can shed light to some generic issues in metal/oxide systems, including the mechanisms and kinetics of particle and thin film growth, the role of surface defects in these processes, and the effect of the size and distribution of the metal clusters on the energetics, thermal stability, electronic properties and functionality in applications.
Pd/MgO(100)
The Pd/MgO(100) system is probably the most studied metal/oxide in fundamental research. Our studies have been focused on the mechanisms and kinetics of Pd cluster/particle growth via the density functional theory calculation. The dynamics is mainly based on the harmonic transition state theory. We have found that small Pd clusters are actually very mobile on the flat MgO(100) surface. This supports the general hypothesis of particle growth mainly on the strong-binding defect sites. Our calculations also suggest the point defects such as oxygen vacancies play the critical role in controlling the particle growth and a kinetic simulation based on charged oxygen vacancies can bring theory and experiment to agreement. We hope the studies of this system can help understand the growth dynamics of late transition metals on MgO(100).
Ca/MgO(100)
The calcium atom is different from transition metals, probably due to the electro-negativity. In collaboration with Professor Campbell's group at the University of Washington, we have systematically studied the Ca thin film growth on the MgO(100) surface. Our theoretical studies rationalize the experimental observations and help build the two-state growth model of Ca growth on MgO(100). We conclude that Ca mainly nucleate around the lower-coordianted defect sites (such as steps and kinks) while ignoring the oxygen vacancies at room temperature. It is remarkable to see the mobility of small clusters extend to a drastically different system like Ca/MgO(100). This may suggest the general nature of small metal clusters on oxides.
Li/MgO
Li is from the active alkaline group. The difference between Li and Ca is mainly on the electronic configuration: Li has one unpaired valence electron while the electrons in Ca are all paired. This fundamental difference can lead to distinctive behaviors in metal growth on MgO(100). Our DFT calculations, complemented with the experiments show that Li can nucleate and grow around both charged oxygen vacancies and extended defects such as steps and kinks at room temperature. In contrast to Ca/MgO(100), the Li adatoms show more electron density shifted to the substrate. The growth dynamics can also be explained through the two-state growth model.
References
L. Xu, J. A. Farmer, C. T. Campbell and G. Henkelman,
Comparison of Ca and Li adsorption on MgO(100) surface,
, (in preparation, 2009).
J. A. Farmer, C. T. Campbell, L. Xu, and G. Henkelman,
Defect Sites and their Distributions on MgO(100) by Li and Ca Adsorption Calorimetry,
J. Am. Chem. Soc 131, 3098-3103 (2009).
S. Li, Z. Zhang, D. Sheppard, B. D. Kay, J. M. White, Y. Du, I. Lyubinetsky, G. Henkelman, and Z. Dohnalek,
Intrinsic diffusion of hydrogen on rutile TiO2(110),
J. Am. Chem. Soc. 130, 9080 (2008).
L. Xu and G. Henkelman,
Calculations of Ca adsorption on a MgO(100) surface: Determination of binding sites,
Phys. Rev. B 77, 205404 (2008).
J. Zhu, J. A. Farmer, N. Ruzycki, L. Xu, C. T. Campbell and G. Henkelman,
Calcium Adsorption on MgO(100): Energetics, Structure and Role of Defects,
J. Am. Chem. Soc., 130, 2314 (2008).
L. Xu, C. T. Campbell, H. Jónsson and G. Henkelman,
Kinetic Monte Carlo simulations of Pd deposition and island growth on MgO(100),
Surf. Sci., 601, 3133, (2007).
L. Xu, G. Henkelman, C. T. Campbell and H. Jónsson,
Pd Diffusion on MgO(100): the role of defects and small clusters mobility,
Surf. Sci., 600, 1351, (2006).
L. Xu, G. Henkelman, C. T. Campbell and H. Jónsson,
Small Pd clusters, up to the tetramer at least, are highly mobile on the MgO(100) surface,
Phys. Rev. Lett., 95, 146103 (2005).