ABSTRACT
The chemistry and physics of under-coordination at a surface, which determines the process of catalytic reactions and growth nucleation, is indeed fascinating. However, extracting quantitative information regarding the coordination-resolved surface relaxation, binding energy, and the energetic behavior of electrons localized in the surface skin from photoelectron emission has long been a great challenge, although the surface-induced core level shifts of materials have been intensively investigated. Here we show that a combination of the theories of tight binding and bond order-length-strength (BOLS) correlation [C. Q. Sun, Prog. Solid State Chem., 2007, 35, 1-159], and X-ray photoelectron spectroscopy (XPS) has enabled us to derive quantitative information, by analyzing the Be 1s energy shift of Be(0001), (1010), and (1120) surfaces, for demonstration, regarding: (i) the 1s energy level of an isolated Be atom (106.416 ± 0.004 eV) and its bulk shift (4.694 eV); (ii) the layer- and orientation-resolved effective atomic coordination (3.5, 3.1, 2.98 for the first layer of the three respective orientations), local bond strain (up to 19%), charge density (133%), quantum trap depth (110%), binding energy density (230%), and atomic cohesive energy (70%) of Be surface skins up to four atomic layers in depth. It is affirmed that the shorter and stronger bonds between under-coordinated atoms perturb the Hamiltonian and hence the fascinating localization and densification of surface electrons. The developed approach can be applied to other low-dimensional systems containing a high fraction of under-coordinated atoms such as adatoms, atomic defects, terrace edges, and nanostructures to gain quantitative information and deeper insight into their properties and processes due to the effect of coordination imperfection.
ABSTRACT
The ability of a catalyst to accept or donate charge is the key to the process of catalytic reaction. However, the determination of the catalytic nature of a specimen as yet remains a great challenge. Here we report an effective yet simple method for this purpose based on the tight binding theory considerations and XPS monitoring of the evolution of valence and core electrons upon alloy formation. Firstly, we measured the valence and core band charge density of the constituent elements of Cu, Ag, and Pd and then the respective states upon alloy formation. A subtraction of the resultant spectrum of the alloy by the composed elemental spectra gives the residual that shows clearly the occurrence of charge trapping or polarization. We found that the valence and the core electrons of the CuPd alloy shift positively to deeper energies, opposite to the occurrences in the AgPd alloy. Findings clarify for the first time that CuPd serves as an acceptor due to quantum trapping and the AgPd as a donor because of charge polarization, which also explain why AgPd and CuPd perform very differently as important catalysts.
ABSTRACT
Incorporating the BOLS correlation algorithm [Y. Sun, J. Phys. Chem. C, 2009, 113, 14696] into high-resolution XPS measurements [J. N. Andersen, et al., Phys. Rev. B: Condens. Matter, 1994, 50, 17525; A. Baraldi, et al., New J. Phys., 2007, 9, 143] has produced an effective way of determining the 3d(5/2) energy levels of isolated Rh(302.163 +/- 0.003 eV) and Pd (330.261 +/- 0.004 eV eV) atoms and their respective bulk shifts (4.367 and 4.359 eV) with a refinement of the effective atomic coordination numbers of the top (100), (110), and (111) atomic layers (4.00, 3.87, and 4.26, respectively). It is further confirmed that the shorter and stronger bonds between under-coordinated atoms induce local strain and skin-depth charge-and-energy quantum trapping and, hence, dictate globally the positive core level binding energy shifts.
