Leed crystallographic studies of structures formed on the (110) and (111) surfaces of Rhodium
Creators: name: Wong, Kin-Chung
Publisher: name: University of British Columbia
Year: 2009

description: Surface crystallographic analyses have been undertaken with the tensor LEED approach in low-energy electron diffraction (LEED) in order to contribute to the development of the principles of surface structural chemistry. For the Rh(110)-c(2x2)-S surface structure, each S atom chemisorbs on a center site of the Rh(110) surface. It bonds to the second layer Rh atom directly below, with a bond distance equal to about 2.27 A, and to four neighboring First layer Rh atoms at close to 2.47 A. A significant feature of this structure is that the second metal layer is buckled; those Rh atoms directly below the S atoms relax down by about 0.11 A compared with the other second layer Rh atoms. This buckling is apparently driven by the need to reduce the difference that would otherwise occur between these two types of S-Rh bond lengths. A component in the observed difference between the S-Rh distances appears to be dependent on the metallic coordination number for the Rh atoms. For the corresponding higher coverage Rh(110)-(3x2)-S surface, the result supports an arrangement of chemisorbed S atoms at 2/3 monolayer (ML) coverage on a basically unreconstructed metallic structure. Alternating [110] channels in the metal surface are occupied differently, although each has two S atoms per unit mesh. In one set, the S atoms occupy long-bridge and center sites with a constant separation along the channel of 4.03 A. In the other set of channels, all S atoms occupy equivalent positions, displaced from regular center sites by 0.39 A, to give successive S to S separations of 3.47 and 4.60 A. The long-bridge site bonding is a novel feature which is facilitated by the neighboring topmost Rh atoms relaxing laterally by about 0.27 A perpendicular to the [110] row. Bucklings of 0.20 and 0.10 A are indicated to occur in the first and the second Rh layers respectively; the latter value essentially equals that (0.11 A) in the corresponding c(2x2) surface. For S atoms at or near center sites in the (3x2) structure, the average S-Rh bond distances to the first and the second Rh layer atoms are 2.42 and 2.29 A respectively; the corresponding values at the long-bridge sites are 2.20 and 2.27 A. New analyses for the Rh(111)-(&gt;^x&gt;/3)R30o-S and Rh(111)-c(4x2)-S surface structures indicate S coverages of 1/3 and 1/2 M L respectively. For the lower-coverage form, S adsorbs on the regular three-coordinate sites which continue the fcc packing sequence; the S-Rh bond lengths are indicated to equal 2.23 A, and relaxations in the metallic structure are negligible. In the c(4x2) form, the adsorption occurs equally on both types of three-coordinate site (fcc and hcp), although some surface Rh atoms bond to two S atoms while others bond to only one, and this sets up some interesting relaxations. Specifically, the S atoms displace laterally from the center of the threefold sites by 0.20 to 0.29 A, and the first metal layer is buckled by about 0.23 A. The first-to-second interlayer spacing in the metal expands to 2.26 A from the bulk value of 2.20 A. The average S-Rh bond length equals 2.22 A, and is not significantly changed from that in the low-coverage form. The structural evolution with increasing coverage for S chemisorbed on the (111) surface of Rh shows a close relation to the corresponding evolution on the Rh(110) surface. ln the Rh(111)-(2x1)-0 surface structure formed by chemisorption of O on the Rh(111) surface, O atoms chemisorb close to the regular fee sites. The structure shows significant relaxations; for example, a buckling of about 0.07 A is indicated in the first metal layer and O appears to displace laterally by about 0.05 A. The individual O-Rh bond lengths are around 2.01 and 1.92 A to top layer Rh atoms which bond to two and one O atoms respectively. Comparison is made with O-Rh bond lengths determined recently by another group for the Rh(110)-p2mg(2x1)-0 surface structure.