Using chemical-state-specific scanned-energy-mode photoelectron diffraction (PhD) from O 1s and C 1s photoemission, we have determined the local structure of the surface species produced on the rutile TiO 2 (110) surface as a result of room temperature exposure to formic acid. The results show clear evidence for the coexistence of formate, HCOO, and hydroxyl, OH, surface species. The formate species is aligned along [001], bridging an adjacent pair of surface 5-fold-coordinated Ti atoms with the formate O atoms nearly atop the Ti atoms with a Ti-O bond length of 2.08 ( 0.03 Å, consistent with scanning tunneling microscopy observations, a number of theoretical calculations, and an earlier very restricted PhD study. The hydroxyl species are formed by H attachment to the surface bridging O atoms and have a Ti-O bond length of 2.02 ( 0.05 Å, significantly longer than for the bridging oxygen atoms on a bulk-terminated surface or as previously reported for the clean surface. Our results exclude the possibility of a large (1/3) fractional occupation by the formate species of a second site azimuthally rotated by 90°and bonded to a surface oxygen vacancy site, as proposed in some earlier infrared and X-ray absorption spectroscopic studies. A much smaller concentration of such a second species cannot be excluded.

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