Formation and evolution of orientation-specific CO chains on nonpolar ZnO(10͞10) surfaces.

Clarifying the fundamental adsorption and diffusion process of CO on single crystal ZnO surfaces is critical in understanding CO activation and transformation over ZnO-based catalysts. By using ultrahigh vacuum-Fourier transform infrared spectroscopy (UHV-FTIRS), we observed the fine structures of CO vibrational bands on ZnO(100) surfaces, which are the combinations of different vibrational frequencies, originated from CO monomer, dimer, trimer and longer polymer chains along [0001] direction according to the density functional theory calculations. Such novel chain adsorption mode results from the relatively large attractive interaction between CO and Zn atoms in [0001] direction. Further experiments indicate that the short chains at low coverage evolve into long chains through Ostwald ripening by annealing. At higher CO coverage (0.7 ML), the as-grown local (2 × 1) phase of chains first evolve into an unstable local (1 × 1) phase below 150 K, and then into a stable well-defined (2 × 1) phase above 150 K.