Formation and evolution of orientation-specific CO₂ chains on nonpolar ZnO(10͞10) surfaces

Clarifying the fundamental adsorption and diffusion process of CO₂ on single crystal ZnO surfaces is critical in understanding CO₂ activation and transformation over ZnO-based catalysts. By using ultrahigh vacuum-Fourier transform infrared spectroscopy (UHV-FTIRS), we observed the fine structures of CO₂ vibrational bands on ZnO(100) surfaces, which are the combinations of different vibrational frequencies, originated from CO₂ 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 CO₂ and Zn_3c 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 CO₂ 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.