Formation and evolution of orientation-specific CO2 chains on nonpolar ZnO(1010) 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 Zn3c 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.

In this paper, based on the high resolution UHV-FTIRS and DFT calculations, we reported the fine structures combined by of CO 2 vibrational levels on ZnO(1010) surfaces with increasing CO 2 coverage, which are attributed to the formation of [0001]-oriented short CO 2 polymer chains consisting of monomer, dimer, trimer and so on. The as-grown chains at high CO 2 coverage further evolve into the unstable local (1 × 1) phase below 150 K, and then relax into the stable (2 × 1) phase above 150 K. Figure 1 presents the polarized infrared reflection absorption spectroscopy (IRRAS) data of adsorbed CO 2 on ZnO(1010) surfaces at 90 K with IR light incident along [1210] direction. At the initial adsorption with 0.1 L (1 L = 1.33 × 10 −6 mbar·s) CO 2 dosage, one vibrational band at 1622 cm −1 first appears in s-polarized spectra (Fig. 1a); in p-polarized spectra, two bands at 1297 and 978 cm −1 (Fig. 1b) emerge simultaneously. These dramatically lowered vibration frequencies of adsorbed CO 2 relative to that of gas-phase CO 2 (2349 cm −1 ) demonstrate evidently that CO 2 have chemically adsorbed on ZnO(1010) surfaces. Based on the IR judgement principle on dielectric substrates 17,18 , the vibrational bands at 1622, 1297 and 978 cm −1 for CO 2 on ZnO(1010) surfaces are assigned respectively to the asymmetrical stretching mode (ν as (OCO), in-plane), symmetrical stretching mode (ν s (OCO), out-of-plane) and the stretching vibration between the carbon atom and the underneath surface O 3c atom (ν (CO 3c ), out-of-plane) for tridentate carbonates. (See Figure S1 for the detailed judgements of the CO 2 vibration direction through the polarized IRRAS.) Our assignment is in accordance with previous HREELS and FTIR reports 5,[6][7][8][9][10][11][12][13][14][15][16][17][18][19] .

Results and Discussion
It is interesting that we observed the fine structures of adsorbed CO 2 vibrational levels with increasing CO 2 coverage. For ν as vibration, besides the 1622 cm −1 band (Fig. 1a), a new band appears at 1582 cm −1 , and they finally converge into one intense band at 1590 cm −1 for saturated CO 2 coverage (2 L). Simultaneously, for ν s vibration (Fig. 1b), besides the 1297 cm −1 band, 1313 and 1337 cm −1 bands appear and finally evolve to one sharp 1340 cm −1 band. The ν (CO 3c ) band evolves from 978 cm −1 to 1008 cm −1 gradually. Therefore, we can divide the fine structures of tridentate carbonate vibration into four different groups as shown in Fig 1: I 1622 To examine whether the fine structures of CO 2 vibration is associated with the surface defects, we treated the ZnO(1010) surface in atomic oxygen atmosphere of 2 × 10 −6 mbar at 750 K and 10 L O 2 at 90 K, but no change in the fine structures is observed. Therefore, the prepared ZnO(1010) surface can be regarded as the stoichiometric surface with negligible surface defects. Recent STM studies also reported that no apparent oxygen vacancies or miss zinc-oxygen dimers were observed on the vacuum-annealed ZnO(1010) surface 20 . Actually, the fine structures of molecular vibrations have already been observed on some single crystal substrates, such as CO and CO 2 on MgO(100) and NaCl(100), which were attributed to the dipole-dipole-coupling in ordered molecular layers [13][14][15][16] instead of the influence of the surface defects.
To clarify the interaction between CO 2 on ZnO(1010), different configurations of two carbonates were designed and checked by DFT calculations. We first calculated the structure of CO 2 monomer on the ZnO(1010) surface of a (2 × 4) supercell with the long axis along [0001] direction. As shown in Fig. 2a, the tridentate configuration was confirmed, which is in well agreement with previous calculation results 5 . Then three distinct configurations of two CO 2 molecules were calculated. The results reveal that when two CO 2 form a chain along [0001] direction, the binding energy per molecule is the lowest, corresponding to the most stable configuration compared to the other distributions of the two CO 2 . The similar results were also calculated in a recent literature 21 .
Based on the above calculation result, we designed CO 2 molecular chains with different length on the ZnO(1010) surfaces of a (6 × 2) supercell and calculated the corresponding vibrational frequencies. The chain contains, respectively, one, two, three, four, five and infinite carbonates arraying end to end along the long axis [0001] direction. All the results are shown in Table 1. We can see that the calculated 1585 cm −1 , 1261 cm −1 and 958 cm −1 for the monomer respectively correspond to the experimental results of Group I: 1622 cm −1 , 1297 cm −1 and 978 cm −1 . Thus the bands of Group I are assigned to the carbonate monomer vibrations. It is easy to understand that most CO 2 are diluted at the initial adsorption on the surface to form carbonate monomers.
As shown in Table 1, the ν as dramatically redshifts from 1585 cm −1 of the monomer to 1546 cm −1 of the dimer, and it slightly changes from 1546 to 1540 cm −1 with increasing CO 2 from dimer to pentamer. Further lengthening the chain to infinite, the ν as blueshifts back to 1563 cm −1 . On the contrary, the ν s of monomer to pentamer increases monotonously from 1261 to 1302 cm −1 , and further to 1310 cm −1 for infinite length. The evolution trend of the calculated results is well consistent with our experimental IR frequencies. Therefore, the fine structures of CO 2 vibrations originate from the short CO 2 chains composed of monomer, dimer, trimer and so on when dosing CO 2 from 0.1 L to 1 L. Accordingly, in Table 1 the strong bands of Group IV measured at saturated CO 2 dosage (2 L) are assigned to the infinitely long chain vibrations.
Generally, the one-dimensional chain formation requires the symmetry loss of substrate surfaces 22 , such as trenches 23,24 or steps 25,26 on surfaces along specific direction, i.e., the space restriction plays a major role in the chain formation. However, on ZnO(1010) surfaces, the CO 2 chains are along [0001] direction, rather than along the surface trench direction [1210]. To explore the formation mechanism of such CO 2 chains, we performed the  Fig. 2e and f, respectively. The bonding formation obviously bents the linear CO 2 and induces the charge redistribution. In the single carbonate, as shown in Fig. 2e, the two O atoms of CO 2 get more electrons while the C and Zn 3c atoms lose more electrons. The charge redistribution induces the extra Coulomb attraction between Zn 3c atoms and O atoms of CO 2 . For the carbonate chain, as shown in Fig. 2f, two O atoms bond to one Zn 3c atom, and the induced positive electricity of Zn 3c atoms is evidently enhanced. As a result, the extra attractive Coulomb interaction between Zn 3c atoms and O atoms of CO 2 is strongly enhanced. Such enhanced attractive interaction makes the chain configuration of adsorbed CO 2 along [0001] direction more stable. Along [1210] direction, on the contrary, the enhanced electrostatic repulsion between CO 2 molecules causes the CO 2 alignment along [1210] direction less stable.
To understand the phase evolution of CO 2 adlayers on ZnO(1010), the temperature dependence of CO 2 chains was studied for fixed CO 2 coverages. For the low CO 2 coverage of 0.2 ML (corresponding to 0.2 L), the IRRAS results are shown in Fig. 3. Slowly annealing to 230 K, the ν as band at 1623 cm −1 gradually converts to 1583 cm −1 in Fig. 3a. At the same time, the three close peaks (1297, 1313, 1325 cm −1 ) of ν s finally convert to one peak at 1337 cm −1 , and the 978 cm −1 band to 1008 cm −1 band, as shown in Fig. 3b. (The corresponding p-polarized spectra with IR light incident along [0001] direction can be seen in Figure S2 in the SI.) Such band conversions reveal that the chain conversions from the monomer to long chains happened upon annealing through Ostwald ripening. Our present study provides an effective way to synthetize long CO 2 chains along [0001] direction on ZnO(1010) surfaces.
For the high coverage of 0.7 ML (corresponding to 2 L), the IRRAS results with annealing are shown in Fig. 4a  and b. As mentioned before we have assigned the 1590 cm −1 to the ν as and 1345 cm −1 to the ν s of long CO 2 chains at 90 K. Slowly annealing to 150 K, the ν as band at 1590 cm −1 unexpectedly converts to 1618 cm −1 gradually, as shown in Fig. 4a. Further annealing to 240 K, the band gradually redshifts back to 1590 cm −1 . On the other hand, the ν s band (1345 cm −1 ) keeps constant from 90 K to 150 K. Over 150 K, its intensity slightly decreases with a weak redshift. (The corresponding p-polarized spectra with IR light incident along [0001] direction can be seen in Figure S3 in the SI). It is easy to know that for the high CO 2 coverage and relative high annealing temperature, the length change of a single long chain will not induce such obvious changes of the CO 2 vibration frequencies. But the change of the separated distance along [1210] direction between two long chains may induce significant changes of the CO 2 vibration frequencies in Fig. 4 due to the interchain interaction.  Thereafter, we calculated a series of CO 2 long chains with different interchain distances, such as the isolated long chain, two neighbouring chains (corresponding to (1 × 1) phase) with the shortest distance of a 0 , and two spacing chains (corresponding to (2 × 1) phase) with 2a 0 . Here a 0 represents the lattice constant along [1210] direction of ZnO(1010) surfaces. The calculated results are shown in Table 1. We found that the calculated ν as of the spacing chains is 1574 cm −1 , which is consistent with the experimental 1590 cm −1 at 90 K and 240 K. The calculated ν s of the spacing chains is 1303 cm −1 , which is in agreement with the vibration frequencies of 1345 cm −1 at 90 K and 240 K. These obviously indicate that the experimentally observed vibration bands at both 90 K and 240 K belong to the spacing chains. Similarly, the experimentally observed vibration frequencies at 150 K belong to the neighbouring chains.
In Fig. 4c, we give a schematic evolution picture of the CO 2 chains with increasing temperature. At low temperature of 90 K, on the one hand, the CO 2 chains with various length are randomly distributed on the surface due to the low kinetic energy; on the other hand, most of the interchain spaces along [1210] are equal to 2a 0 caused by the interchain repulsion, forming the local (2 × 1) phase, as shown in Fig. 4c-I. Annealing to 150 K, the CO 2 diffusion is enhanced to induce the Ostwald ripening between CO 2 chains: the CO 2 molecules detach from the short chains and attach to the long ones. Finally, the lengthened chains become neighbouring with others, forming the local (1 × 1) phase, as shown in Fig. 4c-II. However, due to the strong repulsive interaction between neighbouring chains, the (1 × 1) phase is an unstable intermediate state. Further annealing the intermediate state to 240 K, all the chains will relax to the more stable spacing structure, forming the well-defined stable (2 × 1) phase, as shown in Fig. 4c-III.

Conclusion
In conclusion, the formation and evolution of CO 2 chains on ZnO(1010) surfaces was studied by employing UHV-FTIRS and DFT calculations. We observed the fine structures of CO 2 vibrational levels on ZnO(1010) surfaces, which are attributed to the formation of CO 2 monomer, dimer, trimer and longer polymer chains along [0001] direction. At high CO 2 coverage, the as-grown local (2 × 1) phase composed of chains with various lengths further evolve into the unstable local (1 × 1) phase with annealing to 150 K, and then relax into the well-defined stable (2 × 1) phase above 150 K. The mechanisms of the chain formation and evolution were discussed by DFT calculations and a schematic kinetic model.

Experimental details.
The experiments were carried out using an ultrahigh vacuum (UHV) system 17 (the base pressure better than 6 × 10 −11 mbar) equipped with a vacuum Fourier transform infrared spectroscopy (FTIR) spectrometer (Bruker, VERTEX 80 v), a low energy electron diffraction (LEED)/Auger (AES) spectrometer with gain power of microchannel plates BDL 600IR-MCP. The clean mix-terminated ZnO(1010) (8 × 8 × 1 mm, MTI) surface was prepared by repeated cycles of Ar + sputtering and annealing at 800 K under UHV conditions until no impurities were detected by AES and clear (1 × 1) LEED patterns were obtained. Then, the clean ZnO(1010) was oxidized in oxygen atmosphere (5 × 10 −7 mbar) at 750 K for 20 minutes. The IR measurements were performed using infrared reflection absorption spectroscopy (IRRAS) mode with a fixed incidence angle of 80°. The recorded data, i.e., the absorbance is defined as A = log 10 (R 0 /R), where R 0 and R are the reflected signals from the bare and the adsorbate covered surfaces, respectively. The optical path was evacuated in order to avoid any unwanted IR adsorption from gas phase species. High purity CO 2 (99.99%) and O 2 (99.999%) were dosed via backfilling in the experiments. Computational details. First-principles calculations were performed using the Vienna ab-initio simulation package (VASP) 27,28 with a cut-off energy of 500 eV for the basis set. Γ -point was used for Brillouin zone sampling. The projector-augmented wave method (PAW) 29 with the PBE type exchange-correlation potentials 30 was adopted. To model the ZnO(1010) surface, the optimized lattice parameters of bulk ZnO, a = 3.285 Å and c/a = 1.6131, were used to build slabs with six ZnO layers. Two surface unit cells, which have dimensions of 6 × 2 and 2 × 4 along[0001] and [1210] directions, respectively, were employed to perform the calculations. The atomic positions of top three layers were optimized until the forces are less than 0.03 eV/Å, while the bottom layers were fixed at bulk positions. A vacuum layer with a thickness of 15 Å was used to minimize interactions between adjacent slabs. The vibrational frequencies were derived from Hessian matrix calculated by finite-displacement method.