Bottom graph: total and (filled curve) contributions. Top graph: and (filled curve) contributions to the density of states of bulk ZnO wurtzite, computed at the PBE0 level with the basis set. Fermi level (as dotted lines) set at 0 eV. The red dashed lines outline the and band separation. Contributions originating from the states at lower energies are omitted.
Relaxed structure of the ZnO wurtzite surface with used notations. Light blue and red circles represent Zn and O atoms, respectively.
(a) Band structures of ZnO wurtzite , both considering unrelaxed (left) and fully relaxed (right) eight atomic plane slabs, obtained at the PBE0 level. Fermi level (black dashed line) of the relaxed structure set at 0 eV. Computed direct band gap at in red. (b) Corresponding Brillouin zone with point labeling.
Side views of the (a) bridging , (b) unidentate type , and (c) unidentate type adsorption modes of formic acid on ZnO wurtzite . Zn, O, C, and H atoms as light blue, red, black, and white circles, respectively. Hydrogen bond as dashed line.
Calculated density of states of formic acid on ZnO for all investigated adsorption models displaying the band gap region. Fermi level set at 0 eV in all cases. Total density of states as solid black line. Formic acid-projected contribution as filled red line.
Equilibrium geometry of ZnO wurtzite calculated with different Hamiltonians and basis sets. Lattice parameters ( and ) in angstrom; internal parameter , corresponding to the fractional coordinate of the oxygen atoms along the axis, in fractional units. In parentheses, computed error (in percent), with respect to the all-electron data, obtained when using large core pseudopotentials at the PBE0 level (see Sec. II).
Main properties of the density of states of ZnO wurtzite bulk structure obtained with different Hamiltonians and the basis sets. refers to the width of a band, is the separation between the and bands, and refers to a direct gap. All data are in eV.
Structural parameters of the ZnO surface upon relaxation obtained with the all-electron basis set at the PBE0 level for different numbers of Zn–O dimer planes of the slab. Distances ( and ) in angstrom; angles (from to ) in degrees. Labeling corresponds to that in Fig. 2. denotes the bond contraction (in percent) of the outermost Zn–O dimer when compared to the bulk value, while is that of the bond between the outermost Zn [O] surface atom and the O [Zn] atom is the second plane.
Computed PBE0 atomic displacements (in angstrom) of the ZnO surface upon relaxation with both all-electron and large core pseudopotentials basis sets. refers to the number of atomic planes in the slab. and denote displacements along and , respectively.
Mulliken atomic charges and bond overlap populations of the ZnO wurtzite surface, obtained for eight atomic planes. All data are computed at the PBE0 level with the basis set. refers to the atomic plane number, considering the outermost plane as the first one.
Selected structural parameters of formic acid both adsorbed on a ZnO wurtzite supercell and isolated. The adsorbate labeling corresponds to that in Fig. 4. and are threefold coordinated substrate Zn and O atoms involved in the adsorbate/substrate bonding. Distances in angstrom and angles in degrees.
Additional displacements (in angstrom) of selected outermost substrate atoms upon adsorption of formic acid on ZnO wurtzite when compared to atomic positions of a fully relaxed clean slab. Labeling corresponds to that in Fig. 4. For instance, is the substrate Zn atom bond to the adsorbate atom.
Computed interaction energies of the formic acid molecule with a ZnO supercell in different adsorption modes. For definitions of , , and , see Sec. II. All data are in kJ/mol. refers to the surface coverage.
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