optimized structure of the NH2-ZnO system is shown in Fig. 6.7. The adsorbates bind in a nondissociated form with interatomic distances N1-Zn and N2-Zn of about 2.15 and 2.31 ˚A, respectively. Thus, the N-Zn bond lengths for one of the nonequivalent molecular configurations is in line with the results for the nonpolar surface reported above, whereas, for the second group of ligands, the N-Zn interatomic distance is larger.
Finally, the optimized geometry corresponding to the modification of the NW using MPA is depicted in Fig. 6.8. MPA strongly binds to the nanostructure via two symmetric O-Zn bonds with an O-Zn interatomic distance of 1.92 ˚A. The third oxygen atom of the -PO(OH)2 group, O3, relaxes to a position equidistant from the two hydrogen atoms transferred from the ligand to the surface with an interatomic distancedO3· ··H= 1.67 ˚A. The adsorption energy per adsorbate amounts in this case to 2.69 eV.
Zn O2
OB
OA O1
O3
O O O O O2
O3
O1
OA
Zn P
Figure 6.8: Cross-sectional (left) and side (right) views of the optimized structure of the CH3PO(OH)2-modified ZnO nanowire.
molecules adsorbed per surface unit cell (surface Zn-O pair) and considered coverages of 0.25, 0.5, and 1 by using surface cells with (2 × 2), (2 × 1), and (1 × 1) periodicities, respectively.
The Brillouin zone integration was performed using MP meshes of (1 × 2 × 2) for θ = 0.25 and (1× 4×3) forθ = 0.5 and 1. All atoms were allowed to move until the interatomic forces were smaller than 5×10−3 eV/˚A.
Several molecular orientations were considered during this investigation. The corresponding adsorption energies were calculated using Eq. 6.1. To study the relative thermodynamic stabil-ity of the considered surface coverages, we assume that the ZnO surface is in thermodynamic equilibrium with glycine in the gas phase. The variation of the surface energy of the substrate after adsorption of glycine can then be calculated according to
∆γ = 1
NAA(ET−Ebare−nEGly) = θEads
A , (6.2)
where A is the area of the (1 × 1) surface unit cell and NA is the number of (1 × 1) surface unit cells contained in the supercell.
Table 6.2 summarizes the adsorption energies and the surface energy variations for the most stable configurations of the hybrid interface. Our results show that for θ = 1 glycine binds to the surface through the -COOH (-NH2) group with an adsorption energy ofEads = -1.82 (-1.67) eV. Adsorption through the carboxyl group is, therefore, slightly favored. The corresponding optimized structures are depicted in Fig. 6.9. For the most stable configuration, glycine binds to the substrate in a monodentate mode and dissociatively. Similar investigations on glycine adsorption on Si surfaces found a barrier height for dissociation of only 1 eV [307], suggesting that dissociation is likely to occur at room temperature. One of the carboxylic oxygens, O1, binds to the surface Zn atom with a bond length of 2.15 ˚A, whereas the oxygen atom O2 relaxed to a position equidistant from two neighboring surface Zn atoms at 2.28 ˚A.
θ group mode Eads (eV) ∆γ (eV/A1×1) geometry
1 -COOH monodentate, anion, dissociated -1.82 -1.82 Fig. 6.9 (top)
1 -NH2 dissociated -1.67 -1.67 Fig. 6.9 (bottom)
1 -COOH monodentate, zwitterion, nondissociated -0.84 -0.84 not shown 0.5 -COOH/-NH2 bidentate chelating, dissociated -2.92 -1.46 Fig. 6.10 (middle)
0.5 -COOH/-NH2 monodentate, dissociated -3.12 -1.56 Fig. 6.10 (right)
0.5 -COOH bidentate bridging, dissociated -2.56 -1.28 Fig. 6.10 (left) 0.25 -COOH/-NH2 monodentate, dissociated -3.05 -0.76 Fig. 6.11 (top) 0.25 (VO) -COOH/-NH2 monodentate, dissociated -3.18 -0.80 Fig. 6.11 (bottom)
Table 6.2: DFTB adsorption energies (Eads) and surface energy variations per (1 × 1) ZnO surface (∆γ) corresponding to the stable binding geometries of glycine on (1010) ZnO.
Adsorption through the -NH2 group occurs via the binding of the nitrogen atom to the metal site. One proton is transferred from the functional group to the surface oxygen site during adsorption as shown in Fig. 6.9 (bottom). It is worth mentioning that a metastable zwitterionic form of glycine on ZnO was only observed for a full coverage, with a relatively small adsorption energy ofEads= -0.84 eV. No stable zwitterionic glycine has either been found on Zn-terminated ZnO polar surfaces [308].
Zn O2
N O1
OB
C
O1
N
OB
O2
C
Zn [0001] view [1210] view
Figure 6.9: Optimized structures of the modified (1010) ZnO surface using 1 ML of glycine molecules adsorbed through either the -COOH (top) or the -NH2 (bottom) groups.
For θ = 0.5, two surface Zn sites per molecule are available, and glycine can therefore adsorb through -COOH in a bidentate bridging configuration. The optimized structure corresponding to this adsorption mode is shown in Fig. 6.10 (left). The bond lengths between the oxygen atoms of the molecule and the surface aredO1-Zn1 = 2.03 ˚A anddO2-Zn2 = 2.05 ˚A. The adsorption energy for this configuration is Eads = -2.56 eV. These results are in line with a DFTB investigation where the authors found that acetic acid preferentially attaches to the (1010) ZnO surface in a bidentate bridging mode for a 0.5 ML coverage [305]. However, in the case of glycine, a competition between -NH2 and -COOH groups for attaching to the available binding sites is expected. We found, indeed, that the most stable configurations for the adsorption of glycine involve the attachment of both functional groups. Adsorption in a -COOH bidentate chelating mode occurs through dissociation of the -NH2 group (Fig. 6.10 middle). The adsorption bond lengths for this case are dN-Zn1 =1.95 ˚A, dO1-Zn2 = 2.19 ˚A and dO2-Zn2 = 2.16 ˚A, and the adsorption energy is Eads = -2.92 eV. Slightly favored (Eads = -3.12 eV) is the adsorption in a -COOH monodentate configuration (Fig. 6.10 right). In this case, the -NH2 group does not dissociate. The bond lengths are dN-Zn2 = 2.01 ˚A anddO1-Zn1 = 1.90 ˚A. For each configuration, dissociation of the -COOH group is observed.
The smallest investigated surface coverage is θ = 0.25. For the most stable configuration, the adsorption is produced in a similar manner as for θ = 0.5, that is, the ligand attaches dissociatively through both functional groups in a -COOH monodentate mode (see Fig. 6.11 top). The -NH2 group binds to the surface Zn2 site through the nitrogen atom with a bond
length of 2.01 ˚A, whereas the carboxylic oxygen O2 adsorbs on a neighboring metal site (Zn4) with a bond length of 1.99 ˚A. The corresponding adsorption energy is -3.05 eV. This geometry resembles that found for glycine on polar (0001) ZnO surfaces [308], where the hydrogen of the -COOH group is transferred to the Zn-terminated surface. In contrast, this adsorption mode differs from that found for glycine on TiO2 (110), where the ligand binds favorably through -COOH in a bidentate bridging configuration for a 0.5 ML coverage [309, 310].
O1
N
O2 N
Zn2 Zn1
O2
O1
Zn1
Zn2
O1
Zn1 Zn2
O2
Figure 6.10: Optimized structures of the modified (1010) ZnO surface using 0.5 ML of glycine molecules in different binding modes: -COOH bidentate bridging (left), -COOH bidentate chelating with dissociated -NH2 group (middle) and -COOH monodentate with nondissociated -NH2 group.
N
O2
O1
Zn1
Zn2
O1
Zn4 Zn2
O2
Zn4
O2
O1
Zn3
[0001] view [1210] view
Figure 6.11: Optimized structures of glycine adsorbed on the defect-free (top) and oxygen-reduced (bottom) (1010) ZnO surfaces forθ = 0.25.
To analyze the relative thermodynamic stability of the glycine/ZnO interface for different sur-face coverages, we compare the respective ∆γ values. According to our results, the 1 ML coverage is expected.
Although similar adsorption energies were found for the adsorption of glycine on the ZnO sur-faces through either -COOH or -NH2 groups in high-coverage regimes, the electronic properties of the two modified surfaces are very different. The DOS for the adsorption through -COOH is shown in Fig. 6.12 (middle). Changes with respect to the DOS for the bare surface (Fig. 6.12 top) are observed close to the top of the valence band and the bottom of the conduction band.
The states at the VBM have strong projections onto the OCOOH-2p, surface Zn-3d, and NNH2-2p orbitals. For the adsorption through the -NH2 group, additional levels are inserted close to the VBM, localized mainly at the N-Zn-O3 bonds (see Fig. 6.12 bottom). In addition, unoccupied intragap levels are also seen, with contributions coming mainly from the -COOH group.
bare
-COOH
-NH2
DOS (arb. units)
energy (eV) 0.4
0.2
0 0.4
0.2
0 0.4
0.2
0
total DOS ( x 0.2 ) Zn -3d
N -2p C -2p O1 -2p O2 -2p
OB -2p
2 4 6
0 -2
-4
Figure 6.12: Total and projected density of states for the bare (1010) ZnO surface (top) and the modified surface using glycine adsorbed through the -COOH group (middle) and the -NH2
group (bottom) for θ = 1. The atom notation corresponds to the one employed in Fig. 6.9.
The dashed line indicates the Fermi level position.
6.3.1 The effect of surface oxygen vacancies
The successful modification of ZnO surfaces can be hindered by the presence of intrinsic de-fects and impurities. Surface dede-fects can act as catalysts for adsorption/dissociation of the ligand, thus changing the properties of the nanostructure and affecting the features for device
applications [311–314]. We investigate next the effect of surface oxygen vacancies (VO) on the adsorption of glycine on the ZnO nonpolar surface. VO is a common defect in ZnO and has been extensively investigated in bulk materials [315–317] and (1010) surfaces [318]. The oxygen-reduced surface was modeled by removing one surface oxygen per 2 × 2 surface unit cells. One glycine molecule per oxygen vacancy was considered. The most stable configuration for the adsorption on the defective surface is shown in Fig. 6.11 (bottom). The binding geome-try shows some similarities with that for the defect-free surface. The molecule adsorbs through both functional groups, with interatomic distances ofdN-Zn2 =2.04 ˚A,dO2-Zn4 = 2.06 ˚A,dO1-Zn2
= 2.31 ˚A, and dO1-Zn3 = 2.66 ˚A. However, one of the carboxylic oxygens, O1, relaxes toward the vacancy site compared to the geometry for the defect-free case. This leads to an energy gain of roughly 0.1 eV, thus revealing a small influence of the defect on the glycine adsorption mode and adsorption strength.
total DOS
( Zn1 + Zn2 + Zn3 ) - 4s O1 - 2p
DOS (arb. units)
energy (eV)energy (eV)
0 -2 -4 -6 -8 0
-2 -4 -6 -8 0 -2 -4 -6 -8
Figure 6.13: Electronic band structure for the 0.25 ML coverage of glycine molecules on the defect-free (top) and oxygen-reduced (bottom)(1010) ZnO surfaces (structures shown in Fig. 6.11). The density of states (DOS) for the defective surface is shown to the right of the corresponding band structure. The blue curve denotes the sum of the projections of the DOS onto the 4s orbitals of atoms Zn1, Zn2 and Zn3. The dashed line indicates the Fermi level position.
The densities of states for the glycine adsorption on the defect-free and reduced surfaces for θ = 0.25 are shown in Fig. 6.13. No intragap states appear for the defect-free surface case
(Fig. 6.13 top). However, the adsorption on the defective surface reveals an intragap electronic level with contributions coming mainly from the 4s states of atoms Zn1, Zn2 and Zn3 and the 2pstates of atom O1 (Fig. 6.13 bottom).