Selective Solvent-Induced Stabilization of Polar Oxide Surfaces in an Electrochemical Environment

The impact of an electrochemical environment on the thermodynamic stability of polar oxide surfaces is investigated for the example of ZnO(0001) surfaces immersed in water using density functional theory calculations. We show that solvation effects are highly selective: They have little effect on surfaces showing a metallic character, but largely stabilize semiconducting structures, particularly those that have a high electrostatic penalty in vacuum. The high selectivity is shown to have direct consequences for the surface phase diagram and explains, e.g., why certain surface structures could be observed only in an electrochemical environment.

Figure 1

Pourbaix diagrams, showing the stability of Zn phases as a function of the solution’s $p\mathrm{H}$ and the electrode potential $U$ referenced to the standard hydrogen electrode potential $U_{\mathrm{SHE}}$. (a) Pourbaix diagram for zinc, adapted from Ref. [38]. (b) and (c) Surface Pourbaix diagrams for the ZnO(0001)-Zn surface constructed using energies obtained from DFT calculations (b) without and (c) with a solvent. Colors are used to discriminate the different surface phases: structures with adsorbates on the smooth bulk terminated surface (blue), triangular reconstructions (red), adsorbate-passivated triangular reconstructions (green). The black hashed area shows the water stability region (see text). The stability region of bulk ZnO is superimposed onto the surface diagrams (vertical black dotted lines). The yellow star marks the experimental condition at which triangular reconstructions have been observed at the surface in an in situ AFM study [37]. (d) Top view of the stable surface phases found within the phase diagram. Zn, O, and H atoms are shown as white, red, and blue balls, respectively. The colors used for the frames correspond to the colors used in the surface Pourbaix diagrams.

Figure 2

Solvation energies of ZnO(0001)-Zn surface structures plotted as a function of (a) the excess electrons at the surface and (b) the electrostatic energy of surface phases referenced to that of the clean surface. The excess electrons at the surface correspond to the number of unsaturated electrons in the partially filled surface Zn dangling bonds per unit cell (see text). The colors identify triangular phases for which the electron counting rule (EC) is not fulfilled (red), fulfilled purely by a triangular reconstruction (purple), and fulfilled due to the presence of hydroxyl adsorbates in addition to the triangular reconstruction (green). EC obeying surface phases (i.e., purple and green) are charge neutral [21]. Inset in (a) are electron difference density plots depicting the screening charge at the interface [blue, electron accumulation ($\min \approx -{10}^{-4}{e}^{-}/{\mathrm{Bohr}}^3$); red, electron depletion ($\max \approx {10}^{-4}{e}^{-}/{\mathrm{Bohr}}^3$)] perpendicular to the interface and along the direction indicated by dashed green lines in Fig. 1.