The Role of Surface Orientation in the Photoelectrochemical Behavior of Layer Type d‐Band Semiconductors

The efficiency of photocurrents as well as the photopotential output of electrochemical solar cells based on molybdenum dichalcogenides depends on the crystalline quality of the electrode surface. This phenomenon has been investigated in a systematic way with n‐type crystals by varying the relative proportion of surfaces parallel to the c‐lattice‐vector (van‐der‐Waals surfaces) and surfaces not normal to the c‐vector. We report on anodic reactions studied in the presence of various redox systems and in such electrolytes where the current leads to oxidative dissolution of the solid. — It was found that the two regions react in a characteristically different way. Surfaces not parallel to the c‐axis provide favourable reaction centers for electron transfer reactions in the absence of light while photoreactions predominantly proceed at van‐der‐Waals surfaces. This can be explained by a pronounced difference in the electronic nature of the two sites which results in a different electron density distribution and a different position of electron energy levels in the surface. The metal atoms at surfaces non‐parallel to c have dangling bonds (d‐hybrides) which form surface states and interact strongly with reactants from the solution. At the van‐der‐Waals surfaces, however, the d‐levels are much more shielded from interaction with components of the electrolyte that electronic excitation (generation of holes) is needed to induce anodic oxidation processes.