关键词: 能带排列, 金属-水界面, 氧化物-水界面, 从头算分子动力学

Abstract:

Band alignments of electrode-water interfaces are of crucial importance for understanding electrochemical interfaces. In the scenario of electrocatalysis, applied potentials are equivalent to the Fermi levels of metals in the electrochemical cells; in the scenario of photo(electro)catalysis, semiconducting oxides under illumination have chemical reactivities toward redox reactions if the redox potentials of the reactions straddle the conduction band minimums (CBMs) or valence band maximums (VBMs) of the oxides. Computational band alignments allow us to obtain the Fermi level of metals, as well as the CBM and VBM of semiconducting oxides with respect to reference electrodes. In this tutorial, we describe how to obtain the band alignments using ab initio molecular dynamics simulations. To be simple, we introduce the protocol of computational band alignments through two selected charge-neutral interfaces, i.e., Cu(100)- and SnO₂(110)-water interfaces. It should be bear in mind that one can also apply this protocol to electrified interfaces. The band alignments at charge-neutral interfaces have different meanings for metals and semiconducting oxides. For metals, the alignments amount to Potentials of Zero Charge of metals, under which the metal-water interfaces possess zero net charge. For semiconducting oxides, the alignments show the positions of CBMs and VBMs under a special pH and potential. The special pH is named as Point of Zero Charge and the special potential is called Flat-Band Potential. The oxides-water interfaces have zero net charge if they are at the special pH and potential. It is worth noting that neither the positions of CBMs nor VBMs are directly interpreted as applied potentials. In the protocol, we refer computed levels to standard hydrogen electrode (SHE), and thus directly compare the levels with those from electrochemical experiments. With PBE functional, the computed Fermi level of Cu(100) is -0.726 V with respect to SHE and matches the experimental determination of -0.73 V (SHE). The CBM and VBM of SnO₂(110), however, are computed as 1.76 V and 0.6 V (SHE), respectively, which fails to match the experimental values of 3.747 V and 0.147 V (SHE), respectively. We attribute the failure to the delocalization error of density functional theory. Because of the error, DFT tends to spatially delocalize one-electron orbitals, which occasionally has negligible influences on the Fermi level of metal, but significantly underestimates the band gaps of semiconducting oxides.

Key words: Band alignment, Metal-water interfaces, Oxides-water interfaces, Ab initio molecular dynamics