Establishing the relationship between local interface structure and chemical functionality at aqueous semiconductor interfaces remains very challenging. Important examples include the alignment of electrochemical redox levels with the semiconductor band edges, the identification of catalytic active sites at the interface and pathways for electron and proton transfer. In our approach we initially study the interface structure using ab initio molecular dynamics (AIMD) [1]. We then use the calculated, average interface dipole and the GW approach from many-body perturbation theory, which corrects for errors associated with Kohn-Sham eigenvalues, to calculate the energy offset between the semiconductor band edges and the centroid of the occupied 1b1 energy level of water and thus, the alignment to electrochemical levels [2]. Finally, we use cluster models to study reaction pathways [3]. Our initial studies focused on GaN, ZnO and their alloys, motivated by experiments showing high efficiency for photocatalytic water oxidation under visible light for alloys. At the non-polar (10-10) facet of GaN, a full monolayer of water at the interface spontaneously dissociates forming -N-H and -Ga-OH interface motifs. For ZnO, a mixed state is found, with some -Zn-OH2 motifs as well. These structural motifs have a significant impact on the interface dipole and hence the electrochemical redox alignment. An injected hole tends to localize on the -N-H, but study of reaction pathways shows that the first step of the water oxidation process goes through the -Ga-OH motif, leaving a hole localized on -Ga-O. We have extended these studies to the stable TiO2 anatase (101) and rutile (110) facets. The AIMD calculations reveal interface water dissociation and reassociation processes through several distinct pathways, both direct at the interface and engaging spectator water molecules. Further, the energy landscape for these pathways depends on the interface structure. The impact of these factors on the degree of interface dissociation, interface dipole and hole localization will be discussed. Work performed in part at the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. [1] N. Kharche, et al., Phys. Chem. Chem. Phys. 16, 12057 (2014). [2] N. Kharche, et al., Phys. Rev. Lett. 113, 176802 (2014). [3] M. Z. Ertem, et al., ACS Catal. 5, 2317 (2015).

Impact of dissociated water at photocatalytic aqueous semiconductor interfaces

251st National Meeting (2016)

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