As a model system of 2-D oxide material, layered δ-MnO2 has important applications in Li ion battery systems. δ-MnO2 is also widely utilized as a precursor to synthesize other stable structure variants in the MnO2 family, such as α-, β-, R-, and γ-phases, which are 3-D interlinked structures with different tunnels. By utilizing the stochastic surface walking (SSW) pathway sampling method, we here for the first time resolve the atomistic mechanism and the kinetics of the layer-to-tunnel transition of MnO2, that is, from δ-MnO2 to the α-, β-, and R-phases. The SSW sampling determines the lowest-energy pathway from thousands of likely pathways that connects different phases. The reaction barriers of layer-to-tunnel phase transitions are found to be low, being 0.2–0.3 eV per formula unit, which suggests a complex competing reaction network toward different tunnel phases. All the transitions initiate via a common shearing and buckling movement of the MnO2 layer that leads to the breaking of the Mn–O framework and the formation of Mn3+ at the transition state. Important hints are thus gleaned from these lowest-energy pathways: (i) the large pore size product is unfavorable for the entropic reason; (ii) cations are effective dopants to control the kinetics and selectivity in layer-to-tunnel transitions, which in general lowers the phase transition barrier and facilitates the creation of larger tunnel size; (iii) the phase transition not only changes the electronic structure but also induces the macroscopic morphology changes due to the interfacial strain.

The photocatalytic oxidation of water to molecular oxygen is a key step toward the conversion of solar energy to fuels. Understanding the detailed mechanism and kinetics of this reaction is important for the development of robust catalysts with improved efficiency. TiO2 is one of the best-known photocatalysts as well as a model system for the study of the oxygen evolution reaction (OER). Here we use hybrid density functional based energetic calculations and first-principles molecular dynamics simulations to investigate the pathway and kinetics of the OER on the majority (101) surface of anatase TiO2 in a water environment. Our results show that terminal Ti–OH groups are stable intermediates at the aqueous (101) interface, in accord with the experimental observation that OH radicals are efficiently produced on anatase. Oxidation of Ti–OH gives rise to a second stable intermediate, a surface-bridging peroxo dimer ((O22–)br) composed of one water and one surface lattice oxygen atom, consistent with the surface peroxo intermediates revealed by “in situ” measurements on rutile. Our calculations further predict that molecular oxygen evolves directly from (O22–)br through a concerted two-electron transfer, thus leading to oxygen exchange between TiO2 and the adsorbed species. Oxygen exchange is found to be negligible on rutile, so that different OER pathways are likely to be operative on the two main TiO2 polymorphs. This difference could explain the observed lower OER activity of anatase relative to rutile.Keywords: activation energy; density functional theory; oxygen exchange; photocatalysis; titanium dioxide; water oxidation

Fe-doped NiOx has recently emerged as a promising anode material for the oxygen evolution reaction, but the origin of the high activity is still unclear, due largely to the structural uncertainty of the active phase of NiOx. Here, we report a theoretical study of the structure of β-NiOOH, one of the active components of NiOx. Using a genetic algorithm search of crystal structures combined with dispersion-corrected hybrid density functional theory calculations, we identify two groups of favorable structures: (i) layered structures with alternate Ni(OH)2 and NiO2 layers, consistent with the doubling of the c axis observed in high resolution transmission electron microscopy (TEM) measurements, and (ii) tunnel structures isostructural with MnO2 polymorphs, which can provide a rationale for the mosaic textures observed in TEM. Analysis of the Ni ions oxidation state further indicates a disproportionation of half of the Ni3+ cations to Ni2+/Ni4+ pairs. Hybrid density functionals are found essential for a correct description of the electronic structure of β-NiOOH.Keywords: battery; crystal structure; DFT; genetic algorithm; mosaic texture;