Oxygen Evolution at the BiVO4-Water Interface: Mechanism of the Water Dehydrogenation Reaction

We study the water dehydrogenation reaction at the BiVO4(010)-water interface by combining nudged-elastic-band calculations and electronic structure calculations at the hybrid functional level. We investigate the pathway and the kinetic barrier for the adiabatic reaction going from the hole polaron localized in BiVO4 to the dehydrogenation of the adsorbed water molecule at the interface. The reaction is found to involve the H2O center dot+ radical cation as an intermediate, to have a kinetic barrier of 0.7 eV, and to be initiated by the electron transfer. The calculated kinetic barrier is in good agreement with experiment and is consistent with the slow hole transfer kinetics observed at the surface of BiVO4. To characterize the structural changes occurring during this process, we analyze the O-H distances for three relevant water molecules. We also examine the Wannier functions around the O atom of the adsorbate involved in the reaction to reveal the changes in the electronic structure during the hole hopping. The projected density of states of the lowest unoccupied molecular orbitals allows us to identify the atomic orbitals that are primarily involved in the reaction. We expect that the proposed reaction mechanism generally holds when the surface coverage is dominated by molecularly adsorbed water.

Magnetocrystalline anisotropy energy of Co and Fe adatoms on the (111) surfaces of Pd and Rh

Harald Brune, Markus Etzkorn, Pietro Gambardella, Anne Lehnert, Géraud Moulas, Stefano Rusponi

We performed a combined theoretical and experimental investigation of the orbital magnetism and magnetocrystalline anisotropy of isolated Co and Fe adatoms on Pd(111) and Rh(111). Theoretical calculations of the spin and orbital moments are based on ab initio spin-polarized density-functional theory (DFT) including a self-consistent treatment of spin-orbit coupling. The calculations use a slab model to represent the adsorbate/substrate complex and allow for a complete structural relaxation leading to a strong inward displacement of the adatom and modest vertical and lateral relaxations in the substrate atoms. Compared to an idealized geometry where the atoms are kept on bulk lattice positions up to the surface, relaxation leads to a much stronger adatom/ligand hybridization. This is also reflected in the results for orbital moments and magnetocrystalline anisotropy energy (MAE). The enhanced hybridization leads to strong quenching of the adatom orbital moments but also to the formation of large induced spin and orbital moments in the substrate. As a consequence, we find that the substrate contribution to the MAE is much more important than estimated before on the basis of studies using an idealized geometry. We also find the surprising result that the MAE strongly depends on the adsorption site. The magnitude and even the sign of the MAE change for adatoms on face-centered cubic with respect to the ones on hexagonal close-packed hollow sites on the (111) surface. The dependence of the MAE on the combination of adatom and substrate has been analyzed in terms of the electronic structure, leading to a sound physical picture of the origin of the MAE. A fundamental problem, however, is the correct prediction of the size of the orbital moments of the adatoms. We suggest that this problem can be solved only via post-DFT corrections introducing an orbital dependence of the exchange potential. The theoretical results are compared to site-averaged, element-specific x-ray magnetic circular dichroism (XMCD) measurements. Low-temperature XMCD spectra and magnetization curves reveal weak out-of-plane anisotropy for Fe adatoms on both substrates. Interestingly, Co adatoms on Rh(111) present in-plane anisotropy with MAE of about -0.6 meV, contrary to the known out-of-plane anisotropy of Co on Pd(111) and Pt(111). The orbital to spin magnetic-moment ratio measured by XMCD shows that the Co adatoms present much stronger orbital magnetization components compared to Fe. The connection between orbital moments and MAE is discussed at the theoretical level including the contribution of the induced substrate magnetization.

2010

First-Principles Modeling of Optically Active Organic Molecules in Solar Cell Devices and Biological Environments

Jun Ren

With the increasing cognition of the importance of organic molecules, they are widely applied in printing, biological and pharmacological fields, because of their special capabilities of harvesting solar light, scavenging free radicals, and chelating metal ions. During the past decades, the unique photoelectronic and photochemical properties of organic molecules, such as phthalocyanine, cyanidin, and their relevant derivatives, attract tremendous attention, because they provide an excellent opportunity to solve the worldwide energy crisis by converting directly the solar light to electricity. The surface morphology and electronic interaction of these organic molecules with other molecules, surfaces or interfaces play a critical role in determining the performance of the electronic and optical devices based on organic molecules. In this thesis, we focus on the investigation of several selected organic molecules, and their interaction with molecules, inorganic semiconductors, and organic semi-conductors, by using first-principles electronic structure calculations based on density functional theory, and time-dependent density functional theory. Particular attention is paid to the atomic structure, electronic and optical properties of organic molecules and the corresponding interfaces. The focus of this thesis is on the following aspects of the organic molecules: (i) The complexation mechanism of flavonoids with metal ions. The most likely chelation site for Fe is the 3-hydroxyl-4-carbonyl group, followed by 4-carbonyl-5-hydroxyl group and the 3'-4' hydroxyl (if present) of quercetin. A complex of two quercetin molecules with a single Fe ion is energetically more stable, however, six orbitals of Fe in the three quercetin complex are saturated by three perpendicular molecules to form and octahedral configurations. Furthermore, the optical absorbance spectra serves as signatures to identify various complexes. (ii) The electronic coupling between a dye molecule (Cyanidin) and a TiO2 nanowire. Upon molecular adsorption on TiO2 [010]-wire, cyanidin will be deprotonated into the quinonoidal form. This results in its highest occupied molecular orbitals being located in the middle of TiO2 bandgap and its lowest occupied molecular orbitals being close to the TiO2 conduction band minimum, in turn enhancing visible light absorption. Moreover, the excited electrons are injected into TiO2 conduction band within a time scale of 50 fs with negligible electron-hole recombination. (iii) The atomic and electronic structure of copper (fluoro-)phthalocyanine and graphene. When adsorbed on graphene, F16CuPc molecules prefer to form a close-packed hexagonal lattice with two-ordered alternating α and β stripes, whereas CuPc would like to form a square lattice. In addition, phthalocyanine adsorption modifies the electronic structure of graphene introducing intensity smoothing at 2-3 eV below and a small peak at ∼0.4 eV above the Dirac point in the density of states. And finally, (iv) the electronic interaction between CuPc and fullerene. For CuPc/C60 molecular complex, CuPc prefers to lie flat on the C60 surface rather than taking the standing-up molecular orientation. The favorable adsorption site for CuPc is the bridge site of C60 with one N-Cu-N bond of CuPc being parallel to a C-C bond of C60. Based on the analysis of the molecular complex, we predict that CuPc/C60(001) thin film heterojunction adopting the lying-down molecular orientation should have a higher efficiency of charge transfer in comparison with the relevant CuPc/C60(111) heterojunction with the standing-up arrangement.

EPFL2011

Alignment of energy levels in amorphous oxides and at semiconductor-water interfaces

Zhendong Guo

We conduct a detailed investigation of defects in two representative amorphous oxides: amorphous Al2O3 (am-Al2O3) and TiO2 (am-TiO2), by combining ab initio molecular dynamics (MD) simulations and hybrid functional calculations. Our results indicate that oxygen vacancies and interstitials occur neither in am-Al2O3 nor in am-TiO2 due to structural rearrangements which assimilate the defect structure, and that the injection of extra holes can lead to the formation of O-O peroxy linkages. In am-Al2O3, hydrogen is found to be amphoteric. Based on localized Wannier functions, we identify the nominal charge state and the composition of the defect core units related to C and N impurities in am-Al2O3, which are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively, indicating that none of them gives charge transition levels in am-Al2O3. In addition, those defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy is dependent on the oxygen chemical potential. In addition, we propose an exchange mechanism for hole transport in am-TiO2 that relies on the simultaneous breaking and forming of O-O peroxy linkages that share one O atom. Through the use of nudged-elastic-band calculations, a hopping path as long as 1.2 nm with barriers of 0.3-0.5 eV is demonstrated, suggesting that hole diffusion through O-O peroxy linkages is viable in am-TiO2. In this work, we also determine the band alignment between various semiconductors and liquid water by combining MD simulations of atomistic interface models, electronic-structure calculations at the hybrid-functional and GW levels, and a computational standard hydrogen electrode (SHE). Our study comprises GaAs, GaP, GaN, CdS, ZnO, SnO2, rutile and anatase TiO2. For each semiconductor, we generate atomistic interface models with liquid water at the pH corresponding to the point of zero charge. The MD simulations are started from initial configurations, in which the water molecules are either molecularly (m) or dissociatively (d) adsorbed on the semiconductor surface. The calculated band offsets are found to be strongly influenced by the adsorption mode at the semiconductor-water interface, leading to differences larger than 1 eV between m and d models of the same semiconductor. We then assess the accuracy of various ab initio electronic-structure schemes in determining the band alignment. In the last part, we try to evaluate photocatalysts for water-splitting by considering the surface coverage and the energy alignment. We determine surface concentrations of water molecules, protons, and hydroxyl ions adsorbed at the semiconductor-water interfaces mentioned above as a function of pH. This is achieved through the calculation of the acidity constants at the surface sites, which are derived from ab initio MD simulations and a grand-canonical formulation of adsorbates. It is finally shown how the potential of a semiconductor material as photocatalysts for water splitting can be inferred by combining the nature of the surface coverage and the alignment of the band edges to the relevant redox levels.

EPFL2019