Evaluation of Photocatalysts for Water Splitting through Combined Analysis of Surface Coverage and Energy-Level Alignment

To examine whether suitable conditions occur for the water splitting reaction at their interfaces with liquid water, we determine the pH-dependent surface coverage for a series of semiconductors, including GaAs, GaP, GaN, CdS, ZnO, SnO2, and rutile and anatase TiO2. For this, we calculate acidity constants at surface sites through ab initio molecular dynamics simulations and a Grand Canonical formulation of adsorbates. The resulting pH values at the point of zero charge show excellent agreement with experiment and thereby support the validity of our approach. By combining information concerning the surface coverage with the alignment of the band edges with respect to the relevant redox levels, we scrutinize the potential of the considered semiconductors as photocatalysts and identify the corresponding optimal pH ranges for hydrogen and oxygen evolution. More specifically, our results indicate that GaN stands out among these semiconductors as the most promising candidate for the overall water splitting, with the potential of further improvement through alloying. With the surface coverage, our computational analysis brings a new descriptor that is currently beyond experimental reach.

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

Moisture and energy dynamics in fields soils

Francesco Ciocca

Soil moisture is crucial to water-cycle as it affects infiltration, runoff and land-atmosphere interactions. In dry soils the role of water vapor fluxes becomes essential and causes a tight coupling between soil moisture dynamics and heat transfer, which makes measurements and modeling extremely difficult. Since advances in both theory and experimental techniques are required, we have focused on both aspects: we have performed theoretical-numerical investigations of soil moisture and energy dynamics at the land surface; tested novel measurements techniques, in laboratory and at field scale, to provide new insights into soil moisture dynamics and contribute to a comprehensive theory of coupled energy and moisture transport. As the dynamics of physical systems are dictated by conservation equations, it is common practice to estimate quantities that cannot be directly measured (e.g. vapor fluxes) from the residuals of balance equations. In laboratory as well as in field experiments, residuals are calculated from measurements that are sparse in space and discrete in time. We have theoretically and numerically shown how the use of discrete data may lead to large residuals even without measurements or model errors. We have demonstrated that residuals are very sensitive to the distance between the measurements and that they increase in presence of nonlinear processes, such as moisture transport. A careful error analysis is required to assess the reliability of residuals computed from discrete data and avoid misinterpretations of the underlying physical processes. Another crucial element dictating soil moisture dynamics and evaporation into the atmosphere is soil water-retention. Popular parametric models of the retention curve present a matric potential that becomes infinite at residual saturation, which is the water content below which liquid flow stops. Few extensions of the water-retention curves have been proposed, which remain rarely used in practice. We have studied the effects of different models on liquid and vapor transport. We have shown that parameterizations that allow vapor flux below residual saturation yield larger vapor fluxes that result in a drier soil surface, whereas deep soil remains relatively wet. When diurnal forcing is considered, potential evaporation is reduced in the energy-limited regime and enhanced in the moisture-limited regime. We have established that in this case extended water-retention models predict larger heat fluxes that might critically impact models at whole scales (possibly including global circulation models). Discrimination between different theoretical descriptions and parametric models requires experiments that monitor moisture and energy dynamics. This demands the use of accurate sensors and measurement techniques that are able to simultaneously monitor several quantities (e.g. soil moisture, temperature, soil thermal properties, solute concentration in soil water). As in general different probes are used, problems arise due to different footprints and sampled soil volumes. Multi functional heat pulse probes (MFHPPs) have been conceived to overcome these issues by allowing simultaneous measurements of all required variables. We have assessed MFHPPs ability to measure ground heat flux, which has to be accurately known to avoid significant imbalances in the surface energy budget. We have employed an experimental setup able to generate a non-uniform heat-flux field and we have demonstrated that MFHPPs can reliably capture the magnitudes and directions of the fluxes. This allows more complete information on ground heat flux than that obtained by the classic probes (e.g. ground heat flux plates). To monitor soil state in large-scale field applications we have investigated the possibility to infer distributed thermal conductivity and soil moisture profiles with an optical fiber. Information is obtained by monitoring soil response to an active heat pulse generated from the metal shield armoring the optic cable, and analyzing the temperature differences of the cooling phase, monitored each meter by a distributed temperature sensing system (DTS). We have shown that soil moisture measurements inferred from soil thermal response (recorded by the DTS) are in good agreement with independent measurements in wet and intermediately wet soils; however in dry soils moisture content is underestimated and further investigations are required to improve this technique for dry conditions. In this study we have significantly improved the knowledge of coupled energy and moisture transport on both theoretical-numerical and experimental sides. We have proven that reliable measurements can be obtained at different spatiotemporal scales, and provided both prognostic and diagnostic methods to optimize their use. Finally, we have suggested crucial improvements to numerical models which might have implications beyond field-soils dynamics.

EPFL2013

Structural, Dynamical, and Electronic Properties of Liquid Water: A Hybrid Functional Study

Francesco Ambrosio, Giacomo Francesco Leonardo Miceli, Alfredo Pasquarello

We study structural, dynamical, and electronic properties of liquid water through ab initio molecular dynamics (MD) simulations based on a hybrid functional which includes nonlocal van der Waals (vdW) interactions. The water dimer, the water hexamer, and two phases of ice are studied as benchmark cases. The hydrogen-bond energy depends on the balance between Fock exchange and vdW interactions. Moreover, the energetic competition between extended and compact structural motifs is found to be well described by theory provided vdW interactions are accounted for. Applied to the hydrogen-bond network of liquid water, the dispersion interactions favor more compact structural motifs, bring the density closer to the experimental value, and improve the agreement with experimental observables such as radial distribution functions. The description of the self-diffusion coefficient is also found to improve upon the combined consideration of Fock exchange and vdW interactions. The band gap and the band edges are found to agree with experiment within 0.1 eV.