Nitrogen loss and oxygen evolution reaction activity of perovskite oxynitrides

Perovskite oxynitride photocatalysts were reported by experiment to evolve small amounts of N$_2$ due to the self-oxidation of nitrogen ions by photo-generated holes. The N$_2$ evolution rate was observed to decrease with increasing reaction time and was found to be correlated with a decrease in O$_2$ evolution (OER) activity, the origin of this latter effect however being unknown. Here we investigate, by means of density functional theory calculation, anion vacancies at the TaON-terminated (001) surface of the perovskite oxynitride SrTaO$_2$N. We find an energetic preference for oxygen and nitrogen vacancies to reside at the surface, where they are spontaneously healed by *O and *OH adsorbates under OER conditions. For nitrogen vacancies, this self-healing leads to an altered stoichiometry Ta$_4$O$_{8+x}$N$_{4-x}$ that is accompanied by electron doping. Substitution of N by O at the surface also leads to tensile strain, which confines the excess charge to the very surface layer, affecting the binding energy of reaction intermediates and significantly increasing the OER overpotential. This peculiar change in electronic structure thus provides an atomic scale explanation for the experimentally observed drop in OER activity of perovskite oxynitrides.Comment: 15 pages, 7 figure

Polaron formation and hopping in tantalate perovskite oxides: NaTaO3 and KTaO3

Perovskite tantalates have become potential candidates for water splitting photocatalysts. Therefore, it is of importance to understand the behavior of the photoinduced excess charges in these materials. Herein, we investigate the formation of electron and hole polarons in NaTaO3 and KTaO3. We perform Perdew-Burke-Ernzerhof hybrid density functional PBE0(alpha) calculations, in which we define the fraction alpha of the Fock exchange by enforcing the Koopmans\u27 condition, to properly account for self-interaction corrections in these calculations. We find that the hole polaron mainly localizes on one oxygen site in both materials, leading to a structural distortion where two Ta-O bonds are elongated. The electron polaron, on the other hand, localizes within one atomic plane and exhibits a two-dimensional electron gas nature. Finally, we find that the strong localization of holes leads to a low hole mobility at room temperature similar to 2.94 x 10-6 cm2/Vs and similar to 1.87 x 10-4 cm2/Vs for KTaO3 and NaTaO3, respectively

Water Oxidation Chemistry of Oxynitrides and Oxides: Comparing NaTaO3_3 and SrTaO2_2N

The oxygen evolution reaction (OER) plays an important role in evaluating a photocatalyst and to understand its surface chemistry. In this work we present a comparative study of the OER on the oxide NaTaO$_3$ (113) surface and the oxynitride SrTaO$_2$N (001) surface. Oxynitrides are highly promising photocatalysts due to their smaller band gap and resulting better visible light absorption compared to oxides but our knowledge about their surface structure and chemistry is still very limited. With the goal to compare the surface chemistry of oxides and oxynitrides, we perform density functional theory calculations to obtain the free energy changes associated with the OER reaction steps. For the OER at the Ta site of the clean surfaces, our results predict the rate-limiting step for both materials to be the formation of the *OOH intermediate, with a larger overpotential for the oxide than the oxynitride (1.30 V vs 1.01 V). The Na site is found to be more active than the Ta site on the oxide surface with an OER overpotential of 0.88 V, whereas the OER at the Sr site on the oxynitride has an overpotential of 1.14 V. For the A sites, contrary to the Ta site, the deprotonation of *OH was found to be the rate-limiting step. Computed Pourbaix diagrams show that at relevant (photo)electrochemical conditions all surfaces are covered with oxygen adsorbates. Oxygen adsorbates at A (Na, Sr) sites are however found to couple and desorb as O$_2$, leaving these sites empty under typical operating conditions. Following this desorption, we find the OER to proceed by the conventional *OOH mechanism on the SrO termination of the oxynitride but by a direct coupling of neighbouring *O at Na sites on the oxide surface. This coupling mechanism on the oxide has the smallest overpotential of 0.79 V compared to 0.88 V for the oxynitride, implying that the oxide is a better OER catalyst.Comment: 7 pages, 6 figure

Ab Initio Insights into Charge Localization in Bismuth Oxyhalides BiOX (X = F, Cl, Br, I)

Developing efficient photocatalysts for clean energy generation is crucial to achieving net-zero emissions. To this end, we investigate the behavior of photoexcited charges in bismuth oxyhalides BiOX (X = F, Cl, Br, and I), a family of inexpensive and promising photocatalysts. To model the localization of excess electrons and holes, we use hybrid density functional theory PBE0(α). Our results indicate that electron polarons are unstable in these materials. Concurrently, we find that hole localization is favorable, and we identify two different possible configurations in which polarons are formed. One consists of two dimerized halogen atoms (VK center) and is preferentially formed in BiOBr and BiOI with binding energies that amount to-0.26 and-0.21 eV, respectively. The other corresponds to localization on a single Bi site and the surrounding oxygen and halogen atoms (BiXO). This form of polaron is favorable in BiOF and BiOCl with binding energies that amount to-0.35 and-0.23 eV, respectively. These findings highlight the behavior of photogenerated carriers and may open up avenues for future investigations on carrier transport in bismuth oxyhalides

Real grain shape analysis: characterization and generation of representative virtual grains. application to railway ballast

Grain shape significantly influences the mechanical properties of granular media. In order to explore this effect and to simulate realistic material morphology, we designed a method which well characterizes real grains shape. Starting from a representation of the particle surfaces as a points cloud, this paper presents a method to generate a set of virtual grains that are morphologically representative of real ballast grains. The model relies on a statistical modelling of the ballast grain morphology based on a dimensionality reduction approach (Proper Orthogonal Decomposition) leading to an optimal and nearly exhaustive shape characterization by extracting a hierarchy of shape functions that fully describe the grain sample. We will show the efficiency of the both characterizing and generating methods and describe their advantages, as well as a future outloo