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

Band-gap engineering in AB(Ox_xS1−x_{1-x})3_{3} perovskite oxysulfides: A route to strongly polar materials for photocatalytic water splitting

Polar heterogeneous photocatalysts were shown to lead to enhanced charge-carrier separation that results in superior activity for example for photocatalytic water splitting. Promising photocatalyst materials such as oxynitrides can be rendered polar by epitaxial strain, which however also increases their band gap, making them unsuitable for visible light absorption. This suggests a trade-off between small band gaps and polar distortions - both being crucial for the catalyst's efficiency. In this paper we investigate, using density functional theory calculations, the suitability of strained AB(O$_x$S$_{1-x}$)$_{3}$ perovskites for photocatalytic water splitting. These materials normally have band gaps too small for water splitting but inducing polar distortions via epitaxial strain can increase the band gap to the suitable range. We find perovskite BaZr$_y$Ti$_{1-y}$O$_2$S compounds to be highly promising for photocatalytic water splitting due to large polar distortions and suitable band gaps.Comment: 7 pages, 8 figure

Interplay between strain, defect charge state and functionality in complex oxides

We use first-principles calculations to investigate the interplay between strain and the charge state of point defect impurities in complex oxides. Our work is motivated by recent interest in using defects as active elements to provide novel functionality in coherent epitaxial films. Using oxygen vacancies as model point defects, and CaMnO$_3$ and MnO as model materials, we calculate the changes in internal strain caused by changing the charge state of the vacancies, and conversely the effect of strain on charge-state stability. Our results show that the charge state is a degree of freedom that can be used to control the interaction of defects with strain and hence the concentration and location of defects in epitaxial films. We propose the use of field-effect gating to reversibly change the charge state of defects and hence the internal strain and corresponding strain-induced functionalities.Comment: 4 pages, 4 figure

Public investment and economic growth in Mexico

Mexico's growth rate began to plummet at roughly the same time that its public investment expenditures declined. That decline also appears to coincide with a slowdown in the growth of infrastructure capital in the electricity, transport, and communications sectors. Because of these parallel developments, many economists have attributed at least part of the blame for the decline in Mexico's growth after 1981 to the decline of public infrastructure investment. The empirical results presented in this report provide only limited support for this argument. They also suggest, in turn, that increases in public investment would not automatically translate into faster output and productivity growth. One reason not to take for granted a positive relationship between more public investment and faster growth is public investment's crowding out effect on private investment. Although the time-series regression results for Mexico all point toward a crowding out coefficient of less than unity, the existence limits the growth impact of public investment by reducing its net effect on capital accumulation. The time-series results also suggest that the economy's total factor productivity growth responds positively to increases in the ratio of public to private investment. In light of that result, increases in public investment should have a positive net impact on economic growth, despite significant crowding out effects. Chow breakpoint tests indicate, however, that the positive productivity effect appears to have weakened significantly in the past decade. A third reason for questioning a stable relationship is that the impact of increased public investment is likely to depend on how it is financed. The cross-country regressions reported here indicate that a general increase in the public capital stock has a positive impact on growth only if financed through savings generated through lower public consumption expenditures, but not if financed through higher public debt, which implies higher current and future taxation levels. The scope for reducing public consumption expenditures in Mexico is very limited, however, since they are already at rock bottom levels. Therefore, the only way to assure that the public investment program makes a significant contribution to growth is by improving its"quality"through careful attention to its rate of return and complementarity with private capital. In Mexico the most important reforms to make public investment more productive came from policymakers'recognition of the need to distinguish more clearly between the roles of the public and private sectors. This led to the privatization of most public enterprises and a reorientation of public investment to a more narrowly focused set of activities. In addition, the government took important steps to strengthen the institutional framework within which the public investment program is determined.Macroeconomic Management,Inequality,Economic Theory&Research,Environmental Economics&Policies,Economic Stabilization

Effect of epitaxial strain on cation and anion vacancy formation in MnO

Biaxial strain in coherent epitaxial thin films can have a pronounced effect on the point defect profile in the film material. Detailed fundamental knowledge of the interaction of strain with point defects is crucial in understanding the stoichiometry and resulting properties of strained thin films. Here we investigate the effect of biaxial strain on the formation energy of cation and anion vacancies using MnO as a model system. Our density functional theory calculations show that, as expected from local volume arguments, compressive strain favours the formation of cation vacancies. Interestingly, we find that small compressive and tensile strains lead to ordering of the resulting holes along the in-plane and normal direction respectively, which should manifest in different anisotropic properties in the two strain states.Comment: 6 pages, 5 figure