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

"How Should the Surpluses Be Spent?"

What are the likely economic consequences, particularly on saving, investment, and long-term economic growth, of three alternative uses of budget surpluses: paying down the debt, increasing government spending, and cutting taxes?

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

"How Big Should the Public Capital Stock Be? The Relationship Between Public Capital and Economic Growth"

Investment in infrastructure is necessary for a strong, flexible, and growing economy. However, the relationship between public capital and economic growth is not linear. At a certain level, the tax burden associated with financing and maintaining public capital reduces the returns to private industry, which, in turn, reduces growth; also, different types of spending have different effects on growth. The short- and long-term growth-maximizing effects of public investment increase as the ratio of public to private capital stock rises to an optimal level (found to be about 61 percent); above that level the growth effects decrease. The public to private ratio is below the optimal level throughout much of the country and government spending is not always directed toward the types of investment that have the most positive effects on growth. Good economic policy requires both increasing the public capital stock and reorienting government spending from consumption to investment in physical capital stock.

The Role of Public Infrastructure Capital in Mexican Economic Growth

This paper develops and empirically implements a neoclassical growth model in which output depends on private capital and human capital as well as the quantity, means of financing, and efficiency of use of public capital. The empirical analysis is based on a cross section of 46 developing countries over the period from 1970 to 1990. In general, the paper finds empirical support for the importance of each of the three dimensions of public capital –quantity, financing, and efficiency– for long run standards of living and for transitional growth rates. The empirical results are applied to the recent performance of the Mexican economy.