Syntheses, Crystal Structures, and Electronic Structures of Quaternary Group IV-Selenide Semiconductors.

Early transition-metal chalcogenides have garnered recent attention for their optoelectronic properties for solar energy conversion. Herein, the first Zr-/Hf-chalcogenides with a main group cation, Ba9Hf3Sn2Se19 (1) and Ba8Zr2SnSe13(Se2) (2), have been synthesized. The structure of 1 is formed from isolated SnSe44- tetrahedra and distorted HfSe6 octahedra. The latter condense via face-sharing trimeric motifs that are further vertex-bridged into chains of 1∞[Hf(1)2Hf(2)Se11]10-. The structure of 2 is comprised of SnSe44- tetrahedra, Se22- dimers, and face-sharing dimers of distorted ZrSe6 octahedra. These represent the first reported examples of Hf-/Zr-chalcogenides exhibiting face-sharing octahedra with relatively short Hf-Hf and Zr-Zr distances. Their preparation in high purity is inhibited by their low thermodynamic stability, with calculations showing small calculated ΔUdec values of +7 and +9 meV atom-1 for 1 and 2, respectively. Diffuse reflectance measurements confirm the semiconducting nature of 1 with an indirect band gap of ∼1.4(1) eV. Electronic structure calculations show that the band gap absorptions arise from transitions between predominantly Se-4p valence bands and mixed Hf-5d/Sn-5p or Zr-4d/Sn-5p conduction bands. Optical absorption coefficients were calculated to be more than ∼105 cm-1 at greater than 1.8 eV. Thus, promising optical properties are demonstrated for solar energy conversion within these synthetically challenging chemical systems.

Coordination of copper within a crystalline carbon nitride and its catalytic reduction of CO2.

Inherently disordered structures of carbon nitrides have hindered an atomic level tunability and understanding of their catalytic reactivity. Starting from a crystalline carbon nitride, poly(triazine imide) or PTI/LiCl, the coordination of copper cations to its intralayer N-triazine groups was investigated using molten salt reactions. The reaction of PTI/LiCl within CuCl or eutectic KCl/CuCl2 molten salt mixtures at 280 to 450 °C could be used to yield three partially disordered and ordered structures, wherein the Cu cations are found to coordinate within the intralayer cavities. Local structural differences and the copper content, i.e., whether full or partial occupancy of the intralayer cavity occurs, were found to be dependent on the reaction temperature and Cu-containing salt. Crystallites of Cu-coordinated PTI were also found to electrophoretically deposit from aqueous particle suspensions onto either graphite or FTO electrodes. As a result, electrocatalytic current densities for the reduction of CO2 and H2O reached as high as ∼10 to 50 mA cm-2, and remained stable for >2 days. Selectivity for the reduction of CO2 to CO vs. H2 increases for thinner crystals as well as for when two Cu cations coordinate within the intralayer cavities of PTI. Mechanistic calculations have also revealed the electrocatalytic activity for CO2 reduction requires a smaller thermodynamic driving force with two neighboring Cu atoms per cavity as compared to a single Cu atom. These results thus establish a useful synthetic pathway to metal-coordination in a crystalline carbon nitride and show great potential for mediating stable CO2 reduction at sizable current densities.

Well-Defined Iron Sites in Crystalline Carbon Nitride.

Carbon nitride materials can be hosts for transition metal sites, but Mössbauer studies on iron complexes in carbon nitrides have always shown a mixture of environments and oxidation states. Here we describe the synthesis and characterization of a crystalline carbon nitride with stoichiometric iron sites that all have the same environment. The material (formula C6N9H2Fe0.4Li1.2Cl, abbreviated PTI/FeCl2) is derived from reacting poly(triazine imide)·LiCl (PTI/LiCl) with a low-melting FeCl2/KCl flux, followed by anaerobic rinsing with methanol. X-ray diffraction, X-ray absorption and Mössbauer spectroscopies, and SQUID magnetometry indicate that there are tetrahedral high-spin iron(II) sites throughout the material, all having the same geometry. The material is active for electrocatalytic nitrate reduction to ammonia, with a production rate of ca. 0.1 mmol cm-2 h-1 and Faradaic efficiency of ca. 80% at -0.80 V vs RHE.

Understanding Gut Feelings: Transformations in Coping With Inflammatory Bowel Disease Among Young Adults.

Past studies have revealed a dizzying array of coping techniques employed by persons living with inflammatory bowel disease (IBD). Unfortunately, research has provided little insight into when and why individuals adopt or abandon particular coping strategies. Using a retrospective narrative approach, we explored how participants made sense of changes in their approach to coping over time. Shifts in coping strategies were associated with particular illness experiences that wrought new understandings of IBD and novel identity challenges. They followed a common processual form and were marked by a movement away from techniques of purification, normalization, and banalization toward the development of a more communicative body. This was accompanied by notable shifts in identity work. Notably, participants moved from a preoccupation with maintaining continuity and sameness to permitting their extraordinary bodies to occupy a place in their public and personal identities. Implications of this process for theory and practice are discussed.