Manipulation of the Structure and Optoelectronic Properties through Bromine Inclusion in a Layered Lead Bromide Perovskite.

One of the great advantages of organic-inorganic metal halides is that their structures and properties are highly tuneable and this is important when optimizing materials for photovoltaics or other optoelectronic devices. One of the most common and effective ways of tuning the electronic structure is through anion substitution. Here, we report the inclusion of bromine into the layered perovskite [H3N(CH2)6NH3]PbBr4 to form [H3N(CH2)6NH3]PbBr4·Br2, which contains molecular bromine (Br2) intercalated between the layers of corner-sharing PbBr6 octahedra. Bromine intercalation in [H3N(CH2)6NH3]PbBr4·Br2 results in a decrease in the band gap of 0.85 eV and induces a structural transition from a Ruddlesden-Popper-like to Dion-Jacobson-like phase, while also changing the conformation of the amine. Electronic structure calculations show that Br2 intercalation is accompanied by the formation of a new band in the electronic structure and a significant decrease in the effective masses of around two orders of magnitude. This is backed up by our resistivity measurements that show that [H3N(CH2)6NH3]PbBr4·Br2 has a resistivity value of one order of magnitude lower than [H3N(CH2)6NH3]PbBr4, suggesting that bromine inclusion significantly increases the mobility and/or carrier concentration in the material. This work highlights the possibility of using molecular inclusion as an alternative tool to tune the electronic properties of layered organic-inorganic perovskites, while also being the first example of molecular bromine inclusion in a layered lead halide perovskite. By using a combination of crystallography and computation, we show that the key to this manipulation of the electronic structure is the formation of halogen bonds between the Br2 and Br in the [PbBr4]∞ layers, which is likely to have important effects in a range of organic-inorganic metal halides.

Cation Ordering and Exsolution in Copper-Containing Forms of the Flexible Zeolite Rho (Cu,M-Rho; M=H, Na) and Their Consequences for CO2 Adsorption.

The flexibility of the zeolite Rho framework offers great potential for tunable molecular sieving. The fully copper-exchanged form of Rho and mixed Cu,H- and Cu,Na-forms have been prepared. EPR spectroscopy reveals that Cu2+ ions are present in the dehydrated forms and Rietveld refinement shows these prefer S6R sites, away from the d8r windows that control diffusion. Fully exchanged Cu-Rho remains in an open form upon dehydration, the d8r windows remain nearly circular and the occupancy of window sites is low, so that it adsorbs CO2 rapidly at room temperature. Breakthrough tests with 10 % CO2 /40 % CH4 mixtures show that Cu4.9 -Rho is able to produce pure methane, albeit with a relatively low capacity at this pCO2 due to the weak interaction of CO2 with Cu cations. This is in strong contrast to Na-Rho, where cations in narrow elliptical window sites enable CO2 to be adsorbed with high selectivity and uptake but too slowly to enable the production of pure methane in similar breakthrough experiments. A series of Cu,Na-Rho materials was prepared to improve uptake and selectivity compared to Cu-Rho, and kinetics compared to Na-Rho. Remarkably, Cu,Na-Rho with >2 Cu cations per unit cell exhibited exsolution, due to the preference of Na cations for narrow S8R sites in distorted Rho and of Cu cations for S6R sites in the centric, open form of Rho. The exsolved Cu,Na-Rho showed improved performance in CO2 /CH4 breakthrough tests, producing pure CH4 with improved uptake and CO2 /CH4 selectivity compared to that of Cu4.9 -Rho.

Platinum incorporation into titanate perovskites to deliver emergent active and stable platinum nanoparticles.

Platinum functions exceptionally well as a nanoparticulate catalyst in many important fields, such as in the removal of atmospheric pollutants, but it is scarce, expensive and not always sufficiently durable. Here, we report a perovskite system in which 0.5 wt% Pt is integrated into the support and its subsequent conversion through exsolution to achieve a resilient catalyst. Owing to the instability of most Pt oxides at high temperatures, a thermally stable platinum oxide precursor, barium platinate, was used to preserve the platinum as an oxide during the solid-state synthesis in an approach akin to the Trojan horse legend. By tailoring the procedure, it is possible to produce a uniform equilibrated structure with active emergent Pt nanoparticles strongly embedded in the perovskite surface that display better CO oxidation activity and stability than those of conventionally prepared Pt catalysts. This catalyst was further evaluated for a variety of reactions under realistic test environments-CO and NO oxidation, diesel oxidation catalysis and ammonia slip reactions were investigated.

Multifaceted Study of the Interactions between CPO-27-Ni and Polyurethane and Their Impact on Nitric Oxide Release Performance.

A multifaceted study involving focused ion beam scanning electron microscopy techniques, mechanical analysis, water adsorption measurements, and molecular simulations is employed to rationalize the nitric oxide release performance of polyurethane films containing 5, 10, 20, and 40 wt % of the metal-organic framework (MOF) CPO-27-Ni. The polymer and the MOF are first demonstrated to exhibit excellent compatibility. This is reflected in the even distribution and encapsulation of large wt % MOF loadings throughout the full thickness of the films and by the rather minimal influence of the MOF on the mechanical properties of the polymer at low wt %. The NO release efficiency of the MOF is attenuated by the polymer and found to depend on wt % of MOF loading. The formation of a fully connected network of MOF agglomerates within the films at higher wt % is proposed to contribute to a more complex guest transport in these formulations, resulting in a reduction of NO release efficiency and film ductility. An optimum MOF loading of 10 wt % is identified for maximizing NO release without adversely impacting the polymer properties. Bactericidal efficacy of released NO from the films is demonstrated against Pseudomonas aeruginosa, with a >8 log10 reduction in cell density observed after a contact period of 24 h.

Demonstration of chemistry at a point through restructuring and catalytic activation at anchored nanoparticles.

Metal nanoparticles prepared by exsolution at the surface of perovskite oxides have been recently shown to enable new dimensions in catalysis and energy conversion and storage technologies owing to their socketed, well-anchored structure. Here we show that contrary to general belief, exsolved particles do not necessarily re-dissolve back into the underlying perovskite upon oxidation. Instead, they may remain pinned to their initial locations, allowing one to subject them to further chemical transformations to alter their composition, structure and functionality dramatically, while preserving their initial spatial arrangement. We refer to this concept as chemistry at a point and illustrate it by tracking individual nanoparticles throughout various chemical transformations. We demonstrate its remarkable practical utility by preparing a nanostructured earth abundant metal catalyst which rivals platinum on a weight basis over hundreds of hours of operation. Our concept enables the design of compositionally diverse confined oxide particles with superior stability and catalytic reactivity.

Nanoimprinted distributed feedback lasers of solution processed hybrid perovskites.

Hybrid perovskite materials have considerable potential for light emitting devices such as LEDs and lasers. We combine solution processed CH3NH3PbI3 perovskite with UV nanoimprinted polymer gratings to fabricate distributed feedback (DFB) lasers. The lead acetate deposition route is shown to be an effective method for fabricating low-loss waveguides (loss coefficient ~6 cm-1) and highly compatible with the polymer grating substrates. The nanoimprinted perovskite exhibited single-mode band-edge lasing, confirmed by angle-dependent transmission measurements. Depending on the excitation pulse duration the lasing threshold shows a value of 110 µJ/cm2 under nanosecond pumping and 4 µJ/cm2 under femtosecond pumping. We demonstrate further that this laser has excellent stability with a lifetime of 108 pulses.

Switching on electrocatalytic activity in solid oxide cells.

Solid oxide cells (SOCs) can operate with high efficiency in two ways-as fuel cells, oxidizing a fuel to produce electricity, and as electrolysis cells, electrolysing water to produce hydrogen and oxygen gases. Ideally, SOCs should perform well, be durable and be inexpensive, but there are often competitive tensions, meaning that, for example, performance is achieved at the expense of durability. SOCs consist of porous electrodes-the fuel and air electrodes-separated by a dense electrolyte. In terms of the electrodes, the greatest challenge is to deliver high, long-lasting electrocatalytic activity while ensuring cost- and time-efficient manufacture. This has typically been achieved through lengthy and intricate ex situ procedures. These often require dedicated precursors and equipment; moreover, although the degradation of such electrodes associated with their reversible operation can be mitigated, they are susceptible to many other forms of degradation. An alternative is to grow appropriate electrode nanoarchitectures under operationally relevant conditions, for example, via redox exsolution. Here we describe the growth of a finely dispersed array of anchored metal nanoparticles on an oxide electrode through electrochemical poling of a SOC at 2 volts for a few seconds. These electrode structures perform well as both fuel cells and electrolysis cells (for example, at 900 °C they deliver 2 watts per square centimetre of power in humidified hydrogen gas, and a current of 2.75 amps per square centimetre at 1.3 volts in 50% water/nitrogen gas). The nanostructures and corresponding electrochemical activity do not degrade in 150 hours of testing. These results not only prove that in operando methods can yield emergent nanomaterials, which in turn deliver exceptional performance, but also offer proof of concept that electrolysis and fuel cells can be unified in a single, high-performance, versatile and easily manufactured device. This opens up the possibility of simple, almost instantaneous production of highly active nanostructures for reinvigorating SOCs during operation.

Nano-socketed nickel particles with enhanced coking resistance grown in situ by redox exsolution.

Metal particles supported on oxide surfaces are used as catalysts for a wide variety of processes in the chemical and energy conversion industries. For catalytic applications, metal particles are generally formed on an oxide support by physical or chemical deposition, or less commonly by exsolution from it. Although fundamentally different, both methods might be assumed to produce morphologically and functionally similar particles. Here we show that unlike nickel particles deposited on perovskite oxides, exsolved analogues are socketed into the parent perovskite, leading to enhanced stability and a significant decrease in the propensity for hydrocarbon coking, indicative of a stronger metal-oxide interface. In addition, we reveal key surface effects and defect interactions critical for future design of exsolution-based perovskite materials for catalytic and other functionalities. This study provides a new dimension for tailoring particle-substrate interactions in the context of increasing interest for emergent interfacial phenomena.

In situ growth of nanoparticles through control of non-stoichiometry.

Surfaces decorated with uniformly dispersed catalytically active nanoparticles play a key role in many fields, including renewable energy and catalysis. Typically, these structures are prepared by deposition techniques, but alternatively they could be made by growing the nanoparticles in situ directly from the (porous) backbone support. Here we demonstrate that growing nano-size phases from perovskites can be controlled through judicious choice of composition, particularly by tuning deviations from the ideal ABO3 stoichiometry. This non-stoichiometry facilitates a change in equilibrium position to make particle exsolution much more dynamic, enabling the preparation of compositionally diverse nanoparticles (that is, metallic, oxides or mixtures) and seems to afford unprecedented control over particle size, distribution and surface anchorage. The phenomenon is also shown to be influenced strongly by surface reorganization characteristics. The concept exemplified here may serve in the design and development of more sophisticated oxide materials with advanced functionality across a range of possible domains of application.

Youth suicide prevention: a survey of public school superintendents' acceptability of school-based programs.

From a random sample of members of the 2000-2001 membership directory of the American Association of School Administrators (AASA), public school administrators' acceptability ratings of three school-based programs for the prevention of adolescent suicide were examined. A total of 210 (46%) respondents examined a description of a suicide prevention program and completed a measure designed to evaluate the acceptability of suicide prevention programs. Three suicide prevention programs were evaluated for their acceptability, and included: (a) school-wide curriculum-based programs presented to students; (b) in-service presentations to school staff; and (c) self-report screening programs for students. The results indicated that superintendents rated the staff in-service training and curriculum-based programs as significantly more acceptable than the school-wide screening program. In addition, the school-wide screening program was rated as significantly more intrusive by school psychologists than the staff in-service training or curriculum-based prevention programs. Limitations of the study and future research directions are discussed.