Restructuring of titanium oxide overlayers over nickel nanoparticles during catalysis.

Reducible supports can affect the performance of metal catalysts by the formation of suboxide overlayers upon reduction, a process referred to as the strong metal-support interaction (SMSI). A combination of operando electron microscopy and vibrational spectroscopy revealed that thin TiOx overlayers formed on nickel/titanium dioxide catalysts during 400°C reduction were completely removed under carbon dioxide hydrogenation conditions. Conversely, after 600°C reduction, exposure to carbon dioxide hydrogenation reaction conditions led to only partial reexposure of nickel, forming interfacial sites in contact with TiOx and favoring carbon-carbon coupling by providing a carbon species reservoir. Our findings challenge the conventional understanding of SMSIs and call for more-detailed operando investigations of nanocatalysts at the single-particle level to revisit static models of structure-activity relationships.

Dynamic restructuring of supported metal nanoparticles and its implications for structure insensitive catalysis.

Some fundamental concepts of catalysis are not fully explained but are of paramount importance for the development of improved catalysts. An example is the concept of structure insensitive reactions, where surface-normalized activity does not change with catalyst metal particle size. Here we explore this concept and its relation to surface reconstruction on a set of silica-supported Ni metal nanoparticles (mean particle sizes 1-6 nm) by spectroscopically discerning a structure sensitive (CO2 hydrogenation) from a structure insensitive (ethene hydrogenation) reaction. Using state-of-the-art techniques, inter alia in-situ STEM, and quick-X-ray absorption spectroscopy with sub-second time resolution, we have observed particle-size-dependent effects like restructuring which increases with increasing particle size, and faster restructuring for larger particle sizes during ethene hydrogenation while for CO2 no such restructuring effects were observed. Furthermore, a degree of restructuring is irreversible, and we also show that the rate of carbon diffusion on, and into nanoparticles increases with particle size. We finally show that these particle size-dependent effects induced by ethene hydrogenation, can make a structure sensitive reaction (CO2 hydrogenation), structure insensitive. We thus postulate that structure insensitive reactions are actually apparently structure insensitive, which changes our fundamental understanding of the empirical observation of structure insensitivity.

Understanding carbon dioxide activation and carbon-carbon coupling over nickel.

Carbon dioxide is a desired feedstock for platform molecules, such as carbon monoxide or higher hydrocarbons, from which we will be able to make many different useful, value-added chemicals. Its catalytic hydrogenation over abundant metals requires the amalgamation of theoretical knowledge with materials design. Here we leverage a theoretical understanding of structure sensitivity, along with a library of different supports, to tune the selectivity of methanation in the Power-to-Gas concept over nickel. For example, we show that carbon dioxide hydrogenation over nickel can and does form propane, and that activity and selectivity can be tuned by supporting different nickel particle sizes on various oxides. This theoretical and experimental toolbox is not only useful for the highly selective production of methane, but also provides new insights for carbon dioxide activation and subsequent carbon-carbon coupling towards value-added products thereby reducing the deleterious effects of this environmentally harmful molecule.

A brief, patient- and proxy-reported outcome measure in advanced illness: Validity, reliability and responsiveness of the Integrated Palliative care Outcome Scale (IPOS).

BACKGROUND: Few measures capture the complex symptoms and concerns of those receiving palliative care. AIM: To validate the Integrated Palliative care Outcome Scale, a measure underpinned by extensive psychometric development, by evaluating its validity, reliability and responsiveness to change. DESIGN: Concurrent, cross-cultural validation study of the Integrated Palliative care Outcome Scale - both (1) patient self-report and (2) staff proxy-report versions. We tested construct validity (factor analysis, known-group comparisons, and correlational analysis), reliability (internal consistency, agreement, and test-retest reliability), and responsiveness (through longitudinal evaluation of change). SETTING/PARTICIPANTS: In all, 376 adults receiving palliative care, and 161 clinicians, from a range of settings in the United Kingdom and Germany. RESULTS: We confirm a three-factor structure (Physical Symptoms, Emotional Symptoms and Communication/Practical Issues). Integrated Palliative care Outcome Scale shows strong ability to distinguish between clinically relevant groups; total Integrated Palliative care Outcome Scale and Integrated Palliative care Outcome Scale subscale scores were higher - reflecting more problems - in those patients with 'unstable' or 'deteriorating' versus 'stable' Phase of Illness (F = 15.1, p < 0.001). Good convergent and discriminant validity to hypothesised items and subscales of the Edmonton Symptom Assessment System and Functional Assessment of Cancer Therapy-General is demonstrated. The Integrated Palliative care Outcome Scale shows good internal consistency (α = 0.77) and acceptable to good test-retest reliability (60% of items kw > 0.60). Longitudinal validity in form of responsiveness to change is good. CONCLUSION: The Integrated Palliative care Outcome Scale is a valid and reliable outcome measure, both in patient self-report and staff proxy-report versions. It can assess and monitor symptoms and concerns in advanced illness, determine the impact of healthcare interventions, and demonstrate quality of care. This represents a major step forward internationally for palliative care outcome measurement.

Effect of Electron-Hole Overlap and Exchange Interaction on Exciton Radiative Lifetimes of CdTe/CdSe Heteronanocrystals.

Wave function engineering has become a powerful tool to tailor the optical properties of semiconductor colloidal nanocrystals. Core-shell systems allow to design the spatial extent of the electron (e) and hole (h) wave functions in the conduction- and valence bands, respectively. However, tuning the overlap between the e- and h-wave functions not only affects the oscillator strength of the coupled e-h pairs (excitons) that are responsible for the light emission, but also modifies the e-h exchange interaction, leading to an altered excitonic energy spectrum. Here, we present exciton lifetime measurements in a strong magnetic field to determine the strength of the e-h exchange interaction, independently of the e-h overlap that is deduced from lifetime measurements at room temperature. We use a set of CdTe/CdSe core/shell heteronanocrystals in which the electron-hole separation is systematically varied. We are able to unravel the separate effects of e-h overlap and e-h exchange on the exciton lifetimes, and we present a simple model that fully describes the recombination lifetimes of heteronanostructures (HNCs) as a function of core volume, shell volume, temperature, and magnetic fields.

Structure and rheology of mixed suspensions of montmorillonite and silica nanoparticles.

We present a study of the structure and rheology of mixed suspensions of montmorillonite clay platelets and Ludox TMA silica spheres at pH 5, 7, and 9. Using cryogenic transmission electron microscopy (cryo-TEM), we probe the changes in the structure of the montmorillonite suspensions induced by changing the pH and by adding silica particles. Using oscillatory and transient rheological measurements, we examine the changes in storage modulus and yield stress of the montmorillonite suspensions upon changing the pH and adding silica particles. Cryo-TEM images reveal that changes in pH have a significant effect on the structure of the suspensions, which can be related to the change in charge of the edges from positive at pH 5 to negative at higher pH. Furthermore, at pH 7, the cryo-TEM images show indications of a microphase separation between clay and silica particles. The addition of silica leads to lowering of the storage modulus and yield stress, which we connect to the structural changes of the suspension.

Observation of the full exciton and phonon fine structure in CdSe/CdS dot-in-rod heteronanocrystals.

Light emission of semiconductor nanocrystals is a complex process, depending on many factors, among which are the quantum mechanical size confinement of excitons (coupled electron-hole pairs) and the influence of confined phonon modes and the nanocrystal surface. Despite years of research, the nature of nanocrystal emission at low temperatures is still under debate. Here we unravel the different optical recombination pathways of CdSe/CdS dot-in-rod systems that show an unprecedented number of narrow emission lines upon resonant laser excitation. By using self-assembled, vertically aligned rods and application of crystallographically oriented high magnetic fields, the origin of all these peaks is established. We observe a clear signature of an acoustic-phonon assisted transition, separated from the zero-phonon emission and optical-phonon replica, proving that nanocrystal light emission results from an intricate interplay between bright (optically allowed) and dark (optically forbidden) exciton states, coupled to both acoustic and optical phonon modes.

Learning from the public: citizens describe the need to improve end-of-life care access, provision and recognition across Europe.

BACKGROUND: Despite ageing populations and increasing cancer deaths, many European countries lack national policies regarding palliative and end-of-life care. The aim of our research was to determine public views regarding end-of-life care in the face of serious illness. METHODS: Implementation of a pan-European population-based survey with adults in England, Belgium (Flanders), Germany, Italy, The Netherlands, Portugal and Spain. Three stages of analysis were completed on open-ended question data: (i) inductive analysis to determine a category-code framework; (ii) country-level manifest deductive content analysis; and (iii) thematic analysis to identify cross-country prominent themes. RESULTS: Of the 9344 respondents, 1543 (17%) answered the open-ended question. Two prominent themes were revealed: (i) a need for improved quality of end-of-life and palliative care, and access to this care for patients and families and (ii) the recognition of the importance of death and dying, the cessation of treatments to extend life unnecessarily and the need for holistic care to include comfort and support. CONCLUSIONS: Within Europe, the public recognizes the importance of death and dying; they are concerned about the prioritization of quantity of life over quality of life; and they call for improved quality of end-of-life and palliative care for patients, especially for elderly patients, and families. To fulfil the urgent need for a policy response and to advance research and care, we suggest four solutions for European palliative and end-of-life care: institute government-led national strategies; protect regional research funding; consider within- and between-country variance; establish standards for training, education and service delivery.

Tailoring ZnSe-CdSe colloidal quantum dots via cation exchange: from core/shell to alloy nanocrystals.

We report a study of Zn(2+) by Cd(2+) cation exchange (CE) in colloidal ZnSe nanocrystals (NCs). Our results reveal that CE in ZnSe NCs is a thermally activated isotropic process. The CE efficiency (i.e., fraction of Cd(2+) ions originally in solution, Cdsol, that is incorporated in the ZnSe NC) increases with temperature and depends also on the Cdsol/ZnSe ratio. Interestingly, the reaction temperature can be used as a sensitive parameter to tailor both the composition and the elemental distribution profile of the product (Zn,Cd)Se NCs. At 150 °C ZnSe/CdSe core/shell hetero-NCs (HNCs) are obtained, while higher temperatures (200 and 220 °C) produce (Zn1-xCdx)Se gradient alloy NCs, with increasingly smoother gradients as the temperature increases, until homogeneous alloy NCs are obtained at T ≥ 240 °C. Remarkably, sequential heating (150 °C followed by 220 °C) leads to ZnSe/CdSe core/shell HNCs with thicker shells, rather than (Zn1-xCdx)Se gradient alloy NCs. Thermal treatment at 250 °C converts the ZnSe/CdSe core/shell HNCs into (Zn1-xCdx)Se homogeneous alloy NCs, while preserving the NC shape. A mechanism for the cation exchange in ZnSe NCs is proposed, in which fast CE takes place at the NC surface, and is followed by relatively slower thermally activated solid-state cation diffusion, which is mediated by Frenkel defects. The findings presented here demonstrate that cation exchange in colloidal ZnSe NCs provides a very sensitive tool to tailor the nature and localization regime of the electron and hole wave functions and the optoelectronic properties of colloidal ZnSe-CdSe NCs.

Loosening quantum confinement: observation of real conductivity caused by hole polarons in semiconductor nanocrystals smaller than the Bohr radius.

We report on the gradual evolution of the conductivity of spherical CdTe nanocrystals of increasing size from the regime of strong quantum confinement with truly discrete energy levels to the regime of weak confinement with closely spaced hole states. We use the high-frequency (terahertz) real and imaginary conductivities of optically injected carriers in the nanocrystals to report on the degree of quantum confinement. For the smaller CdTe nanocrystals (3 nm < radius < 5 nm), the complex terahertz conductivity is purely imaginary. For nanocrystals with radii exceeding 5 nm, we observe the onset of real conductivity, which is attributed to the increasingly smaller separation between the hole states. Remarkably, this onset occurs for a nanocrystal radius significantly smaller than the bulk exciton Bohr radius a(B) ∼ 7 nm and cannot be explained by purely electronic transitions between hole states, as evidenced by tight-binding calculations. The real-valued conductivity observed in the larger nanocrystals can be explained by the emergence of mixed carrier-phonon, that is, polaron, states due to hole transitions that become resonant with, and couple strongly to, optical phonon modes for larger QDs. These polaron states possess larger oscillator strengths and broader absorption, and thereby give rise to enhanced real conductivity within the nanocrystals despite the confinement.

Highly luminescent (Zn,Cd)Te/CdSe colloidal heteronanowires with tunable electron-hole overlap.

We report the synthesis of ultranarrow (Zn,Cd)Te/CdSe colloidal heteronanowires, using ZnTe magic size clusters as seeds. The wire formation starts with a partial Zn for Cd cation exchange, followed by self-organization into segmented heteronanowires. Further growth occurs by inclusion of CdSe. The heteronanowires emit in the 530 to 760 nm range with high quantum yields. The electron-hole overlap decreases with increasing CdSe volume fraction, allowing the optical properties to be controlled by adjusting the heteronanowire composition.

Optical Properties of Mn-Doped ZnTe Magic Size Nanocrystals.

In this letter we report successful doping of ZnTe magic size nanocrystals (MSNCs) with Mn(2+). Colloidal ZnTe MSNCs are prepared via a hot-injection method and doped with Mn(2+) via cation exchange. The doped MSNCs show an emission band centered at 620 nm with a radiative decay time of 45 µs, characteristic of Mn(2+) in ZnTe. The excitation spectrum of the Mn(2+) emission shows narrow absorption bands corresponding to different sizes of ZnTe MSNCs providing further evidence that the 620 nm emission originates from Mn(2+) incorporated in the ZnTe host, rather than Mn(2+) bound to the surface. The Mn(2+)-doped ZnTe clusters may serve as nuclei for the growth of larger ZnTe quantum dots doped with a single Mn(2+) ion.

Growth and stability of ZnTe magic-size nanocrystals.

A synthetic method for ZnTe magic-sized clusters (MSCs) is reported, and the stability and growth kinetics of these clusters are investigated. Four distinct MSC families, with lowest-energy absorption peaks at 330, 354, 378, and 392 nm, are observed. The stability and growth kinetics of the MSCs are strongly influenced by the reaction temperature, precursor concentration, and nature of the ligands used as the coordinating solvent. High precursor concentrations result in faster growth and MSC formation at lower temperatures. Higher temperatures accelerate the growth kinetics and lead to a gradual shift from the stepwise MSC growth regime to a continuous growth regime. For temperatures above 260 °C, only continuous growth of nanocrystals is observed. The nature of the ligands also influences the stability and growth of ZnTe MSCs, which are formed with primary alkylamines as ligands, but not when trioctylphosphine, trioctylphosphine oxide, or trioctylamine are used as the sole ligands. This demonstrates the crucial role of ligands in the formation of stable ZnTe MSCs using colloidal synthetic methods. Under optimal synthetic conditions (200 °C, hexadecylamine as ligand, and suitable precursor concentrations), the method presented here allows the synthesis and isolation of a single MSC family absorbing at 330 nm.

Colloidal hard-sphere crystal growth frustrated by large spherical impurities.

Impurities affect the nucleation, growth, and structure of crystals. Here we report the effect of large, spherical, polymethylmethacrylate impurities on the crystal growth of monodisperse, hard, polymethylmethacrylate colloids in a density- and optically matching apolar solvent mixture. Crystal growth, initiated at the bottom of the sample, was studied by imaging sequences of two-dimensional xy slices in the plane of the impurity's center with a laser scanning confocal microscope. Impurities form the center of grain boundaries, and a single fluid particle layer around the impurity persists in all cases. The growth rate sensitively depends on the impurity's size. Crystal growth is inhibited to a greater extent near smaller impurities, pointing to local crystal frustration induced by the curvature of the impurity.