Tailoring Gold Nanoparticle Characteristics and the Impact on Aqueous-Phase Oxidation of Glycerol

Poly(vinyl alcohol) (PVA)-stabilized Au nanoparticles (NPs) were synthesized by colloidal methods in which temperature variations (−75 to 75 °C) and mixed H2O/EtOH solvent ratios (0, 50, and 100 vol/vol) were used. The resulting Au NPs were immobilized on TiO2 (P25), and their catalytic performance was investigated for the liquid phase oxidation of glycerol. For each unique solvent system, there was a systematic increase in the average Au particle diameter as the temperature of the colloidal preparation increased. Generation of the Au NPs in H2O at 1 °C resulted in a high observed activity compared with current Au/TiO2 catalysts (turnover frequency = 915 h–1). Interestingly, Au catalysts with similar average particle sizes but prepared under different conditions had contrasting catalytic performance. For the most active catalyst, aberration-corrected high angle annular dark field scanning transmission electron microscopy analysis identified the presence of isolated Au clusters (from 1 to 5 atoms) for the first time using a modified colloidal method, which was supported by experimental and computational CO adsorption studies. It is proposed that the variations in the populations of these species, in combination with other solvent/PVA effects, is responsible for the contrasting catalytic properties

Morphological Features and Band Bending at Nonpolar Surfaces of ZnO

We employ hybrid density functional calculations to analyze the structure and stability of the (101̅0) and (112̅0) ZnO surfaces, confirming the relative stability of the two surfaces. We then examine morphological features, including steps, dimer vacancies, and grooves, at the main nonpolar ZnO surface using density functional methods. Calculations explain why steps are common on the (101̅0) surface even at room temperature, as seen in experiment. The surface structure established has been used to obtain the definitive ionization potential and electron affinity of ZnO in good agreement with experiment. The band bending across the surface is analyzed by the decomposition of the density of states for each atomic layer. The upward surface band bending at the (101̅0) surface affects mostly the valence band by 0.32 eV, which results in the surface band gap closing by 0.31 eV; at the (112̅0) surface, the valence band remains flat and the conduction band bends up by 0.18 eV opening the surface band gap by 0.12 eV

Deep vs shallow nature of oxygen vacancies and consequent n -type carrier concentrations in transparent conducting oxides

The source of n -type conductivity in undoped transparent conducting oxides has been a topic of debate for several decades. The point defect of most interest in this respect is the oxygen vacancy, but there are many conflicting reports on the shallow versus deep nature of its related electronic states. Here, using a hybrid quantum mechanical/molecular mechanical embedded cluster approach, we have computed formation and ionization energies of oxygen vacancies in three representative transparent conducting oxides: In 2 O 3 , SnO 2 , and ZnO. We find that, in all three systems, oxygen vacancies form well-localized, compact donors. We demonstrate, however, that such compactness does not preclude the possibility of these states being shallow in nature, by considering the energetic balance between the vacancy binding electrons that are in localized orbitals or in effective-mass-like diffuse orbitals. Our results show that, thermodynamically, oxygen vacancies in bulk In 2 O 3 introduce states above the conduction band minimum that contribute significantly to the observed conductivity properties of undoped samples. For ZnO and SnO 2 , the states are deep, and our calculated ionization energies agree well with thermochemical and optical experiments. Our computed equilibrium defect and carrier concentrations, however, demonstrate that these deep states may nevertheless lead to significant intrinsic n -type conductivity under reducing conditions at elevated temperatures. Our study indicates the importance of oxygen vacancies in relation to intrinsic carrier concentrations not only in In 2 O 3 , but also in SnO 2 and ZnO

Morphological Features and Band Bending at Nonpolar Surfaces of ZnO

We employ hybrid density functional calculations to analyze the structure and stability of the (101̅0) and (112̅0) ZnO surfaces, confirming the relative stability of the two surfaces. We then examine morphological features, including steps, dimer vacancies, and grooves, at the main nonpolar ZnO surface using density functional methods. Calculations explain why steps are common on the (101̅0) surface even at room temperature, as seen in experiment. The surface structure established has been used to obtain the definitive ionization potential and electron affinity of ZnO in good agreement with experiment. The band bending across the surface is analyzed by the decomposition of the density of states for each atomic layer. The upward surface band bending at the (101̅0) surface affects mostly the valence band by 0.32 eV, which results in the surface band gap closing by 0.31 eV; at the (112̅0) surface, the valence band remains flat and the conduction band bends up by 0.18 eV opening the surface band gap by 0.12 eV

Double bubbles: a new structural motif for enhanced electron–hole separation in solids

Electron–hole separation for novel composite systems comprised of secondary building units formed from different compounds is investigated with the aim of finding suitable materials for photocatalysis. Pure and mixed SOD and LTA superlattices of (ZnO)12 and (GaN)12, single-shell bubbles are constructed as well as core@shell single component frameworks composed of larger (ZnO)48 and (GaN)48 bubbles with each containing one smaller bubble. Enthalpies of formation for all systems are comparable with fullerenes. Hole and electron separation is achieved most efficiently by the edge sharing framework composed of (GaN)12@(ZnO)48 double bubbles, with the hole localised on the nitrogen within the smaller bubbles and the excited electron on zinc within the larger cages

Polymorphism of L-tryptophan

A new polymorph of L‐tryptophan has been prepared by crystallization from the gas phase, with structure determination carried out directly from powder XRD data augmented by periodic DFT‐D calculations. The new polymorph (denoted β) and the previously reported polymorph (denoted α) are both based on alternating hydrophilic and hydrophobic layers, but with substantially different hydrogen‐bonding arrangements. The β polymorph exhibits the energetically favourable L2‐L2 hydrogen‐bonding arrangement, which is unprecedented for amino acids with aromatic side‐chains; the specific molecular conformations adopted in the β polymorph facilitate this hydrogen‐bonding scheme while avoiding steric conflict of the side‐chains

Solid-state structural properties of alloxazine determined from powder XRD data in conjunction with DFT-D calculations and solid-state NMR spectroscopy : unraveling the tautomeric identity and pathways for tautomeric interconversion

We report the solid-state structural properties of alloxazine, a tricyclic ring system found in many biologically important molecules, with structure determination carried out directly from powder X-ray diffraction (XRD) data. As the crystal structures containing the alloxazine and isoalloxazine tautomers both give a high-quality fit to the powder XRD data in Rietveld refinement, other techniques are required to establish the tautomeric form in the solid state. In particular, high-resolution solid-state 15N NMR data support the presence of the alloxazine tautomer, based on comparison between isotropic chemical shifts in the experimental 15N NMR spectrum and the corresponding values calculated for the crystal structures containing the alloxazine and isoalloxazine tautomers. Furthermore, periodic DFT-D calculations at the PBE0-MBD level indicate that the crystal structure containing the alloxazine tautomer has significantly lower energy. We also report computational investigations of the interconversion between the tautomeric forms in the crystal structure via proton transfer along two intermolecular N–H···N hydrogen bonds; DFT-D calculations at the PBE0-MBD level indicate that the tautomeric interconversion is associated with a lower energy transition state for a mechanism involving concerted (rather than sequential) proton transfer along the two hydrogen bonds. However, based on the relative energies of the crystal structures containing the alloxazine and isoalloxazine tautomers, it is estimated that under conditions of thermal equilibrium at ambient temperature, more than 99.9% of the molecules in the crystal structure will exist as the alloxazine tautomer

Direct monitoring of the potassium charge carrier in Prussian blue cathodes using potassium K-edge X-ray absorption spectroscopy †

Prussian blue is widely utilized as a cathode material in batteries, due to its ability to intercalate alkaline metal ions, including potassium. However, the exact location of potassium or other cations within the complex structure, and how it changes as a function of cycling, is unclear. Herein, we report direct insight into the nature of potassium speciation within Prussian blue during cyclic voltammetry, via operando potassium K-edge X-ray Absorption Near Edge Structure (XANES) analysis. Clear and identifiable spectra are experimentally differentiated for the fully intercalated (fully reduced Fe2+FeII Prussian white), partially intercalated (Prussian blue; Fe3+FeII), and free KNO3(aq) electrolyte. Comparison of the experiment with simulated XANES of theoretical structures indicates that potassium lies within the channels of the Prussian blue structure, but is displaced towards the periphery of the channels by occluded water and/or structural water present resulting from [Fe(CN)6]4− vacancies. The structural composition from the charge carrier perspective was monitored for two samples of differing crystallite size and electrochemical stability. Reproducible potassium XANES spectral sequences were observed for large crystallites (ca. 100 nm) of Prussian blue, in agreement with retention of capacity; in contrast, the capacity of a sample with small crystallites (ca. 14 nm) declined as the potassium became trapped within the partially intercalated Prussian blue. The cause of degradation could be attributed to a significant loss of [Fe(CN)6]–[Fe(NC)6] ordering and the formation of a potassium-free non-conducting ferrihydrite phase. These findings demonstrate the potential of XANES to directly study the nature and evolution of potassium species during an electrochemical process

Materials and Molecular Modelling at the Exascale

Progression of computational resources towards exascale computing makes possible simulations of unprecedented accuracy and complexity in the fields of materials and molecular modelling (MMM), allowing high fidelity in silico experiments on complex materials of real technological interest. However, this presents demanding challenges for the software used, especially the exploitation of the huge degree of parallelism available on exascale hardware, and the associated problems of developing effective workflows and data management on such platforms. As part of the UKs ExCALIBUR exascale computing initiative, the UK-led MMM Design and Development Working Group has worked with the broad MMM community to identify a set of high priority application case studies which will drive future exascale software developments. We present an overview of these case studies, categorized by the methodological challenges which will be required to realize them on exascale platforms, and discuss the exascale requirements, software challenges and impact of each application area